CONTENTS.

PART I.—HISTORICAL INTRODUCTION.
PAGE
Beginning of agricultural chemistry[4]
Early theories regarding plant-growth[4]
Van Helmont[4]
Digby[6]
Duhamel and Stephen Hales[8]
Jethro Tull[9]
Charles Bonnet's discovery of source of plants' carbon[11]
Researches of Priestley, Ingenhousz, Sénébier, on assimilation of carbon[11-12]
Publication of first English treatise by Earl Dundonald[13]
Publication of Theodore de Saussure, 'Chemical Researches on Vegetation,' 1804[14]
Theories on source of plant-nitrogen[15]
Early experiments on this subject[16]
Sir Humphry Davy's lectures (1802-1812)[17]
State of agricultural chemistry in 1812[17]
Beginning of Boussingault's researches (1834)[21]
Publication of Liebig's first report to the British Association[24]
Refutation of "humus" theory[26]
Liebig's mineral theory[26]
Liebig's theory of source of plants' nitrogen[27]
Publication of Liebig's second report to British Association[30]
Liebig's services to agricultural chemistry[31]
Development of agricultural research in Germany[32]
The Rothamsted Experiment Station[33]
Sir J. B. Lawes and Sir J. H. Gilbert, the nature and value of their experiments[33]
Review of the present state of our knowledge of plant-growth[36]
Proximate composition of the plant[36]
Fixation of carbon by plants[37]
Action of light on plant-growth, Dr Siemens' experiments[38]
Source of oxygen and hydrogen in the plant[39-40]
Source of nitrogen in the plant[40]
Relation of the free nitrogen to leguminous plants[42-44]
Relation of nitrogen in organic forms, as ammonia salts, and nitrates to the plant[46-50]
Nitrification and its conditions[51]
Ash constituents of the plant[53]
Methods of research for ascertaining essentialness of ash constituents of plants[53]
(a) Artificial soils, (b) water-culture[53-55]
Method in which plants absorb their food-constituents[55]
Endosmosis[55]
Retention by soils of plant-food[57]
Causes of retention by soils of plant-food[59]
Manuring[60]
"Field" and "pot" experimentation[60]
PART II.—PRINCIPLES OF MANURING.
CHAPTER I.—FERTILITY OF THE SOIL.
What constitutes fertility in a soil[65]
I.Physical properties of a soil[65]
Kinds of soils[66]
Absorptive power for water of soils[67]
Absorptive power for water of sand, clay, and humus[68]
Fineness of particles of a soil[69]
Limit of fineness of soil-particles[69]
Importance of retentive power[70]
Power of plants for absorbing water from a soil, experiments by Sachs[73]
How to increase absorptive power of soils[74]
Amount of water in a soil most favourable for plant growth[75]
Hygroscopic power of soils[75]
Capacity of soils for absorbing and retaining heat[76]
Explanation of dew[77]
Heat of soils[78]
Heat in rotting farmyard manure[78]
Causes of heat of fermentation[79]
Influence of colour on heat-retaining power[80]
Power of soils for absorbing gases[81]
Gases found in soils[81]
Variation in gas-absorbing power of soils[82]
Absorption of nitrogen by soils[82]
Requirements of plant-roots in a soil[83]
Influence of tillage on number of plants in a certain area[86]
Comparison of English and American farming[86]
II.Chemical composition of a soil[87]
Fertilising ingredients of a soil[87]
Importance of nitrogen, phosphoric acid, and potash in a soil[88]
Chemical condition of fertilising ingredients in soils[89]
Amount of soluble fertilising ingredients in soils[90]
Value of chemical analysis of soils[90]
III.Biological properties of a soil[92]
Bacteria of the soil[92]
Recapitulation of Chapter I[96]
APPENDIX TO CHAPTER I.
NOTE
I.Table of absorptive power of soil substances by Schübler[98]
II.Table of rate of evaporation of water in different soils by Schübler[99]
III.Table of hygroscopic power of soils dried at 212° F. (Davy)[99]
IV.Gases persent in soil[100]
V.Amount of plant-food in soils[100]
VI.Chemical composition of the soil[101]
VII.Forms in which plant-foods are present in the soil[107]
CHAPTER II.—FUNCTIONS PERFORMED BY MANURES.
Etymological meaning of word manure[109]
Definition of manures[110]
Different classes of manures[111]
Action of different classes of manures[113]
CHAPTER III.—POSITION OF NITROGEN IN AGRICULTURE.
The Rothamsted experiments and the nitrogen question[115]
Different forms in which nitrogen exists in nature[116]
Relation of "free" nitrogen to the plant[117]
Combined nitrogen in the air[118]
Amount of combined nitrogen falling in the rain[119]
Nitrogen in the soil[120]
Nitrogen in the subsoil[121]
Nitrogen of surface-soil[121]
Amount of nitrogen in the soil[123]
Soils richest in nitrogen[123]
Nature of the nitrogen in the soil[124]
Organic nitrogen in the soil[125]
Differences of surface and subsoil nitrogen[126]
Nitrogen as ammonia in soils[127]
Amount of ammonia in soils[127]
Nitrogen present as nitrates in the soil[128]
Position of nitric nitrogen in soil[128]
Amount of nitrates in the soil[120]
Amount of nitrates in fallow soils[129]
Amount of nitrates in cropped soils[130]
Amount of nitrates in manured wheat-soils[131]
The sources of soil-nitrogen[131]
Accumulation of soil-nitrogen under natural conditions[133]
Accumulation of nitrogen in pastures[134]
Gain of nitrogen with leguminous crops[135]
The fixation of "free" nitrogen[136]
Influence of manures in increasing soil-nitrogen[136]
Sources of loss of nitrogen[137]
Loss of nitrates by drainage[137]
Prevention of loss of nitrogen by permanent pasture and
"catch-cropping"		[138]
Other conditions diminishing loss of nitrates[139]
Amount of loss of nitrogen by drainage[140]
Loss of nitrogen in form of "free" nitrogen[141]
Total amount of loss of nitrogen[142]
Loss of nitrogen by retrogression[142]
Artificial sources of loss of nitrogen[144]
Amount of nitrogen removed in crops[144]
Losses of nitrogen incurred on the farm[146]
Loss in treatment of farmyard manure[146]
Nitrogen removed in milk[147]
Economics of the nitrogen question[147]
Loss of nitrogen-compounds in the arts[148]
Loss due to use of gunpowder[148]
Loss due to sewage disposal[149]
Our artificial nitrogen supply [150]
Nitrate of soda and sulphate of ammonia[150]
Peruvian guano[151]
Bones[151]
Other nitrogenous manures[152]
Oil-seeds and oilcakes[153]
Other imported sources of nitrogen[153]
Conclusion[153]
APPENDIX TO CHAPTER III.
NOTE
I.Determination of the quantity of nitrogen supplied by rain, as ammonia and nitric acid, to an acre of land during one year[155]
II.Nitrogen in soils at various depths[156]
IIINitrogen as nitrates in cropped soils receiving no nitrogenous manures, in lb. per acre (Rothamsted soils)[157]
IV.Nitrogen as nitrates in Rothamsted soils[157]
V.Examples of increase of nitrogen in Rothamsted soils laid down in pastures[158]
VI.Loss by drainage of nitrates[158]
VII.Examples of decrease of nitrogen in Rothamsted soils[159]
VIII.Amount of drainage and nitrogen as nitrates in drainage-water from unmanured bare soil, 20 and 60 inches deep[160]
CHAPTER IV.—NITRIFICATION.
Process of nitrification[161]
Occurrence of nitrates in the soil[162]
Nitre soils of India[162]
Saltpetre plantations[163]
Cause of nitrification[165]
Ferments effecting nitrification[167]
Appearance of nitrous organisms[168]
Nitric organism[169]
Difficulty in isolating them[169]
Nitrifying organisms do not require organic matter[169]
Conditions favourable for nitrification—
Presence of food-constituents[170]
Presence of a salifiable base[171]
Only takes place in slightly alkaline solutions[172]
Action of gypsum on nitrification[173]
Presence of oxygen[173]
Temperature[175]
Presence of a sufficient quantity of moisture[176]
Absence of strong sunlight[176]
Nitrifying organisms destroyed by poisons[176]
Denitrification[177]
Denitrification also effected by bacteria[178]
Conditions favourable for denitrification[178]
Takes place in water-logged soils[179]
Distribution of the nitrifying organisms in the soil[179]
Depth down at which they occur[180]
Action of plant-roots in promoting nitrification[181]
Nature of substances capable of nitrification[181]
Rate at which nitrification takes place[183]
Nitrification takes place chiefly during summer[183]
Process goes on most quickly in fallow fields[184]
Laboratory experiments on rate of nitrification[185]
Certain portions of soil-nitrogen more easily nitrifiable than the rest[187]
Rate of nitrification deduced from field experiments[187]
Quantity of nitrates formed in the soils of fallow fields[188]
Position of nitrates depends on season[188]
Nitrates in drainage-waters[188]
Amount produced at different times of year[189]
Nitrification of manures[190]
Ammonia salts most easily nitrifiable[191]
Sulphate of ammonia the most easily nitrifiable manure[191]
Rate of nitrification of other manures[192]
Soils best suited for nitrification[192]
Absence of nitrification in forest-soils[193]
Important bearing of nitrification on agricultural practice[193]
Desirable to have soil covered with vegetation[194]
Permanent pasture most economical condition of soil[194]
Nitrification and rotation of crops[195]
APPENDIX TO CHAPTER IV.
NOTE
I.Old theories of nitrification[196]
II.Nitrification takes place in solutions devoid of organic matter[196]
III.Oxidising power of micro-organisms in soils[197]
IV.Effect of urine on nitrification in soils[197]
V.Solution used by Professor Frankland in cultivating nitrificative micro-organisms[198]
VI.Experiments by Boussingault on rate of nitrification[198]
VII.Nitrogen as nitrates in Rothamsted soils after bare fallow in lb. per acre[198]
CHAPTER V.—POSITION OF PHOSPHORIC ACID
IN AGRICULTURE.
Occurrence of phosphoric acid in nature[199]
Mineral sources of phosphoric acid[200]
Apatite and phosphorite[200]
Coprolites[201]
Occurrence of phosphoric acid in guanos[202]
Universal occurrence in common rocks[202]
Occurrence in the soil[203]
Condition in which phosphoric acid occurs in the soil[203]
Occurrence in plants[204]
Occurrence in animals[205]
Sources of loss of phosphoric acid in agriculture[205]
Loss of phosphoric acid by drainage[206]
Artificial sources of loss of phosphoric acid[206]
Amount of phosphoric acid removed in milk[207]
Loss of phosphoric acid in treatment of farmyard manure[208]
Loss of phosphoric acid in sewage[208]
Sources of artificial gain of phosphoric acid[208]
APPENDIX TO CHAPTER V.
NOTE
I.Composition of apatite (Voelcker)[210]
II.Percentage of phosphoric acid in the commoner rocks[211]
CHAPTER VI.—POSITION OF POTASH IN AGRICULTURE.
Potash of less importance than phosphoric acid[212]
Occurrence of potash[213]
Felspar and other potash minerals[213]
Stassfurt salts[214]
Occurrence of saltpetre[215]
Occurrence of potash in the soil[215]
Potash chiefly in insoluble condition in soils[216]
Percentage of potash in plants and plant-ash[216]
Occurrence of potash in animal tissue[217]
Sources of loss of potash[217]
Amount of potash removed in crops[218]
Amount of potash removed in milk[218]
Potash manures[218]
APPENDIX TO CHAPTER VI.
NOTE
I.Amount of potash in different minerals[220]
II.Quantity of potash obtained from 1000 lb. of different kinds of vegetation in the manufacture of potashes[220]
PART III.—MANURES.
CHAPTER VII.—FARMYARD MANURE.
Variation in its composition[223]
Made up of three classes of constituents[224]
Solid excreta—
Its nature[224]
Difference in composition of the solid excreta of the different farm animals[224]
Causes of this difference[225]
Percentage of manurial ingredients in solid excreta of different animals[226]
Urine—[228]
Its nature[228]
Variation in its composition[229]
Causes of this variation[229]
Manurial value of the urine of the different farm animals[230]
Percentage of the organic matter, nitrogen, and mineral substances in the food, voided in the solid excreta and urine[232]
Comparison of manurial value of total excrements of the different farm animals[234]
Nature of changes undergone by food in process of digestion[235]
Litter—[236]
Its uses[236]
Straw as litter, and its qualifications[237]
Composition of different kinds of straw[238]
Loam as litter[239]
Peat as litter[240]
Comparison of properties of peat-moss and straw[241]
The bracken-fern as litter[241]
Dried leaves as litter[242]
Manures produced by the different animals—
Horse-manure—
Amount produced[243]
Its nature and composition[243]
Amount of straw used for litter[244]
Sources of loss on keeping[245]
How to prevent loss[245]
Use of "fixers," and the nature of their action[245]
Cow-manure—
Amount produced[248]
Its nature and composition[249]
Amount of straw used as litter[248]
Sources of loss on keeping[249]
Advantages of short dung[249]
Pig-manure—
Amount produced[250]
Its nature and composition[250]
Amount of straw used as litter[251]
Sheep-manure—
Amount produced[251]
Nature and composition[251]
Amount of straw used as litter[252]
Methods of calculating amount of manure produced on the farm[252, note]
Fermentation of farmyard manure—
Action of micro-organic life in producing fermentation[255]
Two classes of bacteria active in this work, aerobies and anaerobies[255]
Conditions influencing fermentation—
Temperature[256]
Openness to the air[256]
Dampness[257]
Composition of manure[257]
Products of fermentation[257]
Analyses of farmyard manure—
Dr Voelcker's experiments[259]
Variation in composition[259]
Amounts of moisture, organic matter (containing nitrogen), and mineral matter[260]
Its manurial value compared with nitrate of soda, sulphate of ammonia, and superphosphate[260]
Comparison of fresh and rotten manure—
The nature and amount of loss sustained in the process of rotting[261]
Ought manure to be appliled fresh or rotten?[262]
Relative merits of covered and uncovered manure-heaps[263]
Methods of application of farmyard manure to the field—
Merits and demerits of the different methods[265]
Setting it out in heaps[265]
Spreading it broadcast, and letting it lie[266]
Ploughing it in immediately[267]
Value and function of farmyard manure—
As a supplier of the necessary elements of plant-food[268]
As a "universal" manure[269]
Proportion in which nitrogen, phosphoric acid, and potash are required by crops[269]
Proportion in which they are present in farmyard manure[270]
Farmyard manure poor in nitrogen[270]
Lawes' and Gilbert's experiments[271]
How it may be best reinforced by the use of "artificials"[271]
Indirect value of farmyard manure as a supplier of humus to the soil[273]
Its influence on soil-texture[273]
Its influence in setting free inert fertilising matter in the soil[274]
Rate at which farmyard manure ought to be applied[275]
Lasting nature of farmyard manure[276]
Its economic value[276]
APPENDIX TO CHAPTER VII.
NOTE
I.Difference in amount of excreta voided for food consumed[279]
II.Solid excreta voided by sheep, oxen, and cows[279]
III.Urine voided by sheep, oxen, and cows[280]
IV.Percentage of food voided in the solid and liquid excrements[281]
V.Pig excrements[281]
VI.Manurial constituents in 1000 parts of ordinary foods[282]
VII.Analyses of stable-manure, made respectively with peat-moss litter and wheat-straw[283]
VIII.Analyses of bracken[283]
IX.Analyses of horse-manure[283]
X.The nature of the chemical reactions of ammonia "fixers"[284]
XI.Analyses of cow-manure[286]
XII.Composition of fresh and rotten farmyard manure[286]
XIII.Comparison of fresh and rotten manure[288]
XIV.Lord Kinnaird's experiments[289]
XV.Drainings of manure-heaps[290]
XVI.Amounts of potash and phosphoric acid removed by rotation from a Prussian morgen (.631 acre)[290]
XVII.Composition of farmyard manure (fresh)[291]
XVIII.The urine (quantity voided)[291]
CHAPTER VIII.—GUANO.
Importance in agriculture[293]
Influence on British farming[294]
Influence of guano not wholly good[295]
Value of guano as a manure[296]
Origin and occurrence of guano[297]
Variation in composition of different guanos[299]
I.Nitrogenous guano—
(a)Peruvian guano[300]
Different deposits of Peruvian guano[301]
Appearance, colour, and nature of Peruvian guano[303]
Composition of Peruvian guano[304]
(b)Other nitrogenous manures: Angamos, Ichaboe[306]
II.Phosphatic guanos—
Occurrence of phosphatic guanos[308]
Inequality in composition of phosphatic guanos[309]
"Dissolved" phosphatic guano[310]
"Equalised" or "rectified" guano[311]
The action of phosphatic guanos as manures[312]
Proportion of fertilising constituents in guano[314]
Mode of application of guanos[315]
Quantity of guano to be used[317]
Adulteration of guano[318]
So-called guanos—
Fish-guano[320]
Value of fish-guano[322]
Mean-meal guano[324]
Value of meat-meal guano[324]
Bat guano[325]
Pigeon and fowl dung[325]
APPENDIX TO CHAPTER VIII.
NOTE
I.Peruvian guano imported into United Kingdom, 1865-1893[327]
II.Guano deposits of the world[327]
III.Composition of concretionary nodules[328]
IV.Table showing gradual deterioration of Peruvian guano, 1867-1881[329]
V.Composition of different guanos[329]
VI.Liebig's theory as to the action of oxalic acid in guano[330]
VII.Analyses of dung of fowls, pigeons, ducks, and geese[331]
CHAPTER IX.—NITRATE OF SODA.
Amount of exports[332]
Date of discovery of nitrate deposits[333]
The origin of nitrate deposits[334]
Forbes and Darwin on the theory of their origin[335]
Source of nitric acid in nitrate of soda[337]
Guano theory of origin of nitrate of soda[337]
Nitric acid in nitrate of soda probably derived from sea-weed[339]
Appearance of nitrate-fields[340]
The method of mining the nitrate of soda[341]
Composition of caliche[342]
Extent of the nitrate deposits[342]
Composition and properties of nitrate of soda[343]
Nitrate applied as a top-dressing[344]
Nitrate of soda encourages deep roots[344]
Is nitrate of soda an exhausting manure?[345]
Crops for which nitrate of soda is suited[346]
Method of application of nitrate of soda[347]
Importance of having a sufficiency of other fertilising constituents[348]
Conclusions drawn[349]
APPENDIX TO CHAPTER IX.
Total shipmentes from South America, 1830-1893[351]
Total imports into Europe and United Kingdom, 1873-1892[351]
CHAPTER X.—SULPHATE OF AMMONIA.
Value of ammonia as a manure[352]
Sources of sulphate of ammonia[353]
Ammonia from gas-works[353]
Other sources[354]
Composition, &c., of sulphate of ammonia[355]
Application of sulphate of ammonia[356]
APPENDIX TO CHAPTER X.
Production of sulphate of ammonia in United Kingdom, 1870-1892[358]
CHAPTER XI.—BONES.
Early use of bones[359]
Different forms in which bones are used[360]
Composition of bones[362]
The organic matter of bones[363]
The inorganic matter of bones[363]
Treatment of bones[364]
Action of bones[365]
Dissolved bones[368]
Crops suited for bones[368]
Bone-ash[369]
Bone-char or bone-black[369]
APPENDIX TO CHAPTER XI.
NOTE
I.Analysis of bone-meal[371]
II.Analysis of dissolved bones[371]
III.Composition of bone-ash[372]
IV.Composition of bone-char[372]
CHAPTER XII.—MINERAL PHOSPHATES.
Coprolites[373]
Canadian apatite or phosphorite[374]
Estremadura or Spanish phosphates[375]
Norwegian apatite[376]
Charlestown or South Carolina phosphate[376]
Belgian phosphate[377]
Somme phosphate[378]
Florida phosphate[378]
Lahn phosphate[379]
Bordeaux or French phosphate[379]
Algerian phosphate[379]
Crust guanos[379]
Value of mineral phosphates as manures[380]
APPENDIX TO CHAPTER XII.
Imports of phosphates[381]
CHAPTER XIII.—SUPERPHOSPHATES.
Discovery of superphosphate by Liebig[382]
Manufacture of superphosphate[383]
Nature of the reaction taking place[385]
Phosphates of lime[385]
Reverted phosphate[389]
Value of reverted phosphate[391]
Composition of superphosphates[391]
Action of superphosphates[392]
Action of superphosphate sometimes unfavourable[395]
Application of superphosphate[395]
Value of insoluble phosphates[396]
Rate at which superphosphate is applied[397]
APPENDIX TO CHAPTER XIII.
NOTE
I.The formulæ, and molecular and percentage composition, of the different phosphates[398]
II.Reactions of sulphuric acid and phosphate of lime[398]
III.Table for conversion of soluble phosphate into insoluble phosphate[399]
IV.Action of iron and alumina in causing reversion[399]
V.Relative trade values of phosphoric acid in different manures[400]
CHAPTER XIV.—PHOSPHATE OR BASIC SLAG.
Its manufacture[401]
Not at first used[403]
Discovery of its value as a manure[403]
Composition of basic slag[404]
Processes for preparing slag[406]
Solubility of basic slag[408]
Darmstadt experiments with basic slag[410]
Results of other experiments[413]
Soils most suited for slag[414]
Rate of application[414]
Method of application[416]
APPENDIX TO CHAPTER XIV.
Analysis of basic slag[417]
CHAPTER XV.—POTASSIC MANURES.
Relative importance[418]
Scottish soils supplied with potash[419]
Sources of potassic manures[419]
Stassfurt potash salts[420]
Relative merits of sulphate and muriate of potash[421]
Application of potash manures[422]
Soils and crops suited for potash manures[423]
Rate of application[423]
CHAPTER XVI.—MINOR ARTIFICIAL MANURES.
Scutch[427]
Shoddy and wool-waste[427]
Soot[428]
CHAPTER XVII.—SEWAGE AS A MANURE.
Irrigation[431]
Effects of continued application of sewage[433]
Intermittent irrigation[434]
Crops suited for sewage[434]
Treatment of sewage by precipitation, &c.[436]
Value of sewage sludge[439]
CHAPTER XVIII.—LIQUID MANURE[442]
CHAPTER XIX.—COMPOSTS.
Farmyard manure a typical compost[446]
Other composts[447]
CHAPTER XX.—INDIRECT MANURES.
Lime[449]
Antiquity of lime as a manure[449]
Action of lime[449]
Lime a necessary plant-food[450]
Lime of abundant occurrence[452]
Lime returned to the soil in ordinary agricultural practice[452]
Different forms of lime[453]
Caustic lime[453]
Lime acts both mechanically and chemically[455]
I.Mechanical functions of lime[455]
Action on soil's texture[455]
Lime renders light soils more cohesive[457]
II.Chemical action of lime[457]
III.Biological action of lime[459]
Action of lime on nitrogenous organic matter[460]
Recapitulation[461]
CHAPTER XXI.—INDIRECT MANURES—GYPSUM, SALT, Etc.
Gypsum[462]
Mode in which gypsum acts[462]
Salt[465]
Antiquity of the use of salt[465]
Nature of its action[465]
Salt not a necessary plant-food[466]
Can soda replace potash?[466]
Salt of universal occurrence[467]
Special sources of salt[468]
The action of salt[468]
Mechanical action on soils[470]
Solvent action[470]
Best used in small quantities along with manures[472]
Affects quality of crop[472]
Rate of application[473]
CHAPTER XXII.—THE APPLICATION OF MANURES.
Influence of manures in increasing soil-fertility[474]
Influence of farmyard manure on the soil[475]
Farmyard manure v. artificials[476]
Farmyard manure not favourable to certain crops[477]
Conditions determining the application of artificial manures[477]
Nature of the manure[478]
Nitrogenous manures[478]
Phosphatic manures[480]
Potash manures[480]
Nature of soil[481]
Nature of previous manuring[482]
Nature of the crop[483]
Amounts of fertilising ingredients removed from the soil by different crops[484]
Capacity of crops for assimilating manures[486]
Difference in root-systems of different crops[488]
Period of growth[489]
Variation in composition of crops[490]
Absorption of plant-food[490]
Fertilising ingredients lodge in the seed[491]
Forms in which nitrogen exists in plants[491]
Bearing of above on agricultural practice[492]
Influence of excessive manuring of crops[492]
CHAPTER XXIII.—MANURING OF THE COMMON FARM CROPS.
Cereals[493]
Especially benefited by nitrogenous manures[494]
Power of absorbing silicates[494]
Barley[495]
Period of growth[495]
Most suitable soil[496]
Farmyard manure not suitable[497]
Importance of uniform manuring of barley[497]
Norfolk experiments on barley[497]
Proportion of grain to straw[498]
Wheat[499]
Rothamsted experiments[500]
Continuous growth[500]
Flitcham experiments[500]
Oats[501]
A very hardy crop[502]
Require mixed nitrogenous manuring[502]
Arendt's experiments[503]
Avenine[503]
Quantities of manures[504]
Grass[504]
Effect of manures on herbage of pastures[505]
Influence of farmyard manure[506]
Influence of soil and season on pastures[507]
Manuring of meadow land[508]
Bangor experiments[508]
Norfolk experiments[509]
Manuring of permanent pastures[509]
Roots[510]
Influence of manure on composition[512]
Nitrogenous manures increase sugar[512]
Amount of nitrogen recovered in increase of crop[513]
Norfolk experiments[513]
Manure for swedes[514]
Highland Society's experiments[515]
Manuring for rich crops of turnips[516]
Experiments by the author on turnips[516]
Potatoes[517]
Highland Society's experiments[518]
The Rothamsted experiments[519]
Effect of farmyard manure[520]
Manuring of potatoes in Jersey[521]
The influence of manure on the composition[521]
Leguminous crops[522]
Leguminous plants benefit by potash[523]
Nitrogenous manures may be hurtful[523]
Clover sickness[524]
Alternate wheat and bean rotation[524]
Beans[525]
Manure for beans[525]
Relative value of manurial ingredients[526]
Gypsum as a bean manure[526]
Effect of manure on composition of crop[527]
Peas[527]
Hops[528]
Cabbages[528]
APPENDIX TO CHAPTER XXIII.
Experiments on bean-manuring[530]
CHAPTER XXIV.—ON THE METHOD OF APPLICATION,
AND ON THE MIXING OF MANURES
Equal distribution of manures[531]
Mixing manures[532]
Risks of loss in mixtures[533]
Loss of ammonia[533]
Effects of lime on ammonia[535]
Loss of nitric acid[536]
Reversion of phosphates[537]
Manurial ingredients should be applied separately[538]
CHAPTER XXV.—ON THE VALUATION AND ANALYSIS OF MANURES.
Value of chemical analysis[539]
Interpretation of chemical analysis[539]
Nitrogen[540]
Phosphoric acid[541]
Importance of mechanical condition of phosphate[542]
Potash[542]
Other items in the chemical analysis of manures[543]
Fertilisers and Feeding Stuffs Act[543]
Different methods of valuing manures[544]
Unit value of manurial ingredients[544]
Intrinsic value of manures[545]
Field experiments[545]
Educational value of field experiments[547]
Value of manures deduced from experiments[548]
Value of unexhausted manures[549]
Potential fertility of a soil[549]
Tables of value of unexhausted manures[551]
APPENDIX TO CHAPTER XXV.
NOTE
I.Factors for calculating compounds from manurial ingredients[553]
II.Units for determining commercial value of manures and cash prices of manures[554, 555]
III.Manurial value of nitrogen and potash in different substances[556]
IV.Comparative manurial value of different forms of nitrogen and potash[557]
V.Lawes' and Gilbert's tables for calculating unexhausted value of manures[559]
CHAPTER XXVI.—THE ROTHAMSTED EXPERIMENTS.
Nature of experiments on crops and manures[561]
Soil of Rothamsted[561]
Table I. List of Rothamsted field experiments[562]
Wheat experiments—
Unmanured plots[565]
Wheat grown continuously on same land (unmanured)[562]
Table II. Results of first eight years[562]
Table III. Results of subsequent forty years[562]
Tablel IV. Wheat grown continuously with farmyard manure (14 tons per annum)[564]
Table V. Wheat grown continuously with artificial manures[565]
Table VI. Experiments on the growth of barley, forty years, 1852-91[566]
Table VIII. Experiments on the growth of oats, 1869-78[567]
Table IX. Experiments on mangel-wurzel[568, 569]
Table X. Experiments with different manures on permanent meadow-land, thirty-six years, 1856-91[570]
Table XI. Experiments on the growth of potatoes— average for five seasons, 1876-80[571]
Table XII. Experiments on growth of potatoes (continued)—average for twelve seasons, 1881-92[572]
_______________
Index[573]

PART I.

HISTORICAL INTRODUCTION

MANURES AND THE PRINCIPLES OF MANURING.

HISTORICAL INTRODUCTION.

Agricultural Chemistry, like most branches of natural science, may be said to be entirely of modern growth. While it is true we have many old speculations on the subject, they can scarcely be said to possess much scientific value. The great questions which had first to be solved by the agricultural chemist were,—What is the food of plants? and,—What is the source of that food? The second of these two questions more easily admitted of answer than the first. The source of plant-food could only be the atmosphere or the soil. As the composition of the atmosphere, however, was not discovered till the close of last century, and the chemistry of the soil is a question which is still requiring much work ere we shall be in possession of anything like a full knowledge of it, it will be at once obvious that the very fundamental conditions for a solution of the question were awanting. The beginning, then, of a true scientific agricultural chemistry may be said to date from the brilliant discoveries associated with the names of Priestley, Scheele, Lavoisier, Cavendish, and Black—that is, towards the close of last century.

Early Theories on Source of Plant-food.

While this is so, and while we must regard the early attempts made towards solving this question as being, for the most part, of little scientific value, it is not without interest, from the historical point of view, to glance briefly at some of these old interesting speculations.

The Aristotelian doctrine, regarding the possibility of dividing matter into the so-called four primary elements, fire, air, earth, and water, which obtained in one form or another till the birth of modern chemistry, had naturally an important influence on these early theories.

Van Helmont's Theory.

Among the earliest and most important attempts made to solve the problem of plant-growth was that by Jean Baptiste Van Helmont, one of the best known of the alchemists, who flourished about the beginning of the seventeenth century. Van Helmont believed that he had proved by a conclusive experiment that all the products of vegetables were capable of being generated from water. The details of this classical experiment were as follows:—

"He took a given weight of dry soil—200 lb.—and into this soil he planted a willow-tree that weighed 5 lb., and he watered this carefully from time to time with pure rain-water, taking care to prevent any dust or dirt falling on to the earth in which the plant grew. He allowed this to go on growing for five years, and at the end of that period, thinking his experiment had been conducted sufficiently long, he pulled up his tree by the roots, shook all the earth off, dried the earth again, weighed the earth and weighed the plant. He found that the plant now weighed 169 lb. 3 ounces, whereas the weight of the soil remained very nearly what it was—about 200 lb. It had only lost 2 ounces in weight."[1]

The conclusion, therefore, come to by Van Helmont was that the source of plant-food was water.[2]

Digby's Theory.

Some fifty years later an extremely interesting book was published bearing the following title: 'A Discourse concerning the Vegetation of Plants, spoken by Sir Kenelm Digby, at Gresham College, on the 23d of January 1660. (At a meeting of the Society for promoting Philosophical Knowledge by Experiments. London: Printed for John Williams, in Little Britain, over against St Botolph's Church, 1669.)' The author attributes plant-growth to the influence of a balsam which the air contains. This book is especially interesting as containing the earliest recognition of the value of saltpetre as a manure. The following is an extract from this interesting old work:—

"The sickness, and at last the death of a plant, in its natural course, proceeds from the want of that balsamick saline juice; which, I have said, makes it swell, germinate, and augment itself. This want may proceed either from a destitution of it in the place where the plant grows, as when it is in a barren soil or bad air, or from a defect in the plant itself, that hath not vigour sufficient to attract it, though it be within the sphere of it; as when the root has become so hard, obstructed and cold, as that it hath lost its vegetable functions. Now, both these may be remedy'd, in a great measure, by one and the same physick.... The watering of soils with cold hungray springs doth little good; whereas muddy saline waters brought to overflow a piece of ground enrich it much. But above all, well-digested dew makes all plants luxuriate and prosper most. Now what may it be that endues these liquors with such prolifick virtue? The meer water which is common to them all, cannot be it; there must be something else enclosed within it, to which the water serves but for a vehicle. Examine it by spagyric art, and you will find that it is nothing else than a nitrous salt, which is dilated in the water. It is this salt which gives fœcundity to all things: and from this salt (rightly understood) not only all vegetables, but also all minerals draw their origine. By the help of plain salt-peter, dilated in water and mingled with some other fit earthy substance, that may familiarize it a little with the corn into which I endeavoured to introduce it, I have made the barrenest ground far out-go the richest, in giving a prodigiously plentiful harvest. I have seen hemp-seed soaked in this liquor, that hath in due time made such plants arise, as, for the tallness and hardness of them, seemed rather to be coppice-wood of fourteen years' growth at least, than plain hemp. The fathers of the Christian doctrine at Paris still keep by them for a monument (and indeed it is an admirable one) a plant of barley consisting of 249 stalks, springing from one root or grain of barley; in which they counted above 18,000 grains or seeds of barley. But do you think that it is barely the salt-peter, imbibed into the seed or root, which causeth this fertility? no: that would be soon exhausted and could not furnish matter to so vast a progeny. The salt-peter there is like a magnet, which attracts a like salt which fœcundates the air, and gave cause to the Cosmopolite to say there is in the air a hidden food of life."[3]

Duhamel and Hales.

The names of the French writer, Duhamel, and of the English, Stephen Hales, may be mentioned in passing as authors of works bearing on the question of vegetable physiology. Both of these writers flourished about the middle of the eighteenth century. The writings of the former contained much valuable information on the effects of grafting, motion of sap, and influence of light on vegetable growth, and also the results of experiments which the author had carried out on the influence of treating plants with certain substances. 'Statical Essays, containing Vegetable Staticks; or an Account of some Statical Experiments on the Sap of Vegetables, by Stephen Hales, D.D.' (2 vols.), was published in London in 1738; and contained, as will be seen from its title, records of experiments of very much the same nature as those of Duhamel.

Jethro Tull's Theory.

Some reference may be made to a theory which created a considerable amount of interest when it was first published—viz., that of Jethro Tull. The chief value of Tull's contribution to the subject of agricultural science was, that he emphasised the importance of tillage operations by putting forward a theory to account for the fact, universally recognised, that the more thoroughly a soil was tilled, the more luxuriant the crops would be. As Tull's theory had a very considerable influence in stirring up interest in many of the most important problems in agricultural chemistry, and as it contained in itself much, the value of which we have only of late years come to understand, a brief statement of this theory may not be without interest.

According to Tull the food of plants consists of the particles of the soil. These particles, however, must be rendered very minute before they become available for the plant, which absorbs them by means of its rootlets. This pulverisation of the soil goes on in nature independently of the farmer, but only very slowly, and the farmer has therefore to hasten it on by means of tillage operations. The more efficiently these operations are carried on, the more abundant will the supply of plant-food be rendered in the soil. He consequently introduced and advocated the system of horse-hoe husbandry. This theory, he informs us, was suggested to him by the custom, which he had noticed on the Continent, of growing vines in rows, and hoeing the intervals between these rows from time to time. The excellent results which followed this mode of cultivation induced him to adopt it in England for his farm crops. He accordingly sowed his crops in rows or ridges, wide enough apart to admit of thorough tillage of the intervals by ploughing as well as by hand-hoeing. This he continued until the plant had reached maturity. As to the exact width of the interval most suitable, he made a large number of experiments. At first, in the cultivation of wheat, he made this interval six feet wide; but latterly he adopted an interval of lesser width, that finally arrived at being between four and five feet. He likewise experimented on each separate ridge as to which was the best number of rows of wheat to be sown, latterly adopting, as most convenient, two rows at ten inches apart. The great success which he met with in this system of cultivation induced him to publish the results of his experiments in his famous work, 'Horse-Hoeing Husbandry.'

While Tull's theory was based on principles at heart thoroughly sound, he was carried away by his personal success into drawing unwarrantable deductions. Thus he came to the conclusion that rotation of crops was unnecessary, provided that a thorough system of tillage was carried out. Manures also, according to him, might be entirely dispensed with under his system of cultivation, for the true function of all manures is to aid in the pulverisation of the soil by fermentation.

The first really valuable scientific facts contributed to the science were made by Priestley, Bonnet, Ingenhousz, and Sénébier.

Discovery of the Source of Plants' Carbon.

To Charles Bonnet (1720-1793), a Swiss naturalist, is due the credit of having made the first contribution to a discovery of very great importance—viz., the true source of the carbon, which we now know forms so large a portion of the plant-substance. Bonnet, who had devoted himself to the question of the function of leaves, noticed that when these were immersed in water bubbles were seen, after a time, to collect on their surface. De la Hire, it ought to be pointed out, had noticed this same fact about sixty years earlier. It was left to Priestley, however, to identify these bubbles with the gas he had a short time previously discovered—viz., oxygen. Priestley had observed, about this time, the interesting fact that plants possessed the power of purifying air vitiated by the presence of animal life.[4] The next step in this highly interesting and important discovery was taken by John Ingenhousz (1730-1799), an eminent physician and natural philosopher. In 1779, Ingenhousz published a work in London entitled 'Experiments on Vegetables.' In it he gives the results of some important experiments he had made on the question already investigated by Bonnet and Priestley. These experiments proved that plant-leaves only gave up their oxygen in the presence of sunlight. In 1782 he published another work on 'The Influence of the Vegetable Kingdom on the Animal Creation.'[5]

The source of the gas, which Bonnet had first noticed to be given off from plant-leaves, Priestley had identified as oxygen, and Ingenhousz had proved to be only given off under the influence of the sun's rays, was finally shown by a Swiss naturalist, Jean Sénébier[6] (1742-1809), to be the carbonic acid gas in the air, which the plant absorbed and decomposed, giving out the oxygen and assimilating the carbon.

Publication of First English Treatise on Agricultural Chemistry.

In 1795, a book dealing with the relations between chemistry and agriculture was published. This work was written by a Scottish nobleman, the Earl of Dundonald, and possesses especial interest from the fact that it is the first book in the English language on agricultural chemistry. The full title is as follows: 'A Treatise showing the Intimate Connection that subsists between Agriculture and Chemistry.'

In his introduction the author says: "The slow progress which agriculture has hitherto made as a science is to be ascribed to a want of education on the part of the cultivators of the soil, and to a want of knowledge, in such authors as have written on agriculture, of the intimate connection that subsists between the science and that of chemistry. Indeed, there is no operation or process not merely mechanical that does not depend on chemistry, which is defined to be a knowledge of the properties of bodies, and of the effects resulting from their different combinations."

In quoting this passage Professor S. W. Johnson remarks:[7] "Earl Dundonald could not fail to see that chemistry was ere long to open a splendid future for the ancient art that had always been and always will be the prime supporter of the nations. But when he wrote, how feeble was the light that chemistry could throw upon the fundamental questions of agricultural science! The chemical nature of the atmosphere was then a discovery of barely twenty years' standing. The composition of water had been known but twelve years. The only account of the composition of plants that Earl Dundonald could give was the following: 'Vegetables consist of mucilaginous matter, resinous matter, matter analogous to that of animals, and some proportion of oil.... Besides these, vegetables contain earthy matters, formerly held in solution in the newly-taken-in juices of the growing vegetables.' To be sure, he explains by mentioning in subsequent pages that starch belongs to the mucilaginous matter, and that on analysis by fire vegetables yield soluble alkaline salts and insoluble phosphate of lime. But these salts, he held, were formed in the process of burning, their lime excepted; and the fact of their being taken from the soil and constituting the indispensable food of plants, his lordship was unacquainted with. The gist of agricultural chemistry with him was, that plants 'are composed of gases with a small proportion of calcareous matter; for although this discovery may appear to be of small moment to the practical farmer, yet it is well deserving of his attention and notice.'"

De Saussure.

The year 1804 witnessed the publication of by far the most important contribution made to the science up till this time. This was 'Recherches Chimique sur la Végétation,' by Theodore de Saussure, one of the most illustrious agricultural chemists of the century. De Saussure was the first to draw attention to the mineral or ash constituents of the plant; and thus anticipate, to a certain extent, the subsequent famous "mineral" theory of the great Liebig. The French chemist maintained that these ash ingredients were essential; and that without them plant-life was impossible. He also adduced fresh experiments of his own in support of the theory, based on the experiments of Bonnet, Priestley, Ingenhousz, and Sénébier, that plants obtain their carbon from the carbonic acid gas in the air, under the influence of the sunlight. He was of opinion that the hydrogen and oxygen of the plant were, probably, chiefly derived from water. He showed that by far the largest portion of the plant's substance was derived from the air and from water, and that the ash portion was alone derived from the soil. To Saussure we owe the first definite statement on the different sources of the plant's food. It may be said that the lapse of nearly a century has shown his views to be, in the main, correct.

Source of Plant-nitrogen.

There was one question, which, even at that remote period in the history of the subject, engaged the attention of agricultural chemists—viz., the question of the source of the plant's nitrogen—a question which may be fitly described at the present hour as still the burning question of agricultural chemistry.[8]

As soon as it was discovered that nitrogen was a constituent of the plant's substance; speculations as to its source were indulged in. The fact that the air furnished an unlimited storehouse of this valuable element, and the analogy of the absorption of carbon (from the same source by plant-leaves), naturally suggested to the minds of early inquirers that the free nitrogen of the air was the source of the plant's nitrogen. As, however, no direct experiments could be adduced to prove this theory, and as, moreover, nitrogen was found in the soil, and seemed to be a necessary ingredient of all fertile soils, the opinion that the soil was the only source gradually supplanted the older theory. Little value, however, must be attached to these early theories, as they can scarcely be said to have been based on experiments of serious value. Indeed it may be safely affirmed, in the light of subsequent experiments, that it was impossible for this question to be decided at this early period, from the fact that analytical apparatus, of a sufficiently delicate nature, was then wholly unknown. Indeed it is only within the last few years that it has been possible to carry out experiments which may be regarded as at all crucial. A short sketch of the development of our knowledge of the relation of nitrogen to the plant will be given further on.

Sir Humphry Davy's Lectures.

A series of lectures on agricultural chemistry, delivered by Sir Humphry Davy during the years 1802-1812, for the Board of Agriculture, and subsequently published in book form in the year 1813,[9] affords us an opportunity of gauging, pretty accurately, the state of knowledge on the subject at the time.

Position of Agricultural Chemistry at beginning of Century.

In his opening lecture Davy says: "Agricultural chemistry has not yet received a regular and systematic form. It has been pursued by competent experimenters for a short time only. The doctrines have not as yet been collected into any elementary treatise, ... and," he adds, "I am sure you will receive with indulgence the first attempt made in this country to illustrate it by a series of experimental demonstrations."

He further on remarks: "It is evident that the study of agricultural chemistry ought to be commenced by some general inquiries into the composition and nature of material bodies, and the law of their changes. The surface of the earth, the atmosphere, and the water deposited from it, must either together, or separately, afford all the principles concerned in vegetation, and it is only by examining the chemical nature of these principles that we are capable of discovering what is the food of plants, and the manner in which this food is supplied and prepared for their nourishment."

Davy goes on further to say: "No general principles can be laid down respecting the comparative merits of the different systems of cultivation and the various systems of crops adopted in different districts, unless the chemical nature of the soil, and the physical circumstances to which it is exposed, are fully known."

He recognises the enormous importance of experiments. "Nothing is more wanting in agriculture than experiments, in which all the circumstances are minutely and scientifically detailed."

In dealing with the composition of plants he says: "It is evident that the most essential vegetable substances consist of hydrogen, carbon, and oxygen, in different proportions, generally alone; but in some few cases combined as carbon and nitrogen. The acids, alkalies, earths, metallic oxides, and saline compounds, though necessary in the vegetable economy, must be considered as of less importance, particularly in their relation to agriculture, than the other principles."

Further on: "It will be asked, Are the pure earths in the soil merely active as mechanical or indirect chemical agents, or do they actually afford food to the plant?"

This question he answers by saying that "water, and the decomposing animal and vegetable matter existing in the soil, constitute the true nourishment of plants; and as the earthy parts of the soil are useful in retaining water, so as to supply it in the proper proportion to the roots of the vegetables, so they are likewise efficacious in producing the proper distribution of the animal or vegetable matter. When equally mixed with it, they prevent it from decomposing too rapidly; and by their means the soluble parts are supplied in proper proportions."

Value of Davy's Lectures.

The chief value of these lectures is due to the fact that they form the first attempt to connect in a systematic manner the various scattered facts, up to that time ascertained, and to interpret their bearing on agricultural practice. We have in them, it is true, a strange mixture of facts belonging rather to botany and physiology than to agricultural chemistry; still they undoubtedly furnished a great impetus to inquiry, and at the same time they did much to popularise the science.

But not merely did Davy summarise and systematise the various results arrived at by others, he also made many valuable contributions to the science himself. The conclusions he drew from the results he obtained were, no doubt, in many cases false, and in other cases exaggerated; still the results possess a permanent interest. He may be said to have worked out many of the most important physical or mechanical properties of a soil, although exaggerating the importance of the influence of these properties on the question of fertility.[10]

These experiments had to do with the heat- and water-absorbing powers of a soil. He experimented on a brown fertile soil, and a cold barren clay, and found at what rate they lost heat. "Nothing," he says, "can be more evident than that the genial heat of the soil, particularly in spring, must be of the highest importance to the rising plant; ... so that the temperature of the surface, when bare and exposed to the rays of the sun, affords at least one indication of the degree of the fertility."

Again he says: "The power of soils to absorb water from air is much connected with fertility.... I have compared the absorbent powers of many soils, with respect to atmospheric moisture, and I have always found it greatest in the most fertile soils; so that it affords one method of judging of the productiveness of land."

Where he erred was in overestimating the functions of the mechanical properties of a soil, and in considering fertility to be due to them alone.

During the next thirty years or so, little progress seems to have been made in the way of fresh experimentation.

Boussingault.

In 1834, Boussingault,[11] the most distinguished French agricultural chemist of the century, began that series of brilliant chemico-agricultural experiments on his estate at Bechelbronn, in Alsace, the results of which have added so much to agricultural science. It was the first instance of the combination of "science with practice," of the institution of a laboratory on a farm; a combination peculiarly fitted to promote the interests of agricultural science, and an example which has been since followed with such magnificent results in the case of Sir John Lawes's famous Rothamsted Experiment Station, and other less known research stations.

Boussingault's first paper appeared in 1836, and was entitled, "The amount of nitrogen in different kinds of foods, and on the equal value of foods founded on these data."

In the year following other papers were published on such subjects as the amount of gluten in different kinds of wheat; on the meteorological considerations of how far various agricultural operations—such as extensive clearings of wood, the draining of large swamps, &c.—influence of climate on a country; and on experiments on the culture of the vine.

Boussingault was the first observer to study the scientific principles underlying the system of rotation of crops. In 1838 he published the results of some very elaborate experiments he had carried out on this subject. He also was the first chemist to carry out elaborate experiments with a view to deciding the question of the assimilation by plants of free atmospheric nitrogen. His first contribution to the subject was published in 1838, but can scarcely be regarded as possessing much scientific value, except in so far as it stimulated further research. Some thirteen years later he returned to this question; and during the years 1851-1855 carried out most elaborate experiments, the results of which, until quite recently, were generally regarded as having, along with the experiments of Messrs Lawes, Gilbert, and Pugh, definitely settled the question.[12]

In 1839 Boussingault was elected a member of the French Institute, an honour paid to him in recognition of his great services to agricultural chemistry.[13]

The foregoing is a brief epitome of the history of the development of agricultural chemistry up to the year 1840, the year which witnessed the publication of one of the most memorable works on the subject, which has appeared during the present century—Liebig's first report to the British Association, a work which may be described as constituting an epoch in the history of the science. Liebig's position as an agricultural chemist was so prominent, and his influence as a teacher so potent, that a few biographical facts may not be out of place before entering upon an estimate of his work.

Liebig.

Liebig was born at Darmstadt in the year 1803. He was the son of a drysalter, and early devoted himself to the study of chemistry in the only way at first at his disposal—viz., in an apothecary's shop. Soon finding, however, his opportunities of study limited, he left the apothecary's shop for the University of Bonn. He did not remain long at Bonn, but in a short time left that university for Erlangen, where he studied for some years, taking his Ph.D. degree in 1822. His subsequent studies were carried on at Paris under Gay-Lussac, Thénard, Dulong, and other distinguished chemists. Through the influence of A. Humboldt, who was at that time in Paris, and whose acquaintance he was fortunate enough to make, he was received into Gay-Lussac's private laboratory. In 1824—that is, when he was only twenty-one years of age—he was appointed Professor Extraordinarius of Chemistry at the University of Giessen. Two years later he was appointed to the post of Professor Ordinarius—an appointment which he held for twenty-five years. In 1845 he was created Baron, and in 1852 appointed Professor at Munich. He died in 1873.

His First Report to British Association.

The report above referred to was made by Liebig at the request of the Chemical Section of the British Association. It was read to a meeting of the Association held in Glasgow in 1840, and was subsequently published in book form, under the title of 'Chemistry in its Application to Agriculture and Physiology,' Liebig's position, past training and experience were such as to peculiarly fit him for the part of pioneer in the new science. As Sir J. H. Gilbert has remarked,[14] "In the treatment of his subject he not only called to his aid the previously existing knowledge directly bearing upon his subject, but he also turned to good account the more recent triumphs of organic chemistry, many of which had been won in his own laboratory."

In his dedication to the British Association at the beginning of the book, Liebig says: "Perfect agriculture is the true foundation of all trade and industry—it is the foundation of the riches of States. But a rational system of agriculture cannot be formed without the application of scientific principles; for such a system must be based on an exact acquaintance with the means of nutrition of vegetables, and with the influence of soils and actions of manure upon them. This knowledge we must seek from chemistry, which teaches the mode of investigating the composition and of studying the characters of the different substances from which plants derive their nourishment."

His criticism of the "Humus" Theory.

The first subject which Liebig discusses is the scientific basis of the so-called "humus" theory. The humus theory seems to have been first promulgated by Einhof and Thaer towards the close of last century. Thaer held that humus was the source of plant-food. He stated in his published writings that the fertility of a soil depended really upon its humus; for this substance, with the exception of water, is the only source of plant-food. De Saussure, however, by his experiments—the results of which he had published in 1804—had shown the fallacy of this humus theory; and his statements had been further developed and substantiated by the investigations of the French chemist Braconnot and the German chemist Sprengel. Despite, however, the experiments of Saussure, Braconnot, and Sprengel, the belief that plants derived the carbonaceous portion of their substance from humus still seemed to be commonly held in 1840.

While Liebig, therefore, can scarcely be said to have been the first to controvert the humus theory, he certainly dealt it its death-blow. He reasserted de Saussure's conclusions, and by some simple calculations showed very clearly that it was wholly untenable. One of the most striking of the arguments he brought forward was the fact that the humus of the soil itself consisted of the decayed vegetable matter of preceding plants. This being so, how, he asked, could it be the original source of the carbon of plants? To reason thus was simply to reason in a circle. He pointed out, further, that the comparative insolubility of humus in water, or even in alkaline solutions, told against its acceptance as correct.

His Mineral Theory.

Having thus controverted the humus theory, he then goes on to deal with the question of the source of the various plant constituents. In treating of the relation of the soil to the plant, he puts forward his "mineral" theory. It cannot be doubted that, while the advance of science since Liebig's time has induced us to considerably modify his mineral theory, it contained the statement of one of the most important facts in the chemistry of plant physiology. He was the first to fully estimate the enormous importance of the mineral portion of the plant's food, and point the way to one of the chief sources of a soil's fertility. Up to this period the ash constituents had been generally considered to be of minor importance. By emphasising the contrary opinion, and insisting upon their essentialness to plant-life, he gave to agricultural research a fresh impetus upon the right lines. His statement of his mineral theory was in the main true, but was not the whole truth.

De Saussure, as has already been pointed out, to a certain extent, anticipated Liebig's mineral theory. He was of the opinion that whatever might be the case with some of the mineral constituents of plants, others were necessary, inasmuch as they were always found in the ash. Of these he instanced the alkaline phosphates. "Their small quantity does not indicate their inutility," he sagaciously remarks. Sir Humphry Davy, as has already been pointed out, missed recognising the true importance of the ash constituents. It was left to Liebig, then, to restate the important doctrine of the essentialness of the mineral matter, already implied to some extent by de Saussure.

Liebig says: "Carbonic acid, water, and ammonia are necessary for the existence of plants, because they contain the elements from which their organs are formed; but other substances are likewise necessary for the formation of certain organs destined for special functions, peculiar to each family of plants. Plants obtain these substances from inorganic nature."

While insisting on the importance of the mineral constituents, he did so in a more or less general way not sufficiently distinguishing one mineral constituent from another.

As all plants contained certain organic acids, and as these organic acids were nearly always found in a neutral state—i.e., in combination with bases, such as potash, soda, lime, and magnesia—the plant must be in a position to take up sufficient of these alkaline bases to neutralise these acids. Hence the necessity of these mineral constituents in the soil. According to him, however, the exact nature of the bases was a point of not so much importance. He assumed, in short, as has been pointed out by Sir J. H. Gilbert, a greater amount of mutual replaceability amongst the bases than can be now admitted.

Passing on to a consideration of the difference of the mineral composition of different soils, he attributes this to the difference in the rocks forming the soils. "Weathering" is the great agent at work in rendering available the otherwise locked-up stores of fertility. He attributes the benefits of fallow exclusively to the increased supply of these incombustible compounds which were thus rendered available to the plant. Treating of this subject, he says: "From the preceding part of this chapter" (in which he has been explaining weathering) "it will be seen that fallow is that period of culture when the land is exposed to progressive disintegration by the action of the weather, for the purpose of liberating a certain quantity of alkalies and silica, to be absorbed by future plants."

His Theory of Manures.

Treating of manures, he showed how the most important constituents of manures were potash and phosphates. In the first edition of his work he also insisted on the value of nitrogen in manures, condemning the want of precautions, in the treatment of animal manures, against loss of nitrogen.

In the later editions of his work he seems to have receded from that opinion, and considered that there was no necessity for supplying nitrogen in manures, since the ammonia washed down in rain was a sufficient source of all the nitrogen the plant required. It was here that Liebig went astray, first in denying the importance of supplying nitrogen as a manure; and secondly, in overestimating the amount of ammonia washed down in rain, which has subsequently been shown to be entirely inadequate to supply plants with the whole of their nitrogen.[15]

His Theory of Rotation of Crops.

In explaining the benefits of the rotation of crops, Liebig propounded a very ingenious theory, but one which was largely of a speculative nature, and which has since been shown to be unfounded on any scientific basis. It was to the effect that one kind of crop excreted matters which were especially favourable to another kind of crop. He did not say whether he considered such excretion positively injurious to the crop which excreted them; but he inferred that what was excreted by the crop was what was not required, and what could, therefore, be of little benefit to a crop of the same nature following it.

The second portion of Liebig's report dealt with the processes of fermentation, decay, and putrefaction.

Publication of Liebig's Second Report to British Association.

In 1842 Liebig contributed his second famous report to the British Association, subsequently published under the title of 'Animal Chemistry; or, Organic Chemistry in its Applications to Physiology and Pathology.' The publication of this report created even greater interest than the publication of his first work. In it he may be said to have contributed as much to animal physiology, as, in his first, he did to agricultural chemistry. His subsequent principal works on agricultural chemistry were—'Principles of Agricultural Chemistry,' published in 1855, and 'On Theory and Practice in Agriculture,' 1856.

Liebig's services to Agricultural Chemistry.

An attempt has been made to sketch in the very briefest manner some of the main points in Liebig's teaching, as contained in his famous report to the British Association in 1840. Agricultural chemistry up till that year can scarcely be described as having a distinct existence as a branch of chemistry. Much valuable work, it is true, had already been done, especially by his two great predecessors, de Saussure and Boussingault; but it was, down to the year 1840, a science made up of isolated facts. Liebig's genius formed it into an important branch of chemistry, supplied the necessary connection between the facts, and by a series of brilliant generalisations formed the principles upon which all subsequent advance has been built.

As has already been indicated, Liebig's chief claim to rank as the greatest agricultural chemist of the century does not rest upon the number or value of his actual researches, but on the formative power he exercised in the evolution of the science. His master-mind surveyed the whole field of agricultural chemistry, and saw laws and principles where others saw simply a confusion of isolated, and, in many cases, seemingly contradictory facts.

But great as the direct value of Liebig's work was, it may be questioned whether its indirect value was not even greater. The publication of his famous work had the effect of giving a general interest to questions which up till then had possessed a special interest, and that for comparatively few. Both on the Continent and in England a very large amount of discussion took place regarding his various theories.

Development of Agricultural Research in Germany.

It was especially in Germany, however, that Liebig's work bore its greatest and most immediate fruit. Thanks to the great chemist, the German Government recognised the importance of forwarding scientific research by State aid. Agricultural Departments were added to some of the universities, largely at State expense, while agricultural research stations were, one after another, instituted in different parts of the country.

The first of the agricultural research stations to be founded was the now famous one of Möckern, near Leipzig. It was instituted in the year 1851. Others followed, until at the present day there are some seventy to eighty of these Versuchs-Stationen scattered throughout Germany, all well equipped and doing excellent work. Some idea of the activity of the German stations may be inferred when it is stated that up to the year 1877 the total number of papers embodying the results of their experiments published by them amount to over 2000.[16]

To trace the development of agricultural chemistry, subsequent to Liebig's time, in the way it has been done prior to the year 1840, is no longer possible. This is due to the enormous increase in the number of workers in the field, as also to the overlapping nature of their work, which renders a strict chronological record wellnigh an impossibility. It will be better, therefore, to attempt to give a brief statement of our present knowledge on the subject, naming the chief workers in the various departments of the subject.

The Rothamsted Experiments.

Before doing so, it is fitting that reference should be made to the work and experiments of two living English chemists, who have done much to contribute to our knowledge in every branch of the science—viz., Sir John Lawes, Bart., and Sir J. H. Gilbert, F.R.S.

The fame of the Rothamsted experiments is now world-wide; and no single experiment station has ever produced such an amount of important work as the magnificently equipped research station at Rothamsted. The Rothamsted station may be said to date from 1843, although Sir John Lawes was engaged in carrying out field experiments for ten years previous to that date.[17] In 1843 Sir John Lawes associated with himself the distinguished chemist Sir J. H. Gilbert, and the numerous papers since published have almost invariably borne the two names. The expense of working the station has been borne entirely by Sir John Lawes himself; who has further set aside a sum of £100,000, the Laboratory, and certain areas of land, for the continuance of the investigations after his death. The fields under experimentation amount to about fifty acres. By a Trust-deed, which was signed on February 14, 1889, Sir John Lawes has made over the Rothamsted Experimental Station to the English nation, to be managed by trustees.

It is impossible to enter, in any detail, into the nature and scope of the Rothamsted experiments.[18] It may be stated that, since the year 1847, some eighty papers have been published on field experiments, and experiments on vegetation; while thirty papers have been published recording experiments on the feeding of animals.[19]

What has all along characterised these valuable experiments has been their practical nature. While their aim has been entirely scientific, the scale of the experiments and the conditions under which they have been carried out, have been such as to render them essentially technical experiments. For this reason their results possess, and will always possess, a peculiar interest for every practical farmer.

The greatest services the Rothamsted experiments have rendered agricultural chemistry have been the valuable contributions they have made to our knowledge of the function of nitrogen in agriculture; its relation in its different chemical forms to plant-life; and the sources of the nitrogen found in plants. Researches of a most elaborate nature have been carried out on what is still one of the most keenly debated questions of the present hour—viz., the relation of the "free" nitrogen in the atmosphere to the plant. Of the very highest value also have been the elaborate researches of Mr R. Warington, F.R.S., on the important question of Nitrification, which have been in course in the Rothamsted Laboratory for the last fifteen years, and to which full reference will be made in the chapter on Nitrification.

To the Rothamsted experiments also we owe the refutation of Liebig's mineral theory. In fact it may safely be said that no experimenters in the field of agricultural chemistry have made more numerous or valuable contributions to the science than these illustrious investigators.

Review of our present Knowledge of Agricultural Chemistry.

Some attempt may now be made to indicate briefly our present knowledge of the more important facts regarding plant physiology, agronomy, and manuring.

Proximate Composition of the Plant.

The great advance made in the direction of the improvement of the accuracy of old analytical processes and the discovery of numerous new ones have furnished us with elaborate analyses of the composition of plants. We now know that the plant-substance is made up of a large number of complex organic substances, formed out of carbon, hydrogen, oxygen, and nitrogen,[20] and that these substances form, on an average, about 95 per cent of the dry vegetable matter; the other 5 per cent being made up of mineral substances. As to the source of these different substances, our knowledge is, on the whole, pretty complete. With regard to the carbon of green-leaved plants, which amounts to from 40 to 50 per cent, subsequent research has confirmed Sénébier and de Saussure's conclusions, that its source is the carbonic acid gas of the air. The decomposition of the carbonic acid gas is effected by the leaves under the influence of sunlight. That a certain quantity of carbon may be obtained from the carbonic acid absorbed by plant-roots, is indeed probable. Especially during the early stages of plant-growth this source of carbon may be of considerable importance. Generally speaking, however, it may be said of all green-leaved plants, that the chief source of their carbon is the carbonic acid gas in the atmosphere.

Carbon Fixation by Plants.

The exact way in which this decomposition of carbonic acid gas is effected by the leaves is not yet clear. It seems to be directly dependent, in some way or other, on the chlorophyll, or green colouring matter. This decomposition of carbonic acid, and the fixation of the carbon by the plant with the formation of starch, takes place only under the influence of sunlight. During the night a reflex action takes place, which is commonly known as respiration, and which is exactly analogous to animal respiration.[21] The rate at which the fixation of carbon takes place depends on the strength of the sun's rays. It seems to take place very rapidly under a strong tropical sun.[22] The action of sunlight on the absorption of carbon has been studied by a number of observers, among others by Sachs, Draper, Cloez, Gratiolet, Caillet, Prillieux, Lommel, &c.

Action of Light on Plant-growth.

Experiments made by several observers, more especially Pfeffer, have shown that the yellow rays of the solar spectrum are the most potent in inducing this decomposition.

Some interesting experiments have been carried out by different observers on the possibility of growing plants under the influence of artificial light. While it would seem that the light from oil-lamps or gaslight is unable to promote growth, except in very exceptional cases, the electric light, or other strong artificial light, seems to be capable of taking the place of sunlight. Heinrich was the first to show that sunlight could be replaced by the magnesium light.

Experiments with the electric light have been carried out by Hervé-Mangon in France and Dr Siemens in England. The plants grown under the influence of the electric light were observed to be of a lighter green colour than those grown under normal conditions, thus indicating a feebler growth; in fact, Siemens was of the opinion that the electric light was about half as effective as daylight.[23]

These experiments are interesting from an industrial point of view; for it is conceivable that at some distant time electricity might be called to the aid of the agriculturist.

Source of Plants' Oxygen.

With regard to the source of the oxygen, which, next to carbon, is the element most largely present in the plant's substance—amounting to, roughly speaking, about 40 per cent—all evidence seems to indicate that it is chiefly derived from water, which is also the source of the plant's hydrogen. In addition to water, carbonic acid and nitric acid may also furnish small quantities. It has been pretty conclusively proved that the atmospheric oxygen, while necessary to plant-growth, and promoting the various chemical vital processes, is not a direct source of the plant's oxygen. The important function played by atmospheric oxygen in certain stages of the plant's growth has been long recognised. Malpighi, nearly two hundred years ago, observed that for the process of germination atmospheric air was necessary; and shortly after the discovery of the composition of the air was made, oxygen was identified as the important gas in promoting this process. Oxygen is also especially necessary during the period of ripening.

Source of Plants' Hydrogen.

Hydrogen, which amounts to about 6 per cent, is, as has already been pointed out, chiefly derived from water. It is possible that ammonia also may form a source.

Source of Plants' Nitrogen.

When we come to treat of the source of the nitrogen, which is found in the plant's substance to an extent varying from a fraction of a per cent to about 4 per cent, we enter on a much more debated question.

What is the source, or, what are the sources, of plant-nitrogen? is a question to the solution of which more time and more research have been devoted than to the solution of any other question connected with agricultural chemistry.

The most obvious source is the free nitrogen, which forms four-fifths of the atmospheric air. Reference has already been made to this question.[24] Priestley was the first of the long list of experimenters on this interesting question.

As far back as 1771 he affirmed that certain plants had the power of absorbing free nitrogen; and this opinion he supported by the results of certain experiments he had made on the subject. Eight years later,—viz., in 1779—Ingenhousz further supported this conclusion, and stated that all plants could absorb, within the space of a few hours, noticeable quantities of nitrogen gas. The first to oppose this theory was de Saussure, who, in 1804, carried out experiments which showed that plants were unable to utilise free nitrogen.

Subsequent experiments, carried out by Woodhouse and Sénébier, supported de Saussure's conclusions. Mention has already been made of Boussingault's elaborate researches on the subject.[25] His first experiments were carried out in 1838. He concluded that plants did not absorb free nitrogen. Georges Ville was the first to reassert the older theory, put forward by Priestley and Ingenhousz. His opinion was founded on experiments he had carried out during the years 1849-52. The subject created so much interest at the time, that a committee of the French Academy—consisting of Dumas, Regnault, Péligot, Chevreul, and Decaisne—were appointed to investigate Ville's experiments. The result of the investigation of the Commission was to confirm Ville's experiments. It is a significant fact, however, that the plant experimented with by the Commission was cress—a non-leguminous plant. It has been commonly assumed that the results of recent experiments have confirmed Ville's experiments. It is only proper to point out that this is not a necessary inference. The assimilation of free nitrogen by the leguminosæ, so far as modern research has revealed, only takes place under the influence of micro-organic life. Ville's experiments, however, were supposed to be conducted under sterilised conditions.

In the meantime the results of Boussingault's second series of experiments, carried out between the years 1851 and 1855, were published, and confirmed his earlier experiments.

The results of a large number of experiments subsequently carried out were in support of Boussingault's conclusions. Among them may be mentioned Mène, Harting, Gunning, Lawes, Gilbert and Pugh, Roy, Petzholdt, and Bretschneider.

Such an amount of overwhelming evidence might naturally have been regarded as conclusively proving that the free nitrogen of the air is not an available source of nitrogen to the plant. The question, however, was not decided. In 1876 Berthelot reopened it. From experiments he had carried out, he concluded that free nitrogen was fixed by various organic compounds, under the influence of silent electric discharges. In 1885 he carried out further experiments, from which he concluded that argillaceous soils had the power of fixing the free nitrogen of the atmosphere. This they effected, he was of opinion, through the agency of micro-organisms. Schloesing has recently shown that this fixation of free nitrogen by soils is extremely doubtful.[26] The gain of nitrogen observed under such conditions can be explained by the absorption by the soil of combined nitrogen—viz., ammonia—from the air.

Berthelot's early experiments in 1876 had the effect of stimulating a number of other experiments, with the result that we now possess the solution of this long-debated and most important problem.

The names of the better known investigators on this subject, in addition to Berthelot's, are those of Hellriegel, Wilfarth, Dehérain, Joulie, Dietzell, Frank, Emil von Wolff, Atwater, Woods, Nobbe, Ward, Breal, Boussingault, Wagner, Schultz-Lupitz, Fleischer, Pagnoul, Schloesing, Laurent, Petermann, Pradmowsky, Beyrenick, Lawes, and Gilbert.

It is impossible to enter into the details of these most important experiments. An attempt may be made, instead, briefly to epitomise them.

Recent Experiments on Nitrogen question.

In the first place, it may be asked, How is it possible that the previous elaborate experiments, published prior to 1876, should now prove unreliable? A satisfactory explanation may be found in the fact, as Lawes and Gilbert have recently pointed out, that the fixation of the free nitrogen by the plant, or within the soil, takes place, if at all, through the agency of electricity or of micro-organisms, or of both. The earlier experiments, however, were so arranged as to exclude the influence of either of those agencies.

The question has further been limited in its scope. It is now supposed that only plants of the leguminous order have the power of drawing upon the free atmospheric nitrogen. Of the experiments above referred to, those of Hellriegel and Wilfarth are the most striking and important. They found in their experiments, that while the legumes have the power of obtaining their nitrogen from the air, cereals have not. Similar experiments by Atwater in America, and others, support this conclusion.

Their conclusions may be briefly epitomised as follows:—

(a) That the leguminous plants—such as peas, &c.—have the power of drawing their nitrogen supplies from the free nitrogen of the air in a way not possessed by other plants; and that they thus possess two sources of nitrogen—the soil and the air.

(b) That this absorption of free nitrogen is not effected directly by the plant, but is the result, so to speak, of the joint action of certain micro-organisms present in certain soils and in the plant itself, (symbiosis).

(c) That this fixation is connected with the formation of minute tubercles on the roots of the plants of the leguminous class; and that these tubercles may be the home of the fixing organism.

(d) That these fixing micro-organisms are not present in all soils.[27]

While the relation of free nitrogen to the plant has long been, and still is, a very obscure problem, it was early recognised that the combined nitrogen present in soils and manures was an important source of plant-food. Reference has already been made to the early theory of Sir Kenelm Digby regarding the value of nitrates.[28] De Saussure, as we have also already seen, was fully impressed with the importance of applying nitrogen to the soil as a manure. Liebig's early attitude on this question was to the effect, that to apply nitrogen in manures was quite unnecessary, as the plant had a sufficient source in the ammonia present in the air, which he erroneously supposed was sufficient in quantity to supply all the needs of the crops. Despite this early recognition of the value of combined nitrogen to the plant, it is only of recent years that we have obtained any definite knowledge as to the respective value of its different compounds as manures, or as to the form in which it is assimilated by the plant. It exists in three forms—(1) as organic nitrogen; (2) as ammonia salts; (3) as nitrates and nitrites. Much experimental work has during late years been devoted to studying the comparative action and merits of these three forms.

Relation of Organic Nitrogen to the Plant.

First, as to the relation of organic nitrogen to the plant. There is a large number of different organic compounds which contain nitrogen. That the plant is able to assimilate certain of these organic compounds, seems, from several experiments, to be extremely probable. From certain researches, carried out as far back as the year 1857, Sir Charles Cameron concluded that the plant could assimilate one of them—viz., urea. From what, however, we have subsequently learned regarding the process of "nitrification," it is quite probable that the nitrogen in these experiments was first converted into nitrates before being assimilated. At any rate, as the plants were not tested for urea, the experiments must be regarded as leaving the problem unsolved.

Other experiments were carried out of a similar nature by Professor S. W. Johnson, the different kinds of nitrogen experimented with being uric acid, hippuric acid, and guanine. But here, again, no definite conclusion can be drawn, as no analyses were made of the plants. More recently, however, Dr Hampe has carried out experiments with urea, uric acid, hippuric acid, and glycocoll. These experiments may be held as demonstrating the fact that at least one organic compound of nitrogen is capable of being assimilated, as urea was actually identified as being present in the plants experimented with. From further experiments, carried out by Dr Paul Wagner and Wolff, glycin, tyrosin, and kreatin are able to be assimilated by the plant.

Plants able to absorb certain Forms of Organic Nitrogen.

We may conclude, then, from these interesting experiments, that plants are able to absorb certain organic forms of nitrogen. That they do so in nature to any extent is extremely improbable, such organic forms of nitrogen being rarely present in the soil, or if present, being converted into ammonia or nitrate salts before assimilation.

Nature of Humus in the Soil.

While on the subject of organic nitrogen, reference may be briefly made to that substance known as humus,—the name applied to the organic portion of soils,—a substance which figures so largely in early theories of plant-nutrition. The most elaborate investigation of the composition of humus has been carried out by Mulder. According to Mulder, it is composed of a number of organic bodies, and he has identified the following substances—ulmin, humin, ulmic, humic, geic acids, &c. These bodies are composed of carbon, hydrogen, and oxygen, which are invariably associated with nitrogen. Detmer and Simon have further investigated the subject. The true function of humus, it would seem, in addition to its numerous mechanical properties, is to furnish, by its decomposition, carbonic acid and nitrogen—in the form of ammonia and nitric acid—to the soil; the former acting as a solvent of the mineral food, the latter as the source of the plant's nitrogen. The old theory, therefore, that the presence of humus in a soil is a condition of fertility, is not so far removed from the truth. Where there is an abundance of humus in the soil there is likely also to be an abundance of nitrogen.

Relation of Ammonia to the Plant.

It seems to be beyond doubt that nitrogen is directly absorbed by plants in the form of ammonia. Liebig, as we have seen, concluded that this was the great source of nitrogen for the plant, and that the ammonia compounds present in the air were an all-sufficient supply. Subsequent research, while confirming his belief so far as regards the capability of plants to assimilate nitrogen in the form of ammonia, has proved that the amount of ammonia present in the air is very minute, and utterly inadequate to supply the plant with the whole of its nitrogen. Investigations have been made on this subject by Graeger, Fresenius, Pierre, Bineau, and Ville. According to Ville's researches, which are among the most recent, the amount does not exceed 30 parts per thousand million parts of air.[29] Some conception of the value of this source of nitrogen may be gained by estimating the quantity falling, dissolved in rain, on an acre of soil throughout the year. Various estimations of the total amount of combined nitrogen, which is in this way brought to the soil, have been made. A certain amount of discrepancy, it is true, is to be found in these various estimations, no doubt largely due to the difference in the circumstances under which the investigations were carried out. Mr Warington has made several investigations at Rothamsted, and, according to his most recently published figures, the total quantity only amounts to 3.37 lb. per acre per annum—of which only 2.53 lb. is as ammonia itself.[30]

As already mentioned, there can be little doubt that plants can absorb nitrogen in the form of ammonia. The question of how far plant-leaves are able to absorb ammonia is a much debated one. It is probable that if they can do so, it is only to a very small extent.[31] The question as to whether the plant's roots can absorb ammonia or not, is also a very keenly debated one. The point is a very difficult one to decide, and is much complicated by the consideration that ammonia, when applied to the the soil, is so speedily converted into nitric acid. Despite, however, these difficulties, and the vast amount of controversy on the point, the experiments of Ville, Hosäus and Lehmann, seem to indicate beyond doubt that ammonia is a direct source of nitrogen. Lehmann's experiments would seem, further, to indicate that there are certain periods of a plant's growth when its preference for ammonia salts seems to be greater than at other times. The point, however, it must be confessed, is still an obscure one. The great difficulty in deciding it, as has just been said, lies in the fact that ammonia salts, when applied to a soil, are, by the process of nitrification, converted into nitrates. In experimenting, therefore, with ammonia, and noting the results, it is wellnigh impossible to say, except by subsequent analyses, whether the nitrogen in the ammonia salts has not been converted into nitrates before assimilation.

Relation of Nitric Acid to the Plant.

Thirdly, as to nitrogen in the form of nitrates. While it is true that plants can absorb nitrogen in certain organic forms and as ammonia salts, it is now a well-known fact that the chief, and by far the most important, source of nitrogen is nitric acid. Probably more than 90 per cent of the nitrogen absorbed by green-leaved plants from the soil is absorbed as nitrates. The tendency of all nitrogen compounds in the soil is towards conversion into nitric acid. It is the final form of nitrogen in the soil. The precise method in which this conversion takes place is a discovery of only a few years' standing. The great economic importance of this discovery, made by the French chemists Schloesing and Müntz, and associated in this country with the names of Warington, Munro, and P. F. Frankland, is only gradually being appreciated. It is without doubt one of the most interesting made in the domain of agricultural chemistry of late years.

Nitrification.

It was in the year 1877 that the two French chemists above referred to published the results of some experiments they had carried out, which proved that nitrification—the name given to the process by which ammonia or other nitrogen salts are converted in the soil into nitric acid—was due to the action of micro-organic life.

The basis of the theory rests upon the fact that dilute solutions of ammonia salts or urine, containing all the necessary constituents of plant-food, if previously sterilised, may be kept for an indefinitely long period of time, provided the air supplied be filtered through cotton wool,—so as to prevent the entrance of micro-organisms—without any formation of nitrates. Introduce, however, into such a solution a little fresh soil, and nitrification will soon follow.

The conditions under which the nitrification ferment acts, as well as the nature of the ferment, or rather ferments, have subsequently been carefully studied by Schloesing and Müntz, Winogradsy, Dehérain, Kellner, and other Continental observers, and especially by Warington, Munro, and P. F. Frankland in this country. These conditions cannot be gone into here. They will be fully discussed in the chapter on Nitrification. Briefly stated, they are a certain range of temperature (between slightly above freezing-point and 50° C., the maximum activity taking place, according to Schloesing and Müntz, at about 30° C.); a plentiful supply of atmosphere oxygen (hence the fact observed by Warington, that nitrification is chiefly limited to the surface-soil); a certain amount of moisture; and the presence of certain of the necessary mineral plant constituents, and the presence of carbonate of lime.

The light which these discoveries throw upon the extremely complicated question of the fertility of the soil is considerable, as it follows that no soil can be regarded as really a fertile one in which the process of nitrification does not freely take place. They furthermore explain many facts, hitherto observed but not well understood, with regard to the action of different nitrogenous manures.

Ash Constituents of the Plant.

We now come to consider the present state of our knowledge on the essentialness of the ash or mineral portion of the plant. While a portion of the plant's substance which, up to Liebig's time, had obtained little notice, it has, since the publication of his famous "mineral" theory, obtained an ever-increasing amount of investigation.

Up till 1800 practically nothing was known of the function of the ash constituents. In 1802 de Saussure wrote that it was unknown whether the constituents of many plants were due to the soils on which they grew, or whether they were the products of vegetable growth. Some two years later, however, he was enabled to carry out a number of experiments which really placed the subject on a firm scientific basis. The essentialness of the ash constituents was only, however, placed beyond all doubt by Wiegmann and Polstorff's researches, carried out in 1840.

Reference has already been made to the great stimulus given to research by the promulgation of Liebig's mineral theory.

Methods of Research.

In epitomising the vast amount of work carried on since 1840, with the view of ascertaining the essentialness of the various substances found in the ash of plants, two methods of experimentation have been followed.

Artificial Soils.

The first of these two methods was that adopted in the famous experiments, carried out by Prince Salm-Horstmar, which have done so much to further our knowledge on this question. It consisted in growing plants on an artificial soil—formed out of sugar-charcoal, pulverised quartz or purified sand—to which were added the different food constituents.

Water-culture.

While the results obtained by Prince Salm-Horstmar by this method were of a most valuable nature, subsequent experimenters have abandoned his method for the other method—viz., "water-culture." The medium used in this process is pure water; and it is from experiments carried out in water-culture that much of our present knowledge, in regard to the relation of the ash constituents to the plant, is due.

The names of those who have worked in this department are very numerous. Among them may be mentioned Knop, Sachs, Stohmann, Nobbe, Rautenberg, Kühn, Lucanus, W. Wolff, Hampe, Beyer, E. Wolff, P. Wagner, Bretschneider and Lehmann. The results obtained by these and other experimenters have demonstrated the following facts.

The substances which have been found in the ash of plants are: potash, soda, lime, magnesia, oxide of iron, oxide of manganese, phosphoric acid, sulphuric acid, silica, carbonic acid, chlorine, lithia, rubidia, alumina, oxide of copper, bromine, iodine, and occasionally even other substances. Of these, however, only six are probably absolutely necessary for plant-growth—viz., potash, lime, magnesia, oxide of iron, phosphoric acid, and sulphuric acid. Three other substances seem also to be almost invariably present, and may possibly be essential—in very minute quantities at any rate—viz., chlorine, soda, and silica. With regard to alumina and oxide of copper, these constituents must be regarded as accidental; while iodine and bromine only occur in the ash of marine plants.

Method of Absorption of Plant-food.

A department of vegetable physiology which has had much work devoted to it is the method in which plant-roots absorb their food. The plant's nourishment is absorbed in solution by means of the roots. Its absorption takes place, according to Fischer and Dutrochet, who have investigated the subject at great length, by the process known as endosmosis. It has also been established by numerous experiments, that different plants require different constituents in different proportions.

Water as a Carrier of Plant-food.

The function performed by water, as the carrier of plant-food, and the motion of the sap of the plant, are questions which have also received much attention. The motion of the plant's sap seems to have attracted a great deal of attention at a very early stage of the study of plant physiology. As far back as 1679, Marriotte studied it. Among other old experimenters were Hales, Guettard, Sénébier, Saint-Martin, de Candolle, and Miguel. In more recent times, it has been investigated by Schübler, Lawes and Gilbert, Knop, Sachs, Unger, and Hosäus. Some idea of the enormous amount of water transpired by plant-leaves may be gained by the statement that from 233 lb. to 912 lb. of water are transpired for every pound of plant-tissue formed.[32]

Agronomy.

When we come to deal with questions relating to the chemistry of the soil, we find that so much investigation has been devoted to this one branch of agricultural chemistry as to constitute it a special branch by itself—known in France under the name of agronomie—and being taught in the large agricultural colleges by special professors of the subject. The value of studying the properties of soils was recognised at an early period. This study was for long largely confined to their physical, or, what are popularly known as their mechanical properties. Thus Sir Humphry Davy ascertained many important facts with regard to the heat and water absorbing and retaining properties of soils.

Retention by Soil of Plant-food.

It was not till a later period that the power soils possess of fixing from their watery solutions various plant-foods, both organic and inorganic, was discovered. The earliest recognition of this most important property of soils was made by Gazzeri, who, in 1819, called attention to the fact that the dark fluid portion of farmyard manure was purified on passing through clay. He concluded that soils, more especially clayey soils, possessed the property of being able to fix from their watery solutions the necessary plant-food constituents, and fix them beyond risk of loss, only affording a gradual supply to the plant as required.

The first experiments carried out on this subject were those by Huxtable and Thompson in 1850. The liquid portion of farmyard manure was filtered through soil and subsequently examined, when it was found to have not only lost its colour, but also to have lost its smell. Ammonia and ammonia salts were also experimented with, and it was found that soils possessed the power of fixing ammonia.

To Thomas Way, however, we are indebted for the most valuable contribution on this important subject made by any one single investigator. His experiments were not merely carried out with regard to ammonia, but also with regard to other bases—such as potash, lime, magnesia, soda, &c. Since Way's experiments much work has been done by Liebig, Stohmann, Henneberg, and Heiden, as also by Voelcker, Eichhorn, Knop, Rautenberg, Pochwissnew, Warington, Beyer, Bretschneider, Sestini, Laskowsky, Strehl, Pillnitz, Peters, W. Wolff, Lehmann, and Biedermann.

Bases and Acids fixed by Soil.

From these experiments it may be taken as proved beyond doubt that soils have the power of fixing, to a greater or less extent, the following bases: ammonia, potash, lime, magnesia and soda; as well as the two acids, phosphoric and silicic. The order in which the different bases are fixed is an important point. It would seem that the soil has a greater affinity for the more valuable manurial substances, such as ammonia, potash, and lime, and that these substances are first fixed. That in fixing any one of the above-mentioned bases from its solution, it can only do so at the expense of another base. Thus, in fixing potash, either lime, magnesia, or soda must be given up. Further, when a base in solution, as sulphate or chloride, is absorbed by a soil, the base is alone fixed, while the sulphuric acid or chlorine is left in solution. Lastly, the amount of base absorbed by a soil depends on the concentration of its solution, on the nature of its combination, and the temperature. Way found in his experiments that a clay soil has more power than a peaty soil, and that a peaty soil has more power than a sandy soil.

Causes of this Fixation.

So much for the fact of soil absorption; as to the cause or causes of this absorption, a great number of theories have been put forward. Those may be divided into two classes—those accounting for it as due to physical properties of the soil; and those, on the other hand, explaining it as due to chemical action.

To the latter class Way's belonged. He explained it as due to the formation in the soil of hydrated double silicates, consisting of a silicate of alumina, along with a silicate of the base fixed. Brüstlein and Peters, on the other hand, were of the opinion that it was purely physical in its nature. A theory has been advanced that it is due to the formation of insoluble ulmates and humates, formed by the union of ulmic and humic acids, along with the bases fixed. Among others who devoted investigation to this interesting question, may be mentioned Rautenberg and Heiden.

On reviewing the evidence, it seems to be pretty well established that it really is mainly a chemical act, due chiefly to the formation of double silicates, and doubtless to a certain extent to the formation of insoluble humates and ulmates. Heiden's experiments would seem to indicate, however, that it is also partly of a physical nature.

With regard to the absorption of phosphoric acid, this has been shown to be a chemical act, and depends on the formation of insoluble phosphates of calcium, iron, aluminium, and magnesium, the percentage of iron especially determining this.

Much analytical work has been accomplished of late years with a view of ascertaining the amount of ash in different kinds of plants, and in the different parts of the plant.

Action of Manures.

The department of agricultural chemistry which has been most largely developed of late years is that connected with the problems of manuring. It is, from a practical point of view, of most value. It is some considerable time since we have recognised that the only three ingredients it is, as a rule, expedient to apply as artificial manures, are nitrogen, phosphoric acid, and potash. The nature, mode of action of the different compounds, and properties of these three substances, and their comparative influence in fostering plant-growth, together with the economic question of which form is, under various circumstances, the most economical for the farmer to use, have together given rise to a large number of "field" and "pot" experiments. As the principles underlying this practice form the subject of the following treatise, any further discussion of the question must be left to the following chapters.

Note.—The reader interested in the historical development of agricultural chemistry is referred to Sir J. H. Gilbert's Presidential Address to the Chemical Section of the British Association, 1880.