FOOTNOTES:
[240] See p. 324.
CHAPTER XVII.
SEWAGE AS A MANURE.
The value of sewage as a manure has been in the past enormously overrated, and much misunderstanding has existed on the part of the public on the question of the profitableness of the disposal of town sewage as an agricultural manure. Not a few of the erroneous opinions prevalent in the past regarding sewage have been due to statements made by scientific and other writers as to the enormous wealth lost to the world by many of the present methods of sewage disposal. Fortunately, however, the sewage question is now increasingly regarded as a question, in the first instance, of sanitary interest. As much has been written on the subject, and many schemes have been devised, at the expense of much ingenuity, for utilising its manurial properties, it may be desirable here to say a few words on the purely agricultural side of the question.
The two most important points about sewage are its enormous abundance and its extremely poor quality. If the most important consideration were not the sanitary one, but its manurial value, then indeed our water system, so universally used in towns, must be regarded as a most wasteful one; for by its means the value of the excrementitious matter from which it derives its manurial ingredients is tremendously lessened. When we reflect that a ton of sewage, such as is produced in many European cities, contains only 2 or 3 lb. of dry matter, and that the total amount of nitrogen in this is only an ounce or two, while the phosphoric acid is considerably less, and that it is on those two ingredients that its value as a manure entirely depends, we see very strikingly how poor a manurial substance sewage is. Various methods have been devised and experimented with for extracting these manurial ingredients, and many methods are in operation in different parts of the world. The methods of utilising sewage for agricultural purposes may be broadly divided into two classes.
Irrigation.
One of these, which may be classed under the heading of irrigation, consists in pouring the sewage on to certain kinds of coarse green crops. Sometimes the land is made to filter large quantities of sewage by special arrangements of drains and ditches. The land is first carefully and evenly graded down a gentle incline. At the top of the field the sewage is conducted along an open ditch from which it is permitted to escape, by the force of gravity, by several smaller ditches running at right angles from the main ditch. By means of stops which may be shifted at will, the sewage can be directed to flow over different parts of the field. Modifications in this plan may be made so as to suit the nature of the ground. In the case, for example, of a steep incline, the field may be sewaged by means of what are known as "catch-work" trenches running horizontally along the hill. In this way the sewage is allowed to pass over the whole of the field, and is caught at the bottom in a deep ditch, whence it is allowed to flow into the nearest river or stream. This is the system which has been employed at the famous Beddington Meadows, near Croydon.
Another method of distributing the sewage is by means of underground pipes, which are laid in a sort of network over the ground to be manured. At certain intervals pipes with couplings for hose are fitted on, and by keeping a certain amount of pressure on the main pipes the sewage may be distributed over the different parts of the field as it is required.
A third modification is subsoil irrigation. This resembles the last-named system, with this difference, that the pipes used are either porous or perforated with small holes.
Total submersion can only be applied in the case of absolutely level lands, and is practised to an enormous extent in Piedmont and Lombardy.
There has been little dispute as to the thorough efficiency of irrigation—when conducted under favourable conditions—as a method of purifying sewage and utilising to the full its constituents of manurial value. It is the only method which has been conclusively shown to extract from sewage that to which it owes most largely its value as a manure—viz., ammonia; and from this fact it deserves a first place in the consideration of agriculturists. For however admirable other methods may be from a sanitary point of view, it is obvious that a method which would allow the ammonia in sewage wholly, or at least to over 90 per cent, to be lost, cannot claim the same place in the judgment of agriculturists as a method which can extract for the soil not only the whole of this valuable constituent, but all else in the sewage which in any way is of value to plant-life.
Effects of continued Application of Sewage.
When sewage is continuously applied to the same land, what generally takes place is this: At first the sewage is purified, and the soil derives corresponding benefit from the valuable fertilising ingredients it thus extracts. After a time, however, the land becomes what has been termed "sewage-sick." The pores in the soil become choked up by the slimy matter the sewage contains in suspension; the aeration of the soil, which, as we have already mentioned, is so necessary, is consequently to a large extent stopped; and the result is, that the land rapidly deteriorates, and the sewage is no longer purified.
This is obviated to some extent by intermittent irrigation. The land, instead of receiving sewage continuously, only receives it at intervals, and is allowed some time to recover between each dose. It is, however, the opinion of those who have given the subject much attention, that land, even although intermittently sewaged, never recovers its original efficacy.
Irrigation, therefore, under favourable conditions, is a most successful method of utilising the manurial value of sewage; but the great difficulty in practice is to obtain those favourable conditions. It has long been known that if soil is properly to discharge its function as a purifier of sewage water, it must be properly aerated; and we now know that in every fertile soil the process of nitrification must be permitted free development. Now the application of large quantities of sewage to a soil is apt to prevent this free development. As we have already seen, absence of air and the lowering of the temperature of the soil distinctly tend to retard nitrification; and these two conditions accompany the application of large quantities of sewage.
Crops suited for Sewage.
Another objection to irrigation has been found in the alleged limited number of crops sewaged land is suited to yield. It has been repeatedly stated that rye-grass is about the only crop it is profitable to grow on it. In opposition to this statement, however, is the opinion expressed in the conclusions arrived at by the committee appointed by the British Association for the consideration of the sewage question. A vast number of experiments were carried out by them between the years 1868-72, and the result they arrived at was as follows: "It is certain that all kinds of crops may be grown with sewage, so that the farmer can grow such as he can best sell; nevertheless, the staple crops must be cattle food, such as grass, roots, &c., with occasional crops of kitchen vegetables and of corn." While, therefore, it is probably a mistake to say that rye-grass is the only crop sewaged land is capable of growing profitably, the bulk of experience goes to show that such a crop is best suited for such land. This being so, the question naturally arises, What is the farmer who uses sewage as a manure to do with the large green crops he obtains from his land? He is, in most cases, unable to use them himself or dispose of them at the time. And while this has hitherto proved to be a most important drawback, now that we have in ensilage a means of preserving our green crops in a condition suitable as fodder for as long a time as is necessary, the grounds on which this objection rests are almost entirely removed.
It will be obvious, of course, that some soils are naturally much better fitted to perform purification of sewage than others; but it must be frankly admitted that even the best of soils can only deal with a certain quantity of sewage. Various calculations have been indulged in as to the amount of sewage an acre of land can successfully deal with. According to one of these, an acre can purify some 2000 gallons per day, or that produced by 100 persons; while other calculations estimate it at 60 persons; and others, again, at 150. The capacity of a sandy soil in this respect will be much greater than that of a heavier soil; and at Dantzic an acre of the sand-dunes is regarded as being capable of purifying the sewage of 600 persons. The late Dr Wallace has calculated that, in order to treat the sewage of Glasgow, over twelve square miles of land would be required. Of course, if the sewage is subjected to previous treatment, which is often the case, by the method immediately about to be described—namely, precipitation—the amount of sewage the soil is capable of purifying will be correspondingly increased. A difficulty which may also be pointed out in connection with irrigation as a means of disposing of sewage, is the impossibility of carrying it on during frosty weather, when the land is frost-bound. In warm climates irrigation has much to recommend it as a means of sewage disposal. In damp and cold climates, on the other hand, there are many objections.
Treatment of Sewage by Precipitation, &c.
We now come to consider the methods grouped under this second heading. Mechanical filtration, of course, only aims at purifying sewage to the extent of removing all insoluble suspended matter which it contains. Different substances have been used as filters, the most generally used being charcoal. Charcoal mixed with burnt clay, gravel, sand, &c., has also been used.
In chemical precipitation, however, we have a method which claims to do more. Beyond the extracting of all solid matters in suspension, it removes (at any rate most chemical precipitants do) nearly all the phosphoric acid, which, next to the ammonia, is the most valuable constituent the sewage contains. Of all precipitants, lime has been the most universally used; and on the whole, it is perhaps the best, for it is both cheap and obtainable almost anywhere. According to an analysis by the late Professor Way, the difference in the percentages of phosphoric acid, potash, and ammonia, before and after treatment with lime, in a sample of sewage, was as follows:—
| Grains per Gallon. | ||
| Before. | After. | |
| Phosphoric acid | 2.63 | .45 |
| Potash | 3.66 | 3.80 |
| Ammonia | 7.48 | 7.50 |
From the above we see that while sludge caused by lime as a precipitant contains nearly all the phosphoric acid, there is not a trace of the potash or ammonia removed. Sulphate of alumina has also been used, both alone and in conjunction with lime. The advantage claimed by it over lime is, that the resulting precipitate is much less bulky. In other respects, however, it does not seem to be any more efficient as a precipitant. In the well-known A, B, C process, a mixture of alum, clay, lime, charcoal, blood, and alkaline salts, in different proportions, has been used. This mixture is said to extract, in addition to the phosphoric acid, a certain proportion of the ammonia; but the amount is so small as scarcely to be worth considering.
Numerous other chemical substances have been used, alone and also in conjunction with one another, such as perchloride of iron, copperas, manganese, &c. All alike, however, have failed to do more than effect partial purification,—the best results, it may be added, being obtained when the sewage thus treated was fresh. With regard to the manurial value of the resulting sludges, much difference of opinion has existed. The small percentage of phosphoric acid and nitrogen they contain has prevented them from being used to any extent as a manure, as their value did not admit of carriage beyond the distance of a few miles. By the introduction a few years ago of the filter-press, their value has been considerably enhanced. The old method of dealing with the sludge at precipitation-works was to allow it to dry gradually by exposure to the atmosphere. The result, however, of leaving sewage-sludge with over 90 per cent of water in it to dry in the air, was to encourage the rapid decomposition and putrefaction of its organic matter, so that in many cases the decomposing sludge proved to be as great a nuisance as the unpurified sewage itself would have been. By the use of Johnson's filter-press, however, a sludge containing 90 per cent of water was at once reduced to 50 per cent or even less. By this means the percentage of its valuable constituents was very much increased, and the sludge-cake, besides being much more portable, was neither so objectionable nor so liable to decomposition as before.
Value of Sewage-sludge.
As to the value of this sludge-cake as a manure, we are happily in possession of some very interesting and valuable experiments by Professor Munro of Downton Agricultural College. The sludge experimented upon was that produced by sulphate of alumina, lime, and sulphate of iron, and contained, after being subjected to Johnson's filter-press, from .6 to .9 per cent of nitrogen, and over 1 per cent of phosphoric acid. It was found that the benefit resulting from the application of the sludge was far from what in theory might have been expected. The experiments were made with turnips; and the results obtained with superphosphate and farmyard manure respectively, in the same field and under exactly the same conditions, were contrasted with those obtained with sludge. Thus it was found that 53 lb. of phosphoric acid as superphosphate, or 60 lb. as farmyard manure, produced a considerably larger crop than 240 lb. of phosphoric acid in the sludge. That is to say, that the phosphoric acid in the sludge did not exert more than one-fifth of its theoretical effect. The explanation of this somewhat strange result Dr Munro finds in the unsuitable physical character of the sludge-cakes. In farmyard manure we have a loose texture and a large amount of soluble constituents when well rotted. It thus quickly distributes its fertilising elements throughout the soil. In the case of the sludge, on the other hand, its composing particles are closely compacted together, and thus offer the greatest resistance to mechanical and chemical disintegration. "As a matter of fact," says Dr Munro, "the sludge-plots in my experimental series were all readily identified, when the roots were pulled, by the presence of unbroken and undecomposed clods of cake, which had evidently given up, at most, a small portion of their valuable ingredients to the soil."
Briefly stated, therefore, the objections to chemical precipitation as a means of dealing with sewage are these—viz., that while it relieves sewage of all its organic matter, and to a large extent of its phosphoric acid, it fails to extract any ammonia, which is thus lost; that the resulting sludge is consequently so poor in fertilising matters as scarcely to make it worth while to remove it any distance for manuring purposes; and that, further, owing to its unfavourable physical character, as at present made, even the small percentage of plant-food it contains is not realisable, within, at any rate, anything like a reasonable time, to its full theoretical extent.
The most profitable method of treating sewage must be determined by various local conditions; and it must be clearly understood that the question of sewage disposal is primarily a sanitary one, and that it must be dealt with from the sanitary aspect. The most profitable way of applying sewage as a manure, however, will doubtless be found by combining chemical precipitation and land irrigation.
CHAPTER XVIII.
LIQUID MANURE.
The adoption of irrigation as a means of utilising sewage, suggests a short consideration of the value of liquid manures. It has been a custom on many farms to apply the liquid manure got from the oozings of manure-heaps, the drainings of the farmyard, byres, stables, piggeries, &c., directly to the soil. Indeed, so strongly has the belief in the superiority of liquid manure over other manure been held by certain farmers, that they have washed the solid animal excreta with water, in order to extract from it its soluble fertilising constituents. The late Mr Mechi was one of the foremost exponents of the value of liquid manure. His farm of Tiptree Hall was fitted up with iron pipes for the distribution of the manure over the different fields. Superphosphate, it may also be added, as first made from bones by Baron Liebig, was applied in a liquid form. As to the general merits of liquid manure, there can be no doubt that it is the most valuable form in which to apply manure. It secures for the manurial ingredients it contains a speedy and uniform diffusion in the soil; but, on the other hand, the expense of distributing it makes its application far from economical. The chief ingredient in liquid manure is urine. Now the removal of urine from the farmyard manure-heap entails a severe loss of the ingredient which is most potent in promoting fermentation. Separation of the urine from the solid excreta is on this very account not to be recommended. Urine, when applied alone, is lacking in phosphoric acid, of which it contains mere traces. It is not, therefore, suitable as a general manure. It has to be pointed out, however, that the drainings from a manure-heap in this respect are superior to pure urine, since they contain the soluble phosphates washed out of the solid excreta. The objections against using liquid manure may be summed up as follows:—
First, it is too bulky a form in which to apply the manure, and hence too expensive; secondly, it is not advisable to deprive the solid excreta of the liquid excreta, as the one supplements the other; thirdly, fermentation is largely fostered in the solid excreta by the presence of the liquid excreta—hence fermentation will not take place properly in the solid excreta when deprived of the liquid excreta.
If, however, the production of liquid manure on the farm is in excess of what can be used for the proper fermentation of farmyard manure, it will be best to utilise it for composts. No better addition to a compost can be made than liquid manure, as it induces speedy fermentation in nearly all kinds of organic matter.
CHAPTER XIX.
COMPOSTS.
The use of composts is an old one. Before artificial manures were so plentiful as they are at present, much attention was paid by farmers to their preparation. A compost is generally made by mixing some substance of animal origin which is rich in manurial ingredients with peat or loam, and often along with lime, alkali salts, common salt, and indeed any sort of refuse which may be regarded as possessing a manurial value. Composting, in short, may be looked upon as a useful method of turning to profitable use refuse of various kinds which accumulate on the farm. The object of composting is to promote fermentation of the materials forming the compost, and to convert the manurial ingredients they contain into an available condition for plant needs. Composts often serve a useful purpose in retaining valuable volatile manurial ingredients, such as ammonia, formed in easily fermentable substances like urine. In fact, we may say that farmyard manure is the typical compost, and its manufacture serves to illustrate the principles of composting.
Farmyard Manure a typical Compost.
Farmyard manure as ordinarily made is not generally regarded as a compost, but in the past it has been widely used for the purpose of making composts. Thus the practice of mixing farmyard manure with large quantities of peat has been in some parts of the world a common one. Peat, as has already been pointed out in a previous chapter, is comparatively rich in nitrogen. When it is mixed with urine or some other putrescible substance, the peat undergoes fermentation, with the result that its nitrogen is to a greater or less extent converted into ammonia. The effect, therefore, of mixing peat with farmyard manure is beneficial to both substances mixed: the escape of ammonia is rendered impossible by the fixing properties of the peat, while the inert nitrogen of the peat is largely converted by fermentation into an available form. The proportion of peat which it is advisable to add in composting farmyard manure will depend on the richness of the quality of the manure: the richer the quality of the manure, the greater the amount of peat it will be able to ferment. Composts of this kind are generally made by piling up the manure in heaps, consisting of alternate layers of peat and farmyard manure. From one to five parts of peat to every one part of farmyard manure is a common proportion. The use of such a manure, containing so much organic matter, will exercise its best effect on light sandy soils.
Other Composts.
But instead of farmyard manure, or in addition to farmyard manure, various other substances may be added, as bones, flesh, fish-scrap, and the offal of slaughter-houses. Sometimes leaves and the dried bracken-fern are used for the manufacture of composts. Some of these substances contain much nitrogen or phosphoric acid, but in their natural condition ferment when applied to the soil at a slow rate. If mixed together before application in pits with peat, leaves, bracken-fern, or some other absorbent material, fermentation proceeds evenly and rapidly. The addition of lime, potash, and soda salts has been found to have a most beneficial effect in promoting fermentation. These substances, as is well known, hasten putrefaction of organic matter. Lime seems especially to be valuable in composting. This is no doubt due to the fact that lime plays a valuable part in promoting the action of various ferments, as has already been illustrated in the case of nitrification. The effect of large quantities of sour organic acids (humic and ulmic), which are the invariable products of the decomposition of organic matter like peat, leaves, &c., is inimical to micro-organic life. The action of lime is to neutralise these acids. There can be no doubt that composting is a useful process for increasing the fertilising properties of different more or less inert manurial substances. But in view of the abundant supply of concentrated fertilisers, the use of composts may considerably decrease in future.
CHAPTER XX.
INDIRECT MANURES.
Lime.
We now come to discuss those manures which we may class under the term Indirect, because their value is due, not to their direct action as suppliers of plant-food—like those manures we have hitherto been engaged in discussing—but to their indirect action. Of these by far the most important is lime.
Antiquity of Lime as a Manure.
Lime is one of the oldest and one of the most popular of all manures. It is mentioned, and its wonderful action commented on, in the works of several ancient writers, more especially Pliny. Of late years, perhaps, its use has become restricted; and, as we shall point out by-and-by, it is well that it is so.
Action of Lime not thoroughly understood.
Despite the fact of the long-established and almost universal use of lime, it can scarcely be said that we as yet clearly understand the exact nature of its action. Much light, however, has been thrown of late years on the subject by the great advance which has been made in our knowledge of agricultural chemistry. Nevertheless, there are many points connected with the action of lime on the soil which are still obscure. Perhaps one reason for the conflicting ideas prevalent with regard to the value of this substance in agriculture is to be found in the fact that it acts in such a number of different ways, and that the nature of the changes it gives rise to in the soil is most complicated. The experience of agriculturists with lime in one part of the country often seems contradictory to the experience of those in other parts of the country. Its action on different soils is very dissimilar. For these reasons, therefore, the discussion of the value of lime as a manure is by no means an easy one.
Lime a necessary Plant-food.
Lime, as we have already pointed out in a former chapter, is a necessary plant-food, and were it present in the soil to a less extent than is actually the case, would be just as valuable a manure as the different nitrogenous and phosphatic manures; and in certain circumstances this is the case. There are soils, though they are by no means of common occurrence, which actually lack sufficient lime for supporting plant-growth, and to which its addition directly promotes the growth of the crop. Poor sandy soils are often of this nature. Another class of soils are also apt to be lacking in lime—at any rate their surface-soil is. These are permanent pasture-soils. Originally there may have been an abundance of lime in the surface portion of the soil; but, as is well known to every practical farmer, lime has a tendency to sink down in the soil. This tendency in ordinary arable soils is largely counteracted by ordinary tillage operations, such as ploughing, &c., by means of which the lime is again brought to the surface. In permanent pasture-soils, however, no such counteracting action takes place, hence impoverishment of the surface-soil in lime eventually results. It is for this reason—partly at any rate—that permanent pasture benefits in an especial degree by the application of lime. We say partly, for there are other important reasons. One is, that lime seems to have a striking effect in improving the quality of pastures by inducing the finer grasses to predominate. It has also a very favourable action in promoting the growth of white clover. Another reason for the favourable effect of lime on pasture-soils is doubtless on account of the action it has in setting potash free from its compounds. Soils, however, which directly benefit from the application of lime in the same way as they benefit from the application of nitrogenous manures, may be safely said to be rare. In the great majority of soils lime exists, so far as the demands of plant-life are concerned, in superabundance.
Lime of abundant Occurrence.
Indeed limestone is one of the most abundant of all rock substances, and it has been calculated that it forms not less than one-sixth of the rock-mass of the earth's crust. Nearly all the commonly occurring minerals contain it, and in the course of their disintegration furnish it to the soil. Vast tracts of country are composed of nothing but limestone; and we have examples, even in this country, of so-called chalk-soils, where it is the most abundant constituent. Nor can it be classed amongst the insoluble mineral constituents of the soil; for although insoluble in pure water, it is soluble in water—such as the soil-water—which contains carbonic acid. This is proved by the fact that it is the chief dissolved mineral ingredient in all natural waters.
Lime returned to the Soil in ordinary Agricultural Practice.
It may be further pointed out, as bearing upon the true function of lime when applied as a manure, that in ordinary agricultural practice nearly all the lime removed from the soil in crops finds its way back again to the farm in the straw of the farmyard manure. For these reasons, then, it is clear that the true function of lime is as an indirect manure.
Let us now proceed to discuss its action. Before doing so, however, it is important that we should clearly understand the different chemical forms in which it occurs.
Different Forms of Lime.
Lime occurs chiefly as carbonate of lime in the forms of limestone, marble, or chalk, which are all chemically the same. It occurs also as sulphate of lime or gypsum, as well as in the forms of phosphate and fluoride. In agriculture it is only used—if we except the phosphate, which is applied not on account of its lime, but its phosphoric acid—in the form of the carbonate or mild lime as it is commonly called, burnt, caustic, or quick lime, and as gypsum. As the value of gypsum as a manure is of such importance, and depends not entirely on its being a compound of lime, we shall consider it by itself. Hence we have only to consider here the action of mild and caustic lime.
Caustic Lime.
When limestone or mild lime is submitted to a great heat, such as is practically done on a large scale in lime-kilns, it is converted into caustic lime or lime proper. Limestone is made up, as we have just mentioned, of lime and carbonic acid. The latter ingredient is expelled in the form of a gas, and the lime is left behind. Lime never occurs naturally as caustic lime, for the simple reason that it is impossible for it to remain in this state, owing to the great affinity it has both for water and carbonic acid.
When lime is burnt, and before it is applied to the field, some time is allowed to elapse in order to permit of its absorbing moisture—or becoming slaked, as it is technically called. This it does more or less slowly by absorbing moisture from the air. As, however, the process would take too long, and as, moreover, the absorption of carbonic acid gas would also take place at the same time, lime is generally slaked in another way. This can be done by simply adding water. An objection to this method is, that the lime is not so uniformly slaked as is desirable. It becomes gritty. The usual method is to cover it up with damp earth in heaps, and allow the moisture of the earth to effect the slaking. When lime absorbs water a new chemical compound is formed, known as lime hydrate; and so rapidly does the lime unite with water, that a great deal of heat is evolved in the operation, the temperature produced being considerably above that of boiling-water. The conversion of slaked lime into carbonate of lime or mild lime is a slower process. Sooner or later, however, it takes place, whether the lime is left on the surface of the soil or buried in it.
A knowledge of these elementary chemical facts is necessary in order clearly to understand the nature of the action of lime in agriculture.
The respective action of quicklime and mild lime is, on the whole, similar, although the former is in every case very much more powerful in its effects than the latter.
Lime acts both mechanically and chemically.
Lime may be said to act on the soil both mechanically and chemically. It alters the texture of the soil, and affects its mechanical properties, such as its absorptive, retentive, and capillary powers with regard to water. It acts upon its dormant fertility, and decomposes its mineral substances as well as its organic matter. Lastly, its influence on the micro-organic life of the soil, which plays such an important part in the preparation and elaboration of plant-food, is of the highest importance. We cannot do better, therefore, than discuss its properties under the headings mechanical, chemical, and biological.
I. Mechanical Functions of Lime.
Action on Soil's Texture.
The effect of lime upon the texture of a soil is among its most striking properties. Every farmer knows well what a transformation is effected in the texture of a stiff clay soil by the application of a dressing of lime. The adhesive property of the soil—its objectionable tendency to puddle when mixed with water—is greatly lessened, and the soil is rendered very much more friable when it becomes dry. Several reasons exist for this change. In the first place, the tendency to puddle in a clayey soil is due to the fine state of division of the soil-particles. The way in which lime counteracts this adhesive property is by causing a coagulation of the fine soil-particles. This flocculation or aggregation of the fine clay-particles, when mixed with water by lime, is strikingly demonstrated by adding to some muddy water a little lime-water. The result will be that the water will speedily be rendered clear, the fine clay-particles coming together and sinking to the bottom of the vessel. Even a very small quantity of lime will effect this change. This property possessed by lime, we may mention, is utilised in the treatment of sewage. As it is the fine clay-particles that are the chief cause of the puddling of clay soils, their flocculation does much to destroy this objectionable property. Another reason why lime renders a clay soil more friable when dry is, that lime does not undergo any shrinkage in dry weather. As clay soils shrink very much in drying, the mixture with such a substance as lime tends to minimise this tendency to cake in hard lumps. The effect of even a very small addition of lime to a clay soil, in the way of increasing its friable nature, is very striking, and can be easily illustrated by taking two portions of clay, into one of which a small percentage of lime is introduced, and working both into a plastic mass with water, and then allowing them to dry. It will be found that while the one is hard and resists disintegration, that portion to which the lime has been added crumbles away easily to a powder. This effect which lime has in "lightening" heavy soils has been known to last for years. The disintegrating effect of quicklime when applied to heavy soils is also due, it may be added, to the change undergone by the lime itself from the caustic state to the mild state.
Lime renders light Soils more cohesive.
Although it may seem somewhat paradoxical, lime, it would appear, in some cases exercises an effect upon the soil exactly the reverse of what has just been stated. That lime should act as a binding agent is only natural when we reflect on the way in which it acts when used as mortar. It is quite to be understood, therefore, that its action on light friable soils should be to increase their cohesive powers, and at the same time to increase the capillary power of the soil to absorb water from the lower layers. The extent of this action, of course, would depend on the form in which the lime is applied, and the amount. A striking example of the binding power of lime is to be found in certain soils extremely rich in lime, in which what is known as a lime-pan has been formed at some distance from the surface.
II. Chemical Action of Lime.
But more important probably than even its mechanical action is the chemical action of lime. It is a most important agent in unlocking the inert fertility of the soil. This it does by decomposing different minerals and setting free the potash they contain. The disintegrating power of lime in this respect depends, of course, on its chemical condition, the caustic form being much more potent than the other forms. Its action in decomposing vegetable matter and rendering the inert nitrogen it contains available for the plant's use, is also one of its most important properties, and accounts for its beneficial action when applied to soils, such as peaty soils, rich in organic matter. Again, its use as a corrective for sour lands has long been practically recognised. The presence of acidity in a soil is hurtful to vegetable life. Lime, by neutralising this acidity, removes the sourness of the land, and does much to restore it to a condition suitable for the growth of cultivated crops. The generation of sourness in a soil is almost sure to give rise to certain poisonous compounds. Lime, therefore, in sweetening a soil, prevents the formation of these poisonous compounds. Badly drained and sour meadow-lands, as every farmer knows, are immensely benefited by the application of this useful manure; for not merely is their sourness removed and their general condition ameliorated, but many of the coarser and lower forms of plant-life, which alone flourish on such soils, are killed out, and the more nutritive grasses are allowed to flourish instead. The action of lime in promoting the formation of a class of compounds of great importance in the soil—viz., hydrated silicates—is worthy of notice. According to the commonly accepted theory, much of the available mineral fertilising matter of the soil is retained in the form of these hydrated silicates. Hence lime, by increasing these compounds, not merely adds to the amount of the available fertility in the soil, but also increases its absorptive power for food-constituents.
III. Biological Action of Lime.
The last way in which lime acts is what we have termed biological. By this we mean the important rôle lime plays in promoting or retarding, as the case may be, the various kinds of fermentative action which go on so abundantly in all soils. The presence of carbonate of lime in the soil is a necessary condition for the process of nitrification. Lime is the base with which the nitric acid, when it is formed, combines; and as we have seen, when discussing nitrification, soils of a chalky nature are among those best suited to promote the natural formation of nitrates. This is one of the reasons for the beneficial effects produced by lime when applied to peaty soils. Not merely does it help to decompose the organic matter so abundant in such soils, but it also furnishes the base with which the nitric acid may combine when it is formed. But while the action of lime is to promote fermentation, it must not be forgotten that there may be cases in which its action is rather the reverse of this. Fermentation of organic matter goes on when there is a certain amount of alkalinity present; while, on the other hand, the presence of acidity seems to retard and check it. Too great an amount of alkalinity, however, would, in the first instance, retard fermentation as much as too great acidity. It has been claimed that the addition of caustic lime to fresh urine may act in this way; and if this were so, the addition of lime to farmyard manure might, to a certain extent, be defended. The experiment, however, would be a hazardous one and not to be recommended, as loss of ammonia would most likely ensue.
Action of Lime on Nitrogenous Organic Matter.
The action of lime on nitrogenous organic matter is of a very striking kind, and is by no means very clearly understood. As we have pointed out, it sometimes acts as an antiseptic or preservative; and this antiseptic or preservative action has been explained on the assumption that insoluble albuminates of lime are formed. Its action in such industries as calico-printing, where it has been used along with casein for fixing colouring matter; or in sugar-refining, where it is used for clarifying the sugar by precipitating the albuminous matter in solution in the saccharine liquor; or lastly, in purifying sewage,—has been cited in support of this theory. While, however, there may be circumstances in which lime, especially in its caustic form, acts as an antiseptic, its general tendency is to promote these fermentative changes, such as nitrification, so important to plant-life.
An important use of lime in agriculture is in preventing the action of certain fungoid diseases, such as "rust," "smut," "finger-and-toe," &c., as well as in killing, as every horticulturist and farmer knows, slugs, &c.
Recapitulation.
We may, in conclusion, sum up in a single paragraph the different ways in which lime acts. Its action is mechanical, chemical, and biological. It acts on the texture of the soil, rendering clay soils more friable, and exerting a certain binding effect on loose soils. It decomposes the minerals containing potash and other food-constituents, and renders them available for the plant's needs. It further decomposes organic matter, and promotes the important process of nitrification. It increases the power of a soil to fix such valuable food-constituents as ammonia and potash. It neutralises sourness, and prevents the formation of poisonous compounds in the soil. It increases the capillary condition of the soil, prevents fungoid diseases, and promotes the growth of the more nutritive herbage in pasture-land.
CHAPTER XXI.
INDIRECT MANURES—GYPSUM, SALT, ETC.
Gypsum.
In the previous chapter mention was made of gypsum as a compound of lime, but no reference to its action as a manure was made. In the past, gypsum was used extensively and highly valued. It was found to be of especial value for clover; and there is a story told of Benjamin Franklin which illustrates the very striking nature of its action on this crop. It is related that he once printed with gypsum the words "This has been plastered" on a field of clover, and that for a long time afterwards the legend was plainly discernible on account of the luxuriance of the clover on the parts of the field which had been thus treated.
Mode in which gypsum acts.
Despite the fact that gypsum is a most ancient manure, it is only of late years that we have come to understand the true nature of its action. For long it was believed that the reason of its striking effect in promoting clover was due to the fact that, as clover was a lime-loving plant, the action of gypsum was owing to the lime it contained. That, however, the action of gypsum is not due to the fact that it supplies lime to the plant, seems evident when it is stated that were this so, any other form of lime would have the same beneficial effect. It is well known, however, that this is not so. Besides, as we have already pointed out, lime is not a constituent which most soils lack, so far as the needs of the crop are concerned. There is a certain amount of truth in the old belief that gypsum enriches the soil in ammonia by fixing it from the air. The power that gypsum has as a fixer of ammonia has already been referred to in the chapter on Farmyard Manure; but in this case the gypsum is brought in contact with the ammonia. The origin of this old belief was due to a misconception as to the amount of ammonia in the atmosphere. No doubt gypsum greatly increases the power of a soil to absorb ammonia from the air; but the quantity of ammonia in the air is so very trifling, that its action in this respect is hardly worth considering. The true explanation of the action of gypsum is to be found in its effect on the double silicates, which it decomposes, the potash being set free. Its action is similar to that of other lime compounds, only more characteristic. As a manure, therefore, its action is indirect, and its true function is to oust the potash from its compounds. Its peculiarly favourable action on clover is due to the fact that clover specially benefits by potash, and that adding gypsum practically amounts to adding potash. Of course it should be borne in mind that the soil must contain potash compounds if gypsum is to have its full effect. Now, however, that potash salts suitable for manuring purposes are abundant, it may well be doubted whether it is not better to apply potash directly. Further, it must be borne in mind that gypsum is applied to the soil whenever it receives a dressing of superphosphate of lime, as gypsum is one of the products formed by treating insoluble phosphate of lime with sulphuric acid.
It is possible that gypsum may act as an oxidising agent in the soil, just as iron in the ferric condition does. It has a large quantity of oxygen in its composition, and under certain conditions may act as a carrier of oxygen to the lower layers of the soil. When it is used, it should be applied some months before the crop is sown.
Gypsum, therefore, although it contains two necessary plant-constituents, lime and sulphuric acid, cannot be regarded as a direct manure; and as its action comes to be more fully understood, its use, which was never very abundant in this country, will probably decrease. We have already, in the chapter on Nitrification, referred to the action of gypsum in promoting nitrification.
The action of salt as a manure presents a problem which is at once of the highest interest and surrounded with the greatest difficulties. In view of the large quantities now used for agricultural purposes, a somewhat detailed examination of the nature of its action is not out of place in a work such as the present.
Antiquity of the Use of Salt.
The recognition of the manurial functions of salt dates back to the very earliest times. Its use among the ancients is testified by numerous allusions in the Old Testament; while, according to Pliny, it was a well-known manure in Italy. The Persians and the Chinese seem also to have used it from time immemorial, the former more especially for date-trees.
Nature of its Action.
Despite, however, the great antiquity of its use, much difference of opinion seems always to have existed as to the exact method of its action, and as to its merits as a manure in promoting vegetable growth. It furnishes, in fact, a good example of the difficulty which exists in the case of many manures, whose action is chiefly indirect, of fully understanding their influence on the soil and on the crop. In fact, the action of salt is probably more complicated than that of any other manurial substance.
Salt not a necessary Plant-food.
We have already seen that neither sodium nor chlorine—the two constituent elements of salt—are in all probability absolutely necessary plant-foods. If they are necessary, the plant only requires them in minute quantities. Despite this fact, soda is an ash-constituent of nearly every plant, and in many cases one of the most abundant. In amount it is one of the most variable of all the ash-constituents, being present in some plants only in minute quantities, while in others it occurs in large quantities. Mangel and plants of the cabbage tribe may be cited as examples of plants containing large amounts of soda in their composition. But the plants which contain it in largest quantity are those which thrive on the sea-coast, and it has been thought that for them at least salt is a necessary manure. This, however, does not seem to be the case. In fact, the amount of soda in a plant seems to be largely a matter of accident. It may be added that the succulent portions of a plant are generally richest in soda.
Can Soda replace Potash?
Again, it has been believed that soda is capable of replacing potash in the plant; but this does not seem to be the case to any extent. The view that soda is able to replace potash, it has been thought, is supported by the variation which exists in the proportion of soda and potash in different plants. It must be remembered, however, that it is highly probable that most plants contain a larger quantity of ash-constituents than is absolutely necessary for their healthy growth. Especially is this the case with such a necessary plant-food as potash, of which there is generally present, in all likelihood, an excess. The variation in the quantity of potash and soda present in many plants under different circumstances can scarcely, therefore, be regarded as furnishing a proof of the replacement of potash by soda. Incidentally we may mention, as a fact worthy of notice, that cultivated plants have more potash and less soda in their composition than wild plants. What has been said of soda may be held to apply equally to chlorine, as it seems to be chiefly in the form of common salt that soda enters the plant. The amount of salt, therefore, present in plants must be regarded as largely accidental and dependent on external circumstances, such as the nature of the soil, &c.
Salt of universal Occurrence.
But even were salt a necessary plant-food, its occurrence in the soil is already of sufficient abundance to obviate any necessity for its application. It may be said to be of almost universal occurrence. Even the air contains it in traces. That this is the case in the neighbourhood of the sea-coast is well known; but even in air far inland, accurate analysis of the air would probably demonstrate its presence in greater quantity than is commonly believed. It is a wise provision that plants absorb salt, for it increases their efficiency as food,—the function of salt as a constituent of animal food being of the very highest importance. It is an indispensable food-ingredient for animal life. With regard to ordinary farm-stock, the amount of salt which naturally occurs in their food is quite sufficient. In the case, however, of pastures in countries far removed from the sea, the custom of specially supplying stock with salt is common. This is done by placing a piece of rock-salt in the fields.
Special Sources of Salt.
The salt of commerce is obtained from various sources. Besides the sea, we have ample sources of salt in the large saline deposits found in many parts of Europe, especially in Austria, and in England in Cheshire.
The Action of Salt indirect.
From what has been said above, it is clear that the action of salt as a manure is indirect and not direct. What the nature of that indirect action is we shall now proceed to discuss.
In considering the evidence of the manurial value of salt, we are at once brought face to face with the fact that the experience of its action in the past has as often been unfavourable as favourable. Salt, it is well known, is both an antiseptic and a germicide. It is, indeed, one of the most commonly used of preservatives. When applied in large quantities to the soil, it has a most deleterious action on vegetation. This hurtful action of salt has long been known; and it is as often mentioned in the writings of antiquity on account of its unfavourable as on account of its favourable action. Thus, for example, among the ancient Jews it was customary, after the conquest of a hostile town, to strew salt on the enemy's fields, for the purpose of rendering them barren and unfertile. And again, among the Romans, for the same purpose, salt was often spread on a spot where some great crime had been committed.
While, therefore, its unfavourable action has long been known, the fact that there are circumstances under which its action is, on the contrary, favourable for promoting vegetable growth has also been long recognised. The difficulty for the agricultural student is to reconcile these two seemingly contradictory experiences. For the English agriculturist the subject possesses especial interest, since in England it has been in the past most generally used and its action most discussed since the time of Lord Bacon, who discusses in his writings the action of solutions of it on different plants.
The true explanation of salt being so different in its action is to be found in the quantity applied, the nature of the soil, the crop to which it is applied, and the conditions under which it is applied—i.e., whether it is applied alone or along with other manures.
In the first place, it must be noted that salt exerts a mechanical action on the soil of a very similar kind to that exercised by lime. When applied to clay soils it causes a flocculation or coagulation of the fine clay-particles, and thus prevents the soil from puddling to the same extent as would otherwise be the case. In fact, an example of this action of salt when in solution causing the precipitation of fine suspended clayey matter, is afforded by the formation of deltas at the mouths of rivers. The power of clarifying muddy water is common indeed to saline solutions. Schloesing attributes the clarifying power of a soil to the presence of the saline matters it contains; and from this point of view it would appear that manures containing any saline substance may exert an important mechanical influence on the soil.
Solvent Action.
But a much more important property of salt is its solvent action on the plant-food present in the soil. Its action in decomposing the minerals containing lime, magnesia, potash, &c., is similar to the action of gypsum. By acting upon the double silicates it liberates these necessary plant-foods. It is not only on the basic substances upon which it acts, but also on the phosphoric and silicic acids, which it sets free. Its power of dissolving ammonia from the soil is considerable. Experiments with a weak solution of salt on a soil by Peters and Eichhorn to test its solvent power, showed that the salt solution dissolved more than twice as much potash and nearly thirty times as much ammonia as an equal quantity of pure water did. When applied to the soil, it seems chiefly to liberate lime and magnesia. The exact nature of the chemical action taking place is a point of some dubiety. According to some, it is changed into nitrate of soda; according to others, into carbonate of soda. The latter theory seems to be the more probable one. Its action on the lime and magnesia compounds is to convert them into chlorides; and this chemical reaction explains the action that salt has in increasing the water-retaining and water-absorbing power of the soil; for the chlorides of magnesia and lime are salts which have a great power of attracting water from the air.
Again, the very fact that salt acts as an antiseptic may serve to explain its beneficial action in certain cases where it prevents rankness of growth. No doubt this was its function when applied along with Peruvian guano. This it might do by preventing too rapid fermentation (nitrification) of the manure, or by actually weakening the plant. Its action when applied with farmyard manure may also be similar. But while its effect in many cases may be towards retarding fermentation, on the other hand its action, when applied along with lime to compost-heaps, is towards promoting more rapid decomposition. Probably a reaction takes place between the lime and the salt, the result of which is the formation of caustic soda.
Such are some of the ways in which salt may act. It must at once be seen how its action in one case will be favourable and in another case unfavourable. There must be fertilising matter present in the soil if it is to act favourably. Again, it will only be under such circumstances, where rankness of growth is likely to ensue, that its antiseptic properties will act favourably and not unfavourably.
Best used in small Quantities along with Manures.
Probably it is for these reasons that its action has been found to be most favourable when applied along with other manures and not alone. Applied along with nitrate of soda, as is commonly done, it doubtless increases the efficiency of the nitrate. Some plants seem to be undoubtedly benefited by salt: of these flax may be mentioned. The application of salt to plants of the cabbage tribe seems also to be highly beneficial. On mangels, along with other manures, it has also been found to have a very favourable effect. But with many crops its action has been proved to be less favourable.
Affects Quality of Crop.
Although salt has often been found to increase the quantity of a crop, the quality of the crop has been made to suffer. Its action on beetroot has been more especially studied. The effect of its application is to lessen the total quantity of dry matter and sugar in the plant. This has been found to be the case both when the salt was applied alone and along with nitrate of soda and other manures. On potatoes, again, its action has been found to be deleterious, lessening their percentage of starch. The deleterious action of chlorides on the quality of potatoes is also seen when potassium chloride is applied. It is for this reason that potash should never be applied to the potato crop in the form of chloride.
In the late Dr Voelcker's opinion, the conditions under which salt had the most favourable action on the mangel crop was in the case of a light sandy soil, and applied at the rate of 4 to 5 cwt. per acre. Its action when applied to clay soils was not so favourable.
Rate of Application.
Lastly, the rate at which it may be applied will naturally vary. From 1 cwt. and even less, up to 6 cwt. or even more, has been the rate at which it has been commonly applied in the past. From what has been said, it will be seen that it is more likely to exert a favourable influence when applied only in small quantities.
CHAPTER XXII.
THE APPLICATION OF MANURES.
The conditions which regulate the application of manures are many and varied, and the subject, it must be admitted, despite the large amount of investigation already carried out, is most imperfectly understood. For these reasons it is impossible to do little more than lay down certain general principles which may be of service to the agriculturist in guiding him in carrying out the manuring of his crops.
Influence of Manures in increasing Soil-fertility.
In the first place it may be asked, How far can what we may call the permanent fertility of a field be influenced by the application of manures? And to this question the answer must be made, that the influence of manuring in increasing soil-fertility is very slight and only very gradually felt. This is illustrated by the difficulty experienced in attempting to restore to a fertile condition a soil which has long been treated by an exhaustive system of cultivation. In such a case it will be found impossible to restore the fertility of the soil, except very gradually. Farmers who farm in new countries, and in rich virgin soils, little realise sometimes how quickly they may impoverish the fertility of their soils by exhaustive treatment, and how slow the process of restoration is. Nor is this strange when we reflect on the relatively small quantities of fertilising ingredients we are in the habit of adding to the soil by the application of manures, and the nature of their action. The small rate at which they are applied, and the impossibility of distributing them equally in the soil, explain how comparatively limited their action must necessarily be. Some manures, it is true—viz., those which are soluble—are more equally distributed; but then such manures, from their very nature, are little likely to affect the permanent fertility of the soil.
Influence of Farmyard Manure on the Soil.
Of manures which have the best effect in improving a soil's permanent fertility, farmyard manure is undoubtedly the most important. This is owing partly to the fact that it is applied in such large quantities, and partly on account of its composition. Liberal manuring with farmyard manure, systematically carried out, will in time do much to build up a soil's fertility. But liberal manuring with artificial manures will also effect the same end. This it does in an indirect manner by means of the increased crop residues obtained under such treatment. Indeed one of the speediest methods of bringing a soil into good condition is by heavily manuring certain green crops, and then ploughing them in.
Farmyard Manure v. Artificials.
The question how far farmyard manure may be supplanted by artificials is one often discussed. We have already referred to this question in the chapter on Farmyard Manure. It is possible that, with our increasing knowledge of agricultural science, we may in the future be able to dispense with farmyard manure, and make shift to do with artificials alone. At present, however, all our experience points to the fact that the most satisfactory results are obtained from manures by using artificials in conjunction with farmyard manure. It is better both for farmyard manure and artificial manures to be applied together,[241] so that they may mutually act as supplementary the one to the other. While this is so, there may be circumstances in which it will be best to use artificials alone. Where, for example, fields, owing to their situation, are inaccessible, and where the expense of conveying the bulky farmyard manure would be very considerable, it may be found more economical to apply the more concentrated artificial manures. With few exceptions, however, it will be found most desirable to use artificial manures as supplementary to farmyard manure, and not as substitutes for it.
Farmyard Manure not favourable to certain Crops.
While the above is true, it may be well to point out one or two facts regarding the nature of the influence of farmyard manure on certain crops. For instance, it has long been recognised as inadvisable in strong rich soils to apply it directly to certain grain crops, such as barley and wheat, since such a practice is apt to encourage rankness of growth—an undue development of straw at the expense of the grain. It is consequently customary to apply farmyard manure to the preceding crop. The direct application of farmyard manure to wheat, however, according to Sir J. B. Lawes, is not fraught with unfavourable results where the soil is a light one; it is only when the soil is of a heavy nature that it is best to apply it to the preceding crop. Potatoes are another crop to which it is best not to apply it directly. On the other hand, many are of the opinion that mangels seem to be able to benefit from large applications of farmyard manure.
Conditions determining the Application of Artificial Manures.
In the application of artificial manures a large number of considerations have to be taken into account. Among these may be mentioned the nature of the manure itself, and its mechanical and chemical condition; the nature of the soil and its previous treatment with manures, as well as the nature of the climate, the nature of the crop, and the previous cropping. It may be well, therefore, to examine somewhat in detail some of these considerations.
Nature of the Manure.
Nitrogen, phosphoric acid, and potash exist in the common manures, as has already been pointed out, in different states of availability. Nitrogen, for example, may exist in a soluble or insoluble condition, as nitrates, as ammonia, or in various organic forms. Phosphoric acid, similarly, may exist in a soluble form, as it does in superphosphate of lime, or in an insoluble form, as it does in bones or basic slag. Potash, on the other hand, exists—or should exist—in artificial manures only in a soluble form. Now a correct knowledge of the behaviour of these different forms of the common manurial ingredients when applied to the soil is, in the first place, necessary for their successful and economical use.
Nitrogenous Manures.
Thus our knowledge of the inability of the soil-particles to retain nitrogen in the form of nitric acid, as well as our knowledge of the fact that nitrogen is in this form immediately available for the plant's needs, teaches us that nitrate of soda should never be applied before the plant is ready to utilise it—in short, that it should only be applied as a top-dressing; and further, that the use of such a fertiliser in a damp season is less likely to be economical than in a dry one. Again, with regard to nitrogen in the form of ammonia salts, our knowledge of the fact that ammonia is retained by the soil-particles, and that before it becomes available for the plant's needs it has to undergo the process of nitrification, teaches us the desirability of applying it a short time before it is likely to be used. While, lastly, with regard to the nitrogen in the various organic forms in which it occurs, our knowledge of the rate at which these are converted into an available form in the soil will determine when they are best applied. Some forms of organic nitrogen are in a soluble condition, and are quite as speedy in their action as sulphate of ammonia. This is the case with a considerable proportion of the different organic forms of nitrogen present in guano. Other forms of organic nitrogen are only slightly less so—as, for example, dried blood, which ferments very speedily. With regard, therefore, to nitrates and ammonia salts, as well as the more quickly available organic forms of nitrogen, they should either be applied as a top-dressing after the plant has started growth, or only shortly before seed-time. Bones, shoddy, and the various so-called native guanos, should be applied a considerable period before they are likely to be required—not later than the previous autumn.
Phosphatic Manures.
With regard to phosphatic manures the same considerations hold good. Inasmuch as phosphoric acid, whether applied in the soluble condition, as in superphosphate, or the insoluble form, as in bones, basic slag, &c., is not liable to be washed out of the soil, the risk of loss is very slight, and need not be taken into account. As we have pointed out in considering the action of superphosphate, phosphoric acid in this latter form is more speedily available to the crop, and the necessity of applying it much before it is likely to be used does not exist. Hence superphosphate and manures which contain any appreciable amount of soluble phosphoric acid, such as guano, should only be applied shortly before seed-time. Bones, basic slag, or mineral phosphate ought to be applied, on the other hand, a long time before they are likely to be used. Hence an autumn application is to be recommended in the case of such manures.
Potash Manures.
Lastly, with regard to potash manures, as these are soluble, there is no necessity for applying them much before they are likely to be absorbed by the plant. Some are of the opinion that potash is, except in the case of sandy soils, best applied some little time before it is likely to be used, so as to permit of its being washed down into the soil—a process which takes place only comparatively slowly. As potash manures have often been found to give a better result on pastures during the second year than during the first, they are best applied in the autumn.
The above statement as to the behaviour of the different fertilisers when applied to the soil, has a not unimportant bearing on the quantities in which they may safely be respectively applied. The rate at which manures may be applied depends, as we shall immediately see, on other conditions; but what it is here desirable to point out is, that it is not safe to apply such manures as nitrate of soda, or, for that matter, sulphate of ammonia, in large quantities at a time. In fact these manures, especially the former, will best be applied in very small quantities, and rather in several doses. With regard to other manures, more especially phosphatic manures, the same reasons for small application do not exist.
The truth of the above statements is so obvious that it may be regarded as superfluous to make them. As, however, their clear apprehension is essential to understanding the conditions of successful manuring, no apology need be made for making them.
Nature of Soil.
Another condition which has to be taken into account in considering the application of manures is the nature of the soil, as well as its previous treatment. Soils poor in organic matter are those which are most likely to be benefited by the application of nitrogenous manures. Soils of a dry light character require less phosphoric acid than they do of nitrogen and potash; while on a damp and heavy soil phosphatic manures are more likely to be beneficial than nitrogenous or potassic manures. Lastly, a soil rich in organic matter generally requires phosphates, and possibly potash. A point of considerable importance to notice is, that a soil rich in lime can stand a larger application of phosphoric acid than one poor in lime. As a rule, it will be found that the best results with potash will be obtained when applied to a sandy soil. The nature of the soil is an important consideration in determining how far it is advisable to apply readily soluble manures. To a very light and non-retentive soil the risk of loss in applying an easily soluble manure is considerably increased. The nature of the climate is also of importance. Thus, in a dry climate, manures of a soluble nature will have a better effect than in a wet climate, while the opposite will be the case with the more slowly acting manures.
Nature of previous Manuring.
A consideration of equal importance is the previous treatment of the soil with manure. For example, where a soil has been liberally treated with farmyard manure, it has been found that mineral manures have a very inferior effect to that obtained by nitrogenous manure. Lawes and Gilbert have found this to be strikingly the case in their experiments on the growth of wheat. In these experiments it was found that the application of mineral manures was accompanied with little or no benefit to the crop, whereas very striking results followed the application of nitrogen. This they attributed to the fact that the supply of mineral fertilisers in the straw of the farmyard manure is largely in excess of the supply of nitrogen. The nature of the action of the manure previously applied is also to be taken into account in determining how long its influence may probably last. Where, for example, the manure has been nitrate of soda or sulphate of ammonia, it may be safely concluded that its direct influence is no longer felt a year after application. The influence of superphosphate of lime, while scarcely so temporary, may be said to last only for a comparatively short time.[242] On the other hand, when the manure applied is of a slow-acting nature, such as bones or basic slag, its influence will probably be felt for a number of years.
Nature of the Crop.
But more important than any of the above-mentioned conditions is the nature of the crop itself. Our knowledge of the requirements of the different farm crops is still very imperfect. A very wide experience, however, of the effect of different manures on different crops, has conclusively proved that their manurial requirements differ very considerably. The subject is complicated by other considerations, such as the nature of the soil, &c.; but notwithstanding this fact, certain points seem to be pretty well established.
In seeking to understand the respective requirements of the different crops for different fertilisers, two important considerations must be borne in mind. These are—(1) the quantities of the three fertilising ingredients—nitrogen, phosphoric acid, and potash—which different crops remove from the soil; and (2) the different power crops possess of assimilating these ingredients.
Amounts of Fertilising Ingredients removed from the Soil by different Crops.
The most convenient way of instituting a comparison between the requirements of the different crops in this respect is by calculating the amount, in pounds, of nitrogen, phosphoric acid, and potash, which average amounts of the different crops remove per acre. The following table shows this for the common crops:—
| Nitrogen. | Phosphoric Acid. | Potash. | ||
| Mangels | Root, 22 tons | 87 | 36.4 | 222.8 |
| Leaf | 51 | 16.5 | 77.9 | |
| Total crop | 138 | 52.9 | 300.7 | |
| Turnips | Root, 17 tons | 63 | 22.4 | 108.6 |
| Leaf | 49 | 10.7 | 108.6 | |
| Total crop | 112 | 33.1 | 148.8 | |
| Beans | Grain, 30 bushels | 77 | 22.8 | 24.3 |
| Straw | 29 | 6.3 | 42.8 | |
| Total crop | 106 | 29.1 | 67.1 | |
| Red clover hay, 2 tons | 102 | 24.9 | 83.4 | |
| Swedes | Root, 14 tons | 70 | 16.9 | 63.3 |
| Leaf | 28 | 4.8 | 16.4 | |
| Total crop | 98 | 21.7 | 79.7 | |
| Oats | Grain, 45 bushels | 38 | 13.0 | 9.1 |
| Straw | 17 | 6.4 | 37.0 | |
| Total crop | 55 | 19.4 | 46.1 | |
| Meadow hay, 1-1/2 ton | 49 | 12.3 | 50.9 | |
| Wheat | Grain, 30 bushels | 33 | 16.0 | 9.8 |
| Straw | 15 | 4.7 | 25.9 | |
| Total crop | 48 | 20.7 | 35.7 | |
| Barley | Grain, 30 bushels | 35 | 16.0 | 9.8 |
| Straw | 13 | 4.7 | 25.9 | |
| Total crop | 48 | 20.7 | 35.7 | |
| Potatoes, 6 tons | 47 | 21.5 | 76.5 | |
| Maize | Grain, 30 bushels | 28 | 10.0 | 6.5 |
| Stalks, &c. | 15 | 8.0 | 29.8 | |
| Total crop | 43 | 18.0 | 363 | |
From the table it will be seen that the crops which remove the largest quantities of all three fertilising ingredients are the root crops—mangels and turnips; that beans remove twice as much nitrogen as the cereals—oats, barley, and wheat—which, in this respect, practically differ very little from one another; while potatoes remove about the same quantity of nitrogen as the cereals. It will further be noticed that the amounts of phosphoric acid removed by the different crops differ very much less than those of nitrogen and potash. Mangels remove slightly more, and turnips slightly less, than double the amount removed by cereals. Meadow-hay, it will be seen, of all crops removes the least phosphoric acid.
In looking at the amounts of potash, we are at once struck by their great discrepancy. Such a crop as mangels removes more than six times as much potash from the soil as the cereals. Turnips also make large demands on this ingredient, removing over four times as much as the cereals. Leguminous crops, such as red clover and beans, remove about twice as much.
Capacity of Crops for assimilating Manures.
Instructive though these figures undoubtedly are, they must not be regarded, as often erroneously they are, as furnishing by themselves sufficient data upon which to base the practice of manuring. A consideration which is of much greater importance is the capacity that different crops possess for assimilating the various manurial ingredients from the soil. Considered from the point of view of absolute amount, there is in most soils an abundant supply of plant-food; but of this amount only a small proportion is available. Further, the amount of this available plant-food will vary with different crops—one crop being able to grow where another crop would starve. As illustrative of this, in the Norfolk experiments it was found that the turnip was able to assimilate potash from a soil on which the swede was practically starved. It is on this fact more than any other that the principles of manuring are based. Several explanations of the different capacities crops possess of assimilating their food may be put forward. And we may here point out that crops belonging to the same class exhibit, on the whole, a certain amount of similarity in their manurial requirements. Thus, for example, we may say that gramineous crops so far resemble one another in possessing small capacity for assimilating nitrogen, root crops for assimilating phosphoric acid, and leguminous crops for assimilating potash, and that, consequently, these crops are generally most benefited by the application, respectively, of nitrogen, phosphoric acid, and potash. But while a certain general resemblance exists, crops belonging to the same class differ in many cases very considerably, as we shall immediately see.
Difference in Root Systems of different Crops.
One explanation of the different capacity possessed by different crops for absorbing plant-food from the soil is to be found in the difference of their root systems. Every agriculturist knows that crops in this respect differ very widely. Crops having deep roots will naturally have a larger surface of soil from which to draw their food-supplies than crops having shallower roots. Such crops as red clover, wheat, and mangels are able to draw their food-supplies from the subsoil to an extent not possessed by shallower-rooted crops, such as barley, turnips, and grass. Crops having surface-roots, on the other hand, have often greater capacity for assimilating nitrogen,—this ingredient, as has already been pointed out, being chiefly located in the surface-soil. The tendency of growing shallow-rooted crops will therefore be towards impoverishing the surface-soil; whereas the occasional growth of a deep-rooted crop brings the plant-food in the subsoil into requisition. In this connection it may be well to draw attention to the singular capacity possessed by certain crops for absorbing nitrogen. Of these the case of clover is the most striking, and has long puzzled agriculturists. The discovery, which has been repeatedly referred to in these pages, that the leguminous order of crops, to which clover belongs, have the power of absorbing the free nitrogen of the air through the agency of micro-organic life in the plant and in the soil, has furnished an explanation of this long-debated problem.
Period of Growth.
A further reason is the difference in the period of a crop's growth. A crop which grows quickly, and consequently occupies the ground during a comparatively short period, will naturally require a richer soil, and therefore a more liberal treatment with manure, than one whose growth is more gradual.
Another consideration is the season of the year during which active growth of the crops takes place. For example, in the case of the wheat crop, active growth takes place in spring and ceases early in the summer. Since, however, nitrification goes on right through the summer, and nitrates are most abundant in the soil in late summer and autumn, such a crop as wheat is ill suited to obtain any benefit from this bountiful provision of nature, and is consequently particularly benefited by the application of nitrogenous manures. Root crops, on the other hand, sown in summer, continue their active growth into autumn, and are thus enabled to utilise the nitrates formed in the process of nitrification. The custom of sowing a quickly growing green crop, such as rye, mustard, rape, &c., after a wheat crop, is a practice which aims at conserving the nitrates and preventing their loss by autumn and winter rains. The name "catch crop" has been applied to such a crop. By ploughing under the green crop, the nitrogen removed from the soil in the form of easily soluble nitrates is restored in an insoluble organic form, and the soil is at the same time enriched by the addition of much valuable organic matter.[243]
It is chiefly the above facts that form the scientific basis of the long-pursued practice of the rotation of crops.
Variation in Composition of Crops.
A point of considerable interest is the influence exerted by manures on the composition of crops. It has been assumed in the preceding pages that the composition of crops of the same plant is uniform; but this is not strictly the case, as it has been proved that not merely the manure and soil have an appreciable influence on the crop's composition, but so also has the climate.
Absorption of Plant-food.
The laws regulating the absorption of plant-food are most interesting, although, unfortunately, very imperfectly understood as yet. The fertilising ingredients are capable of considerable movement in the plant, and are only absorbed up to a certain period of growth. This in many plants is reached when they flower. After this period they are no longer capable of absorbing any more food. The popular belief that plants in ripening exhaust the soil of its fertilising matters is consequently a fallacy.
Fertilising Ingredients lodge in the Seed.
The tendency of fertilising matters is to move upward in the plant as it matures, and finally to become lodged in the seed. It is for this reason that the cereals prove such an exhaustive crop. That nature, however, can in certain cases be very economical of her food-supplies, is strikingly illustrated by the fact that much of the fertilising matter contained in the mature leaves in autumn passes back into the tree before the leaves fall from it.
Forms in which Nitrogen exists in Plants.
The form in which nitrogen is present in the plant is chiefly as albuminoids. As, however, albuminoids belong to that class of bodies known as colloids, which cannot easily pass through porous membranes like those forming the walls of plant-cells, they are changed during certain periods of the plant's growth into amides, which are crystalloids, and consequently able to move freely about in the plant. Amides are most abundant in young plants during the period of their most active growth, and as the plant ripens the amides seem to be largely converted into albuminoids.
While the subject is not very clearly understood, it would seem to be pretty conclusively proved that there is a direct relation between the amount of the phosphoric acid and of the nitrogen absorbed.
Bearing of above Facts on Agricultural Practice.
The bearing of these facts upon practice is obvious. In the first place, they show how important it is that plants should be well fed when they are young, and that in the practice of green manuring it is best to plough in the crop when it is in flower, as no additional benefit is gained by allowing it to ripen, seeing that no further absorption of fertilising ingredients takes place after the period of flowering.
Influence of excessive Manuring of Crops.
The influence of large quantities of manures is seen in the case of certain root crops. It is found, in such a case, that while the roots are larger, they are more watery in composition and of less nutritive value. Again, it seems to be a fact pretty generally known to practical men, that nitrate of soda seems to have a bad effect on the quality of hay. It would seem, further, that the influence of nitrogenous fertilisers on cereals is to increase the percentage of nitrogen in the grain, but that they have no such influence in the case of leguminous crops. Phosphatic manures, on the other hand, in the case of leguminous crops, seem to have the effect of diminishing the amount of nitrogen in the seed.