FOOTNOTES:
[62] See Chapter on Basic Slag.
CHAPTER III.
THE POSITION OF NITROGEN IN AGRICULTURE.
Of manurial ingredients, nitrogen is by far the most important, and on the presence and character of the nitrogen it contains, the fertility of a soil may be said to be most largely dependent. Most soils, as a rule, are better supplied with available ash ingredients than with available nitrogen compounds. The expensive nature of most artificial nitrogenous manures also gives to nitrogen the first position from an economic point of view. A thorough study, therefore, of the different forms in which it exists in nature, of the numerous and complicated changes it undergoes in the soil, by which it is prepared for the plant's needs, of the relation of its different forms to plant-life, and of the natural sources of its loss and gain, is of the highest importance if we are to hope to understand the difficult question of soil-fertility.
The Rothamsted Experiments and the Nitrogen question.
The position of nitrogen in agriculture is a question of great difficulty and complexity. It has engaged much attention, and has had devoted to its elucidation much elaborate and painstaking research. To the Rothamsted experiments we owe most of the information we possess on the subject, and the facts contained in this chapter are almost entirely derived from the results of these famous experiments, as embodied in the memoirs and writings of Messrs Lawes, Gilbert, and Warington.
Different forms in which Nitrogen exists in Nature.
We have already referred to the nitrogen question in the historical introduction. In order, however, to have a comprehensive view of the subject, it may be well to recapitulate some of the facts there mentioned.
Nitrogen, as we have already seen, exists in the "free" or elementary condition, as nitrates and nitrites, as ammonia, and in a large number of different organic forms.
Nitrogen in the Air.
It occurs in greatest abundance (amounting to about 80 per cent) in the first of these forms in the air. That this free nitrogen, which is practically unlimited in quantity,[63] has originally been the source of all its other forms, is of course obvious. But this conversion of free nitrogen into the various compound forms in which it occurs throughout the mineral, vegetable, and animal kingdoms, has been a process effected by a variety of indirect methods, and only at the expense of a vast amount of time. For practical purposes, the free nitrogen of the air may be regarded chiefly as a non-available source for most bodies containing it. It may be described as of all forms of nitrogen the least active, as far as plant-life is concerned.
Relation of "free" Nitrogen to the Plant.
The relation of the "free" nitrogen to the plant has formed the subject of much research, more especially during the last few years, and a brief epitome of the main results arrived at has already been given in the Introductory Chapter.[64]
That this source of nitrogen is not so inaccessible to the plant as was formerly believed, has now been abundantly proved. As the considerations which have led to this conclusion, and have suggested the very recent elaborate experiments on the fixation of free nitrogen by the plant—the results of which bid fair, it would seem, to largely revolutionise our agricultural practice—have been due to the study of the relation of the soil-nitrogen to the plant, it will be best to defer further discussion of this question till we have dealt with the other sources of nitrogen.
In addition to nitrogen in the free state, air contains very small quantities of this element in combined forms. We have it in minute traces as nitrates and nitrites, as ammonia,[65] and also in still smaller traces as organic nitrogen in the minute dust-particles which modern researches have revealed as being present in such enormous numbers in our atmosphere. What the sources of these nitrates and nitrites (which exist in quantities so minute that accurate determination of their amount is rendered extremely difficult) are is a disputed point. That nitrogen and oxygen unite together to form nitric and nitrous oxides under the influence of intense heat, such as the electric spark, has been proved beyond doubt. One source, therefore, is probably the electrical discharges which are taking place more or less frequently on different parts of the earth's surface. Nitrates may also be formed in the combustion of nitrogenous bodies.[66] In the burning of coal-gas, for example, it is probable that small quantities of nitrates may be produced. Similarly the slow combustion or decay of nitrogenous organic matter, which constantly takes place all over the earth's surface, may be regarded as another source of this form of combined nitrogen. Ammonia may be similarly formed by the combustion, either quick or slow, of nitrogenous organic matter. It exists in the air as nitrate or nitrite of ammonia, and also as carbonate of ammonia.[67]
Amount of combined Nitrogen falling in the Rain.
The importance of the combined nitrogen in the air as a source of soil-nitrogen is best gauged by the amount falling annually on the soil dissolved in rain. This has been found to vary considerably. In the rain falling in the vicinity of large towns the amount is greater than in rain falling in the country. Thus at Rothamsted, in England, the average amount for several years was only 3.37 lb. nitrogen per annum per acre, of which 2.53 lb. were as ammonia,.84 being as nitric acid. At Lincoln, in New Zealand, 1.74 lb. fell annually per acre—as ammonia,.74, as nitric acid, 1.00; while at Barbadoes the amount was 3.77 lb., of which .93 was as ammonia, and 2.84 as nitric acid.[68] That the combined nitrogen derived from the air by the soil may be considerably in excess of this is highly probable. Soils, especially when damp, may absorb much larger quantities from the air of the combined nitrogen it contains. We must remember that the air in contact with the soil-surface is constantly being changed, and that there is thus a constant renewal of the air passed over the ground. The result is that the amount of air from which combined nitrogen may be removed is very great.[69]
Nitrogen in the Soil.
It has been remarked as a fact worthy of notice that nitrogen is essentially a superficial element. By this is meant that it is only found, as a rule, on the earth's immediate surface. This statement can only be admitted to be true within certain limits. The chief source of nitrogen, in addition to the atmosphere, is, of course, vegetable and animal tissue.[70] As vegetable and animal tissue are only found to any extent on the earth's surface, nitrogen is therefore chiefly found there. The natural deposits of nitrogen salts, such as the nitrate-fields of Chili and the saltpetre soils of India, &c., also only occur superficially. Notwithstanding these facts, however, the amount of nitrogen which exists at probably considerable depths from the surface must be very great. There are few sedimentary rocks which do not contain it. At Rothamsted a sample of calcareous clay, taken from a depth of 500 feet, contained .04 per cent—that is, as much as is found, on an average, in the Rothamsted clay subsoils.
Nitrogen in the Subsoil.
On the whole, however, as we have said, nitrogen is chiefly found in the surface-soil. The amount found in the subsoil at Rothamsted seems to vary very slightly at different depths, the percentage amounting to from .06 to .03.[71] Unlike the nitrogen of the surface-soil, that in the subsoil seems to be of very ancient origin, being probably derived from the remains of animal and vegetable life in the mud deposited at the bottom of the ocean. It is more abundant in the case of a clay subsoil than in a sandy subsoil.
Nitrogen of Surface-Soil.
Nitrogen has a tendency to collect on the top layers of the surface-soil, the first 9 inches or foot containing by far the largest proportion of it. In the table given in the Appendix,[72] the rate at which it decreases in amount the further down we go is clearly shown. Determinations of the respective amounts of nitrogen in every 3 inches of the soil, taken to a depth of one foot of the experimental wheat-field at Rothamsted, showed that the percentage between the first 3 inches and the second 3 inches varied very slightly. A more marked difference, however, was shown to exist between the nitrogen in the second and third 3 inches; while the fourth 3 inches were distinctly poorer—differing very little in their percentage of nitrogen from the subsoil. This was the case in unmanured soil. In the case of heavily manured soil, the increase in the soil's percentage, due to manure, was shown to be felt to the depth of a foot, but not much below it.[73]
A careful perusal of the tables in the Appendix will show that the quantity of nitrogen in the case of both arable and pasture soils steadily decreases for the first 3 feet, but that below this depth little decrease is seen, the percentage evidently becoming fairly constant.
The amount of Nitrogen in the Soil.
Very considerable difference exists in the amount of nitrogen present in different soils. The majority of analyses refer only to the amount found in the surface-soil—generally in the first 9 or 12 inches. As the soil, further, is not a body exactly homogeneous in its character, very considerable difficulty exists in obtaining reliable results. A great deal depends, therefore, on the method of sampling and the basis of calculation adopted; and it may be that this may occasionally explain, to some extent at least, the great discrepancies in the estimation of the quantities of nitrogen present in different soils as found by different investigators.
Peat-soils richest in Nitrogen.
Of all soils, peat-soils are richest in nitrogen. Professor S. W. Johnson found the nitrogen in fifty separate samples of peat to range from .4 per cent to 2.9 per cent, the average being 1.5 per cent. On the other hand, marls and sandy soils are poorest, the analyses of a number of these soils showing only from .004 to .083 per cent for the former, and .025 to .074 for the latter. As a general rule most arable soils contain over one-tenth per cent of nitrogen, or, say, over 3500 lb. per acre. A good pasture-soil, taken to a depth of 9 inches at Rothamsted, was found to contain about a quarter per cent. In ten samples of soil, taken to a depth of 9 inches, from different parts of Great Britain and Ireland, Munro found from .128 to .695 per cent of nitrogen, the average being .3278 per cent. The Rothamsted soils, it may be pointed out, are probably poor in nitrogen compared with most soils. A. Müller's investigations showed that in some of the soils he has analysed, the nitrogen amounted to little short of one per cent, while for the others the average was over half a per cent; even the poorer soils he examined contained about one quarter per cent on an average. Anderson's analyses of Scottish wheat-soils showed a variation of from .074 to .22 in the surface-soil, while he found in their subsoil from .15 to .92 per cent. Boussingault's results are also very much higher. The amount of nitrogen in a number of loams coming from widely different localities he examined contained from 6000 to 30,000 lb. per acre—the soil taken to a depth of 17 inches.[74]
Nature of the Nitrogen in the Soil.
When we compare the amount of nitrogen removed by different crops (which, even in the case of those most exhaustive of nitrogen, does not often amount to more than 150 lb. per acre), with the amount contained in the soil, the former amount seems very insignificant when compared to the latter. Such being the case, it would seem at first sight that the addition of nitrogen in the form of manures is quite superfluous. We must remember, however, that while the total amount of nitrogen is relatively large when compared to that removed by crops, only a very small proportion is in a condition available to the plant. This leads us to consider the different forms in which nitrogen is present in the soil, and their respective quantities.
Organic Nitrogen in the Soil.
Nitrogen occurs in the soil as organic nitrogen, nitric acid, nitrous acid, and ammonia. By far the largest proportion is present in the first of these forms. This is a wise provision, for otherwise the soil would be apt to become very speedily impoverished in nitrogen; for that present as nitrates it has scarcely any power to retain, while that present as ammonia is soon converted into nitrates by the process of nitrification.
The organic nitrogen of the soil, although we are apt to think of it as such, is by no means of a homogeneous character, or of equal value as a source of plant-food. Some of it, it would seem from recent investigations, is in a condition more susceptible of being converted into an available form than the rest. Thus in the process of nitrification, a process which we shall consider at length immediately, there seems to be generally a certain small proportion more ready to undergo this change than the rest; so that when this small amount is used up nitrification proceeds more slowly. In short, although we as yet know very little of the nature of the organic nitrogen of soils, we cannot doubt but that there is a constant series of changes in its composition taking place, resulting in the gradual elaboration of more available forms, until ultimately these are converted into ammonia and nitrates.
The great bulk of the organic nitrogen, however, in the soil must be regarded as in an inert condition, and by no means available for the crop. What the exact chemical form of this nitrogen is it is extremely difficult to say. Mulder was of the opinion that a considerable proportion was in the form of humate of ammonia. This opinion, as we shall have occasion to see immediately, was based on false grounds. It is highly probable that it may be in some form approximating to amide nitrogen. Its inert character is against the belief that it long remains as albuminoid nitrogen.
Different Character of Surface and Subsoil Nitrogen.
A point of very considerable importance to notice is, that the nitrogenous organic matter of the surface-soil is very different from that found in the subsoil. This difference is shown by the variation in the ratio of nitrogen to carbon, which points to the fact that, just as we should naturally suppose, the origin of the latter is very much more ancient than the origin of the former. Thus in the first 9 inches of old pasture-soil at Rothamsted, the ratio was 1:13; while in the subsoil, 3 feet from the surface, it was only 1:6. In the surface-soil it thus approaches more nearly in composition ordinary vegetable matter.
Nitrogen as Ammonia in Soils.
The second form in which nitrogen is present in soil is as ammonia. A very considerable misapprehension has existed in the past as to the amount of nitrogen in this form in soils. This mistake was due to the method adopted in estimating it, which consisted in treating the soil with boiling caustic alkalies and counting as ammonia what was given off as such. It is now known that certain forms of organic nitrogen—as, for example, amides—if treated in this way are slowly converted into ammonia. Statements, therefore, which are found in the older text-books, representing the amount of ammonia in soils as at over a tenth per cent, must be regarded as utterly unreliable. Indeed it is highly probable that ammonia only occurs in most soils in very minute traces. From what we know of the process of nitrification, we see how it is wellnigh impossible that ammonia should exist to any extent in the soil except under very exceptional circumstances.
Amount of Ammonia present in the Soil.
In ordinary soils it probably does not amount to more than from .0002 per cent to .0008 per cent, or an average of .0006 per cent.[75] In rich soils, or in garden-soils, the amount may be considerably more. Thus Boussingault found in a garden-soil .002 per cent. In peat and in peat-mould even a higher percentage has been found—viz.,.018 for the former and .05 for the latter.
Nitrogen present as Nitrates in the Soil.
The third form of nitrogen in the soil is nitric acid. It is more abundant in this form than as ammonia; but still, compared with the organic nitrogen, its amount is trifling. Probably not more than 5 per cent of the total nitrogen of a soil is ever present as nitrates. The reason of this is twofold. First, as we have already remarked, the soil has very little power to retain nitrogen in this form; and secondly, where the soil is covered with growing vegetation the nitrates are quickly assimilated by the plant as they are formed. It is for this reason that we find the quantity of nitrogen as nitrates very much greater in fallow soils than in those covered with a crop.
Position of Nitric Nitrogen in Soil.
As we shall have occasion to see more fully in the following chapter on Nitrification, the formation of nitrates is chiefly limited to the surface-soil, the largest proportion being formed within the first 9 or 12 inches. For this reason we find the largest quantity of nitrates in the surface-soil. But inasmuch as they are easily washed into the lower layers of the soil after formation, we often find a considerable proportion beyond the first 9 inches. The position of nitrates in the soil thus depends very considerably on the season of the year and the weather. In dry weather, where the evaporation of the soil-water takes place at a considerable rate, the tendency will be to concentrate the nitrates in the superficial portion of the soil. In wet weather, on the other hand, the tendency will be to wash the nitrates into the lower layers.
Amount of Nitrates in the Soil.
The determination of the amount of nitrates in a soil is not of very great economic importance; as this varies so much, and depends on such a number of different conditions, such as the season, the condition of the land, and prevailing weather. A point of very much greater economic importance is the total amount formed in the year, and the rate at which nitrification takes place. These questions will be discussed elsewhere, and therefore need not here be referred to. Some interesting analyses made at Rothamsted, however, of the amount of nitrates in soils at different depths, merit careful consideration.
Nitrates in Fallow Soils.
In the Appendix to the chapter on Nitrification,[76] will be found a table containing the amounts of nitrates found in the first 27 inches of fallow soils. The amounts vary from 33.7 lb. to 59.9 lb. per acre. The analyses were made in September or October. In four out of the six analyses, it will be found that by far the largest proportion is found in the first 9 inches. In these cases the preceding summer had been dry, and thus the nitrates had not been washed down to any depth. In the other two cases the largest amount is found in the second 9 inches of soil, and a considerable amount is also found in the third 9 inches.
Nitrates in Cropped Soils.
In the case of cropped soils we find the amount of nitrates very much less. A table containing an elaborate series of determinations of nitrates in cropped soils, receiving, however, no manure, and taken to a depth of 9 feet, will be found in the Appendix.[77] The first 27 inches only contain some 5 to 14 lb. per acre, and the most of that is found in the first 9 inches. This shows how speedily nitrates are assimilated by the growing crop. An interesting point shown by these analyses is that nitrates almost entirely cease in cropped soils a certain depth down, but that at a still lower depth they again occur in small quantities.
Nitrates in manured Wheat-soils.
Lastly, we give in the Appendix[78] the amount of nitrates found in wheat and barley soils, differently manured, at Rothamsted. From a perusal of these tables, it will be seen that the amount (under various conditions of manuring) of nitrates in the first 27 inches varies from 21.2 lb. per acre to 52.2 lb. for the wheat-soils, and 20.1 to 44.1 lb. per acre for the barley-soils.
The Sources of Soil-nitrogen.
We shall now consider the sources of soil-nitrogen, the conditions which determine its increase, and the amount of that increase, as well as the sources of loss, and the conditions which determine this loss.
That dissolved in Rain.
The natural sources of the soil-nitrogen are several. We have first of all the atmospheric nitrogen. Of this let us first consider that present as combined nitrogen. This, as we have already seen, consists chiefly of nitrates, nitrites, and ammonia, and reaches the soil dissolved in rain or in other meteoric forms of water, such as snow, hail, fog, hoar-frost, &c.
That absorbed by the Soil from the Air.
It is also absorbed by the soil from the air, especially when the soil is in a damp condition, as has been proved by Schloesing's experiments, already referred to. The total amount which falls dissolved in the rain, per acre per annum, varies very considerably in different parts of the world, but in any case only amounts yearly to a few pounds per acre.[79] That absorbed by the soil from the air may be probably very much more considerable. Schloesing in his experiments found that this latter might amount to 38 lb. per acre per annum. These results, however, were obtained under circumstances most favourable for absorption—viz., with a damp soil and in the vicinity of Paris, where the air is presumably richer in combined nitrogen than it is in the country. The nitrogen absorbed, it may be mentioned, was almost entirely in the form of ammonia. It is to be noted that the nitrogen the soil obtains in this way from the combined nitrogen of the air is not all pure gain. With regard to the nitrates and nitrites, no doubt most of these are formed by electrical discharge, although a small portion of them may be formed by the oxidation of ammonia by means of ozone and peroxide of hydrogen. With regard to the ammonia and the combined nitrogen present in the organic particles in the air, a not inconsiderable proportion is probably derived from the soil. Schloesing considers the chief source of the ammonia present in the air to be the tropical ocean; but we must remember that the source of much of the nitrogen in the tropical ocean is, after all, the soil.
Leaving aside for a moment the question of the availability of the free nitrogen of the air, let us consider the other sources of soil-nitrogen.
Accumulation of Soil-nitrogen under Natural Conditions.
The chief source is of course the remains of vegetable and animal tissue.[80] Plants are the great conservers of soil-nitrogen. By assimilating such available forms of it as nitrates, and converting them into organic nitrogen, they prevent the loss of this most valuable of all soil constituents that would otherwise take place.
They also serve to collect the nitrogen from the lower soil-layers and concentrate it in the surface portion. In a state of nature, where the soil is constantly covered with vegetation, the process going on, therefore, will be one of steady accumulation of nitrogen in the surface-soil. To what extent this accumulation goes on, and how far it is limited by the conditions of loss, will be considered immediately. That it may go on to a very great extent is amply proved by the existence of the so-called virgin soils of countries like America and Australia. There are cases, also, where the accumulation of nitrogen is practically unlimited, although the result in such cases is not necessarily a fertile soil. Such cases are peat-bogs. But let us pass on to the accumulation of soil-nitrogen under the ordinary conditions of husbandry.
Accumulation of Nitrogen in Pastures.
The case which, under the conditions of ordinary farming, most resembles a state of nature, is that of permanent pasture. It will be best, therefore, to study first the conditions under which gain of nitrogen takes place in this case.
Increase of Nitrogen in the soil of Pasture-land.
That there is a steady increase of nitrogen in the soil of land under pasture is a fact of universal experience. The older a pasture is the richer is its soil in nitrogen. The comparison of the analyses of the soil of arable land with the soil of pastures of different ages shows this in a striking way.[81] Thus at Rothamsted it was found that while the amount of nitrogen in an ordinary arable soil was .140 per cent, that in pastures eight, eighteen, twenty-one, and thirty years old was respectively .151, .174, .204, and .241 per cent. In the last two analyses we have a record of the actual gain in nitrogen made by the same pasture, this being .04 per cent in nine years' time. From these statistics it may be inferred that the surface-soil of a pasture may increase at the rate of 50 lb. per acre per annum. A point of great interest in connection with this subject is the fact that there seems to be a limit to the accumulation of nitrogen in pastures; for it would seem that pastures centuries old are not any richer in nitrogen than those thirty to forty years old.
Gain of Nitrogen with Leguminous Crops.
Another case where the gain of nitrogen to the surface-soil is very striking is in that of leguminous crops, such as clover, beans, peas, &c. This fact has been long recognised—especially with regard to clover—by farmers, and has been largely instrumental in leading to the investigation of the "free" nitrogen question. That a soil bearing a leguminous crop increases in nitrogen at a very striking rate is a problem that requires to be solved. A partial explanation of the phenomenon is found in the extraordinary capacity such a crop as clover has, by means of its multitudinous and ramifying roots, for collecting nitrogen from the subsoil. This, however, would only account for the increase in nitrogen to a certain extent. There must be some other source, and the only other source is the air. That the free nitrogen of the air is, after all, available for the plant's needs, is a supposition which has long seemed extremely probable, and which, within the last few years, has been proved beyond doubt to be a fact in the case of leguminous plants.
The Fixation of "Free" Nitrogen.
The method in which these plants are able to make use of the free nitrogen is still a point requiring much research. So far as the question is at present investigated, it would seem that the fixation is effected by means of micro-organisms present in tubercles or root excrescences found on the roots of leguminous plants.[82] Not merely has this been placed beyond doubt, but attempts have been made to isolate and study the bacteria effecting this fixation. From Nobbe's exceedingly interesting experiments, recently carried out, it would seem that the different kinds of leguminous plants have different bacteria. Thus the bacteria in the tubercle on the pea seems to be of a different order from the bacteria in the tubercles of the lupin, and so on. This discovery is of great importance, it need scarcely be pointed out, as it throws much light on the principles of the rotation of crops.
Influence of Manures in increasing Soil-nitrogen.
It may be doubted, however, if under any other conditions there is a positive gain of soil-nitrogen. In other cases the amount in the soil is only maintained under liberal manuring. In connection with this point a very striking fact has been observed with regard to the effect of continuous large applications of farmyard manure. It has been found at Rothamsted that in such a case, after a while, the manure does not seem to increase the soil-nitrogen, although where the nitrogen goes to remains a mystery. In the case of the application of artificial manures, there does not seem to be almost any appreciable gain to the soil-nitrogen. The soil-nitrogen is only increased by means of the residue of crops. In this way, of course, by increasing the amount of this crop-residue, artificial manures may be said indirectly to increase the soil-nitrogen.[83]
Sources of Loss of Nitrogen.
We now come to consider the sources of loss. The chief source, of course, is that by drainage. Land under cultivation will suffer very much more from this source of loss than in a state of nature. Our modern system of husbandry, involving as it does thorough drainage, can scarcely fail to very considerably increase this source of loss.
Loss of Nitrates by Drainage.
The form in which nitrogen is lost in this way is as nitrates. It is a somewhat striking fact, and one worthy of note, that of the three important manurial ingredients—nitrogen, phosphoric acid, and potash, the first of these, in its final and most valuable form, is alone incapable of being fixed by the soil, and thus retained from loss by drainage.
As nitrates are constantly being formed in the soil, the loss to its total nitrogen must be considerable. It is due to the fact of the great solubility of nitrates, as well as to the fact, as already mentioned, of the incapacity of the soil-particles to fix them. To this one exception must be made. According to Knop, small quantities of nitric acid are held in the insoluble condition in soils in the form of highly basic nitrates of iron and alumina. The quantity, however, of these insoluble compounds probably amounts to a very minute trace indeed.
Permanent Pasture and "Catch-cropping" prevents Loss.
The amount of loss varies, and will depend on a number of different circumstances—thus the nature of the soil, climate, and season of the year will all influence its quantity. The way in which the soil is cultivated is also another important factor. Where it is constantly covered with vegetation, as in the case of permanent pasture, the loss will be at a minimum. Under such conditions, plant-roots are always there ready to fix, in the insoluble organic form, the soluble nitrates as they are formed. A consideration of this fact forms one of the strongest arguments in favour of the practice of what is known as "catch-cropping." The practice consists in sowing some quickly-growing green crop—e.g., mustard, vetches, &c.—so as to occupy the soil immediately after harvest, and subsequently to plough it in. The nitrates, which it is known are most abundantly formed towards the end of summer,[84] and which are allowed to accumulate in the soil from the period at which the active growth of, and consequently assimilation of nitrates by, the cereal crop have ceased, are thus fixed in the organic matter of the plant, and removed from danger of loss by drainage incidental to autumn rains.
Other Conditions diminishing Loss of Nitrates.
The nature of the soil is another important condition regulating this loss. Some soils are very much opener and more porous than others; in such soils, of course, the loss by drainage will be greatest. We are apt at first sight, however, knowing the great solubility of nitrates, to overrate this source of loss. We have to remember that while nitrates are constantly being washed down to the lower layers of the soil, there is likewise an upward compensating movement of the soil-water constantly taking place. This is due to the evaporation of water from the surface of the soil, which induces an upward capillary movement of water from its lower to its higher layers.[85] This upward movement of water is very much increased, in the case of soil covered with vegetation, by the transpiration of the plants. The climate and the season of the year will affect the extent of this upward movement. Where there is a heavy rainfall it will be very much less than in dry climates. After a long period of drought the nitrates will be found to be concentrated in the top few inches of the soil; and in hot climates this sometimes takes place to such an extent that the surface of the soil has been actually covered with a saline crust, caused by the rapid evaporation of soil-water under the influence of a burning tropical sun. From this point of view it will be seen how very much less powerful a single shower of rain is—even although at the time it is heavy—in causing loss of nitrates by drainage, than a continuance of wet weather. In the former case, where the showers are separated by an interval of dry weather, the nitrates washed down into the lower layers of the soil are slowly brought up again by the capillary action caused by evaporation.
Amount of Loss by Drainage.
What the actual amount of loss is which takes place in this way it is wellnigh impossible to say. What it amounts to under certain definite circumstances has been discovered by actual experiment at Rothamsted. Taking the circumstances most favourable to extreme loss—viz., unmanured fallow land—the highest amount registered at Rothamsted for a year is 54.2 lb. per acre from soil 20 inches deep, while the smallest amount is 20.9 lb. In the former case, the drainage-water was equivalent to 21.66 inches, while in the latter, to 8.96 inches. The average for thirteen years on unmanured fallow soil has been 37.3 lb. (for 20 inches), 32.6 lb. (for 40 inches), 35.6 lb. (for 60 inches). The point of especial interest in this connection is that an annual loss of nitrogen, equal to over 2 cwt. of nitrate of soda, may take place from a comparatively poor arable soil lying fallow.
The loss on cropped soils is of course very much less—in short, should amount to very little—especially in permanent pasture, where it is reduced to a minimum. Taking an average, Mr Warington is of opinion that the loss in England may be put at 8 lb. per annum per acre.[86]
Loss in Form of Free Nitrogen.
The other chief natural source of loss of nitrogen is due to its escape from the soil in its "free" state. This source of loss is very much less important than that by drainage, and probably amounts to very little. That, however, it takes place is beyond a doubt; and that it may—as we shall see by-and-by—under certain circumstances amount to something very considerable is also proved. Where large quantities of nitrogenous organic matter decay, and where, consequently, the supply of atmospheric oxygen is insufficient to effect complete oxidation, "free" nitrogen may be evolved in considerable quantities. Similarly, it may be evolved in the case of vegetable matter decaying under water. In soils rich in organic matter the reduction of even nitrates may take place, accompanied with the evolution of free nitrogen, which is thus lost.
Total Amount of Loss of Nitrogen.
What the rate of total loss of nitrogen is from these different sources does not admit of easy calculation. Sir John Lawes, in dealing with the question of soil-fertility, estimated some years ago, by comparing the soil of old pasture at Rothamsted with that which had been under arable culture for 250 years, that during that period some 3000 lb. of nitrogen per acre had disappeared from the arable land. Examples of decrease of nitrogen in Rothamsted soils, under various conditions of culture, will be found in the Appendix.[87]
Loss of Nitrogen by Retrogression.
A source of loss of nitrogen may be here mentioned which has to do with diminution of amount of available nitrogen, rather than absolute loss of nitrogen to the soil, and which we may term loss by retrogression. Nitrogen in an available form, such as nitrates, has been found to be converted into a less available form. This retrogression may be effected, as in the case of nitrates, by reduction—i.e., by removal of the oxygen in combination with the nitrogen, which in many cases may be set free, and thus partially although not necessarily entirely lost. Such reduction is due to the action of bacteria of the denitrifying order.[88] Or, on the other hand, nitrogen may be converted into some kind of insoluble form which seems to resist decomposition and lies in an inert condition in the soil utterly unavailable for the plants' needs. A striking example of this retrogression of nitrogen seems to be afforded in the case of farmyard manure. It has been found in the Rothamsted experiments, as has been pointed out in the preceding pages, that when farmyard manure is applied, year after year, to the same land in large quantities, a very considerable percentage of its nitrogen does not (i.e., within a reasonable number of years) become available for the crop's uses. What, indeed, becomes of the nitrogen is a mystery; but it is highly probable that some such kind of retrogression as that above referred to, whereby the nitrogen is converted into some inert organic form, takes place.
Artificial Sources of Loss of Nitrogen.
So far, the sources of loss of nitrogen considered have been what we may term natural sources. By this is meant that the loss of nitrogen from the above sources takes place in a state of nature, and not merely under conditions of cultivation. No doubt the loss due to drainage is very much greater under arable farming than would be the case where artificial drainage does not obtain; still, under any conditions, this loss must be reckoned with. On the other hand, by artificial sources of loss are meant those entirely dependent on our modern system of agriculture and our modern system of sewage disposal, whereby the nitrogen contained in that portion of the produce of the farm which goes to supply our food is not returned to the soil, but is totally lost.
Amount of Nitrogen removed in Crops.
The modern tendency towards centralisation in large towns has rendered this loss—despite all that has been said to the contrary—a necessity. It is extremely difficult, however, to form any estimate of its amount. We know, of course, the amount of nitrogen removed from the soil by different crops. We cannot, however, estimate how much of this may find its way back again to the soil. The amount of nitrogen contained in the different crops will be fully dealt with in the chapter on the manuring of different crops. It may be, however, not without interest to give here some approximate indication of the amount of this loss, in order to render the view of the subject as comprehensive as possible.
Recent agricultural returns for Great Britain give the total produce of wheat at over 76 million bushels, that of barley at over 69 million, and that of oats at over 150 million. Calculating the amount of nitrogen, these quantities of wheat, barley, and oats respectively and collectively contain, and calculating also how much sulphate of ammonia and nitrate of soda these amounts of nitrogen represent, the following are the results:—
| Nitrogen. | Sulphate of Ammonia. | Nitrate of Soda. | ||
| Bushels. | Tons. | Tons. | Tons. | |
| Wheat | 76,224,940 | 37,432 | 176,465 | 227,266 |
| Barley | 69,948,266 | 27,324 | 128,813 | 165,896 |
| Oats | 150,789,416 | 56,835 | 267,936 | 345,068 |
| Total | 296,962,622 | 121,591 | 573,214 | 738,230 |
Of course these figures, so far as the amounts of nitrogen are concerned, can only be regarded as approximate, as it is only possible in such calculations to obtain approximate results. Accepting these calculations as merely approximate, they are, nevertheless, of the highest interest and importance. It is of great importance to understand that in the annual produce of our three common cereal crops—supposing them to be all consumed off the farm—there is removed from the soil a quantity of nitrogen equal to that contained in over half a million tons of sulphate of ammonia, and three quarters of a million tons of nitrate of soda.
As has already been remarked, it is impossible to estimate exactly what proportion of this total nitrogen finds its way back to the soil. In the case of wheat, it may be pointed out that the portion which is used as a feeding-stuff—viz., bran—is very much richer in nitrogen than the flour. While, then, we are unable to estimate with any exactitude this source of loss of nitrogen, it cannot for a moment be doubted that it is enormous, from what has been already stated. We must remember that the portion of the crop richest in nitrogen is that which is generally removed—the straw which is grown in producing a bushel of wheat, barley, or oats, containing less than half the amount of nitrogen contained by a bushel of the grain itself.
Losses of Nitrogen incurred on the Farm.
In addition to the loss due to removal of crops from the farm, there are one or two other sources of loss which it may be well to briefly refer to.
Loss in Treatment of Farmyard Manure.
There can be little doubt that in the past a very considerable source of loss was the improper treatment of farmyard manure. The way in which this loss may take place will be fully considered in the chapter on farmyard manure. Suffice it to say here, that this may take place by volatilisation of the nitrogen as carbonate of ammonia, caused by carelessness in allowing the temperature of the manure-heap to rise too high; or by drainage of the soluble nitrogen compounds, caused by allowing the rich black liquor of the manure-heap to be washed away, and not properly conserved.
Nitrogen removed in Milk.
Another source of loss which is apt to be overlooked is the amount of nitrogen removed in milk. Professor Storer has calculated that in the case of a cow giving 2000 quarts, or 4300 lb., of milk in a year, and the milk being all sold as such, there would be carried away from the farm 22 lb. of nitrogen.[89]
Economics of the Nitrogen question.
And here, before concluding our survey of the different sources of loss of nitrogen, it may be well to regard for a moment the subject from a somewhat wider standpoint than that from which we have been considering it. The total supply of nitrogen in a combined form is limited. As we have pointed out, it may be regarded as the element on which, more than any other, life, animal as well as vegetable, depends. To animal life it is alone available in combined form; to vegetable life it is chiefly also only available in combined form. In the air we have an unlimited quantity of nitrogen, but it is almost entirely in an uncombined form, and therefore largely unavailable. The conversion of nitrogen from the free state to a combined form is a process which takes place only very slowly. Any source which diminishes the sum-total of our already all too limited supply of combined nitrogen must be regarded as worthy of most serious consideration. The question, therefore, of the artificial waste of nitrogen daily taking place around us, is one which ought to possess for economists a very great interest indeed. This waste has, of late years, enormously increased, and would seem to threaten us at no very distant date with a nitrogen famine. It is incidental to the use of certain nitrogenous substances in the manufacture of various articles, and to our present system of sewage disposal.
Loss of Nitrogen-compounds in the Arts.
The articles referred to are such as explosives, starch, textile substances, malt liquors, &c. The question is strikingly dealt with in an able paper on "The Economy of Nitrogen" in the 'Quarterly Journal of Science.'[90]
Loss due to Use of Gunpowder.
The explosives—more particularly gunpowder—are the most important of these articles. Gunpowder contains 75 per cent of saltpetre, which in its turn contains about 10 per cent of nitrogen. When gunpowder explodes, practically the whole of this nitrogen is converted into "free" nitrogen. The loss is thus in a sense irreparable. In the paper above, referred to, our total annual exports of this substance are estimated at 19,000,000 lb.; while the total annual production of the world is estimated at not less than 100,000,000 lb. The annual loss of nitrogen due to this source alone would amount to about 10,000,000 lb.[91] Similarly, loss of nitrogen, although to a less extent, is caused by the use of other explosives, as well as in the manufacture of the other articles above mentioned.
Loss due to Sewage Disposal.
The loss due to our present system of sewage disposal has been already taken into account in dealing with the loss due to removal of crops. It may be well, however, to treat it from the sewage aspect. Taking the amount of nitrogen in the excreta of every individual as, on an average, half an ounce, the annual amount voided in the excreta of the total population of the British Isles would amount to 365,000,000 lb.[92]—of this, the amount in the London sewage alone being 91,000,000 lb.[93] By the water system, which is almost universally adopted in this country, the above quantity of nitrogen is entirely lost to the soil. A small portion of it, it may be argued, is eventually recovered in sea weed and fish, which may be used for manure. This, however, is to argue too much sub specie æternitatis. Not all the nitrogen originally present in the excreta finds its way into the sea; for it is highly probable that a considerable quantity escapes in the process of the decomposition of the sewage as "free" nitrogen.
From the above statement of the sources of loss and gain of nitrogen taking place in the soil, it may be pretty safe to conclude that while in a state of nature the gain balances the loss, if indeed it does not do more, under conditions of arable farming such is very far from being the case; and that if fertility of the land is to be maintained, recourse to nitrogenous manures must be had,—in short, that the application of artificial nitrogenous manures is a necessary condition of modern husbandry.
Our Artificial Nitrogen Supply.
Before concluding this chapter, it may be interesting to enumerate very briefly the chief sources of our artificial nitrogen supply.
Nitrate of Soda and Sulphate of Ammonia.
The most important artificial nitrogenous manures in use at present are nitrate of soda and sulphate of ammonia. Of the former, the annual exportation from Chili is close on one million tons, of which quantity about 120,000 tons is imported into the United Kingdom. Of sulphate of ammonia, on the other hand, the total production in this country is about 130,000 tons per annum,[94] the greater proportion of which is exported, leaving only from 30,000 to 40,000 tons for consumption. Nitrate of soda, it must be remembered, is not entirely used for manurial purposes, a small proportion of the above imports being used for chemical manufacturing purposes.
Peruvian Guano.
Peruvian guano is another important nitrogenous manure very much less abundant now than formerly, as the different guano-beds have become nearly exhausted. While the imports of this important manure into the United Kingdom amounted in 1870 to nearly 250,000 tons, at present not more than 11,000 tons are being imported.
Bones.
A further source of nitrogen is bones, which, of course, are chiefly valuable as a phosphatic manure, but which contain also some 3 to 4 per cent of nitrogen. Of this valuable manure we import at present about 30,000 tons, while about 60,000 tons are collected in this country, bringing up our total consumption to 100,000 tons.
The above mentioned are the most important of nitrogenous manures; there are, however, a number of other nitrogenous manures used in this country in very much smaller quantities. As most of these substances are made in this country, it is very difficult to estimate the amount of their annual production with exactness. These substances are as follows: fish-guano, meat-meal guano, dried blood, shoddy, scutch, horns and hoofs, hair, bristles, feathers, leather-scrap, &c. Of fish-guano, the total consumption per annum may be put down at about 8000 tons, of which a fourth is imported into this country, the remaining 6000 tons being manufactured at home. Of meat-meal guano, dried blood, hoof-guano, &c., about 2500 tons are annually imported, the home production bringing up the total amount to some 10,000 tons. Of shoddy, some 12,000 tons are manufactured in this country; while scutch—the name given to a manure manufactured from the waste products incidental to the manufacture of glue and the dressing of skins—is produced only to the extent of a few thousand tons annually.
It is a fact worthy of notice, that while the use of phosphatic manures has increased very considerably of late years, the same cannot be said of nitrogen. According to Mr Hermann Voss, some 34,000[95] tons of nitrogen were used in the form of artificial manures in 1873, while now only about 28,000 tons are used—i.e., some 6000 tons less.
Oil-seeds and Oilcakes.
There still remains a very important source of nitrogen which has not yet been mentioned, in the shape of oil-seeds and oilcakes, used for feeding purposes. Oilcakes are both manufactured in this country and imported in large quantities. Recent Agricultural Returns show the total imports of oilcakes at 256,296 tons; that of linseed at 370,000 tons; that of rape-seed at 80,000 tons; and that of cotton-seed at 289,413 tons.
Other imported Sources of Nitrogen.
We have further, in considering this question, to take into account the large amount of maize, peas, beans, wheat, and oats which are imported into this country, a certain quantity of which is used as cattle-food, and will therefore go to enrich their manure. Also the imported straw used for purposes of litter must not be forgotten. In 1887 this amounted to 52,393 tons.
Conclusion.
In conclusion, it may be asked how far are the artificial sources of nitrogen able to make good the loss? In the opinion of such a reliable authority as Sir John Lawes, they do not. There are some soils which depend almost entirely upon imported fertility, and could not be cultivated without it. Upon some of them it is possible that the imports of nitrogen are in excess of the exports. Taking the agricultural acreage as a whole, however, he is of opinion that there is a decided loss of nitrogen, which he estimates at from 15 lb. to 20 lb. per acre per annum.[96]