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 Nitrate of Ammonia. 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 aeternitatis. 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.

Other Nitrogenous Manures.

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]

FOOTNOTES:

[63] The total amount of nitrogen in the air has been estimated approximately at four million billion tons.

[64] See Introductory Chapter, pp. 40 to 45.

[65] Although ammonia is more abundant than nitrates and nitrites, it only amounts to a few parts per million of air. According to Muentz, the air at great heights contains more ammonia than in its lower strata. The opposite, however, is the case with regard to nitrates, which are only found in air near the surface of the earth. See p. 49.

[66] Nitric acid may also be formed by the oxidation of ammonia by ozone, or peroxide of hydrogen.

[67] According to Schloesing, the chief source of the ammonia present in the air is the tropical ocean, which yields gradually to the atmosphere, under the action of the powerful evaporation constantly going on, a large amount of nitrogen in this form. The sources of the nitrogen of the ocean are the nitrates which it receives from the drainage of land, animal and vegetable matter, sewage, &c.

[68] See Appendix, Note I., p. 155.

[69] To illustrate this point, it may be mentioned that on the least windy of days, when the wind is only moving at the rate of two miles an hour—and this, it may he added, is so slow as to be scarcely noticeable—the air in a space of 20 feet is changed over five hundred times in an hour. The combined nitrogen thus absorbed is probably entirely in the form of ammonia. It would seem so at any rate, from some experiments by Schloesing. See p. 132.

[70] No vegetable or animal cell exists which does not contain nitrogen.

[71] This is less on the whole than what has been found in subsoils by Continental investigators. Thus, for example, A. Mueller found the average of a number of analyses of subsoils to be .15 per cent., and the late Dr Anderson found the nitrogen in the subsoil of different Scottish wheat-soils to run from .15 per cent to .97 per cent.

[72] See Appendix, Note II., p. 156.

[73] "Under prolonged kitchen-garden culture the subsoil becomes enriched with nitrogenous matter to a far more considerable depth; this has been shown by the analyses of the soil of the old kitchen-garden at Rothamsted. This is doubtless due to the practice of deep trenching employed by gardeners."—R. Warington, 'Lectures on Rothamsted Experiments.' U.S.A. Bulletin, p. 24.

[74] The comparatively insignificant effect the addition of various nitrogenous manures have in increasing the total soil-nitrogen is strikingly illustrated in the tables given in the Appendix, Note IV., p. 157.

[75] See Storer's Agric. Chem., vol. i. p. 357.

[76] See Chapter IV., Appendix, Note VII., p. 198.

[77] See Appendix, Note III., p. 157.

[78] See Appendix, Note IV., p. 157.

[79] See Appendix, Note I., p. 155.

[80] The original source of the nitrogen in the soil must have been the nitrogen in the air. When plants first begin to grow on a purely mineral soil, they must obtain nitrogen from some source. The small traces washed down in the rain will supply sufficient nitrogen to enable a scanty growth of the lower forms of vegetable life; whereas these by their decay furnish their successors with a more abundant source, which rapidly increases, until we have a fair percentage of humus accumulated.

[81] See Appendix, Note V., p. 158.

[82] See Historical Introduction, pp. 40-45.

[83] The evidence demonstrating this is to be found in the fact that the amount of carbon found in different soils rises or falls in proportion to the nitrogen. See p. 126.

[84] See Chapter IV. on Nitrification.

[85] Diffusion as well as capillary attraction is a means of bringing nitrates again to the surface-soil after rain.

[86] See Appendix, Note VI., p. 158, and Note VIII., p. 160; also p. 154.

[87] See Appendix, Note VII., p. 159.

[88] See following Chapter on Nitrification, p. 178.

[89] According to the Agricultural Returns for 1888, the number of cows in milk in Great Britain amounted to 2,450,444. If we multiply this number by 22 the result is 54,000,000 lb., or in tons 24,107. This quantity represents 154,067 tons of ordinary commercial nitrate of soda.

[90] For 1878 (p. 146 et seq.) The reader interested in the subject is referred to the paper itself.

[91] In tons 4464, and represents 28,530 tons of nitrate of soda.

[92] This in tons 162,946, which represents 1,041,384 tons of nitrate of soda.

[93] This in tons 40,625, which represents 259,633 tons of nitrate of soda. See paper in 'Journal of Science' already referred to.

[94] Europe's total production may be stated at 200,000 tons.

[95] 10,500 tons of which were as guano.

[96] Mr Warington estimates this at about 8 lb. See p. 141.



APPENDIX TO CHAPTER III.

NOTE I. (p. 119).

DETERMINATIONS OF THE QUANTITY OF NITROGEN SUPPLIED BY RAIN, AS AMMONIA AND NITRIC ACID, TO AN ACRE OF LAND, DURING ONE YEAR.

(From Dr Fream's 'Soils and their Properties,' p. 62.)

+ + -+ + Nitrogen per million, as Total + + -+Nitrogen Year. Rainfall. per Ammonia. Nitric acre. Acid. + + -+ + -+ lb. Kuschen 1864-65 11.85 0.54 0.16 1.86 " 1865-66 17.70 0.44 0.16 2.50 Insterburg 1864-65 27.55 0.55 0.30 5.49 " 1865-66 23.79 0.76 0.49 6.81 Dahme 1865 17.09 1.42 0.30 6.66 Regenwalde 1864-65 23.48 2.03 0.80 15.09 " 1865-66 19.31 1.88 0.48 10.38 " 1866-67 25.37 2.28 0.56 16.44 Ida-Marienhuette, mean of six years 1865-70 22.65 9.92 Proskau 1864-65 17.81 3.21 1.73 20.91 Florence 1870 36.55 1.17 0.44 13.36 " 1871 42.48 0.81 0.22 9.89 " 1872 50.82 0.82 0.26 12.51 Vallombrosa 1872 79.83 0.42 0.15 10.38 Montsouris, Paris 1877-78 23.62 1.91 0.24 11.54 " 1878-79 25.79 1.20 0.70 11.16 " 1879-80 15.70 1.36 1.60 10.52 + -+ + -+ Mean of 22 years 27.63 10.23 + + -+ + -+

NOTE II. (p. 122).

NITROGEN IN SOILS AT VARIOUS DEPTHS.

(1) Rothamsted Soils.

+ -+ - Depth. Arable soil. Old pasture soil. + -+ -+ + per cent. lb. per acre. per cent. lb. per acre. 1st 9 inches 0.120 3,015 0.245 5,351 2d 9 " 0.068 1,629 0.082 2,313 3d 9 " 0.059 1,461 0.053 1,580 4th 9 " 0.051 1,228 0.046 1,412 5th 9 " 0.045 1,090 0.042 1,301 6th 9 " 0.044 1,131 0.039 1,186 - - - - Total, 54 inches - 9,554 - 13,143 -+ -+ + 7th 9 inches 0.042 1,049 - - 8th 9 " 0.041 1,095 - - 9th 9 " 0.044 1,173 - - 10th 9 " 0.043 1,076 - - 11th 9 " 0.043 1,112 - - 12th 9 " 0.045 1,198 - - - - - Total, 9 feet 16,257 + -+ -+ +

(2) Manitoba Soils.

+ -+ -+ Depth. Brandon. Niverville. Winnipeg. Selkirk. + + -+ -+ per cent. per cent. per cent. per cent. 1st foot 0.187 0.261 0.428 0.618 2d " 0.109 0.169 0.327 0.264 3d " 0.072 0.069 0.158 0.076 4th " 0.019 0.038 0.107 0.042 + + -+ -+

NOTE III. (p. 130).

NITROGEN AS NITRATES IN CROPPED SOILS RECEIVING NO NITROGENOUS MANURE, IN LB. PER ACRE (Rothamsted Soils).

- - - -+ Wheat. + - Bokhara White After After clover, Vetches, Lucern, clover, Depth. fallow, clover, 1882. 1883. 1885. 1885. 1883. 1883. - - - - - lb. lb. lb. lb. lb. lb. 1st 9 inches 3.4 6.1 3.4 10.2 8.9 11.5 2d 9 " 3.1 4.4 1.0 2.7 1.1 1.4 3d 9 " 0.8 1.6 0.6 1.1 0.8 0.9 4th 9 " 1.0 1.3 1.0 1.5 0.8 1.9 5th 9 " 0.8 1.5 0.8 2.5 1.0 7.1 6th 9 " 0.6 0.8 1.7 4.4 0.9 11.3 7th 9 " 0.8 2.2 4.5 0.6 13.1 8th 9 " 0.9 1.7 4.9 0.8 12.6 9th 9 " 0.7 2.4 4.8 0.7 11.2 10th 9 " 2.0 2.1 5.1 0.6 10.7 11th 9 " 1.5 2.1 6.4 0.4 11.1 12th 9 " 3.8 2.8 6.5 0.4 10.0 - - - - -

NOTE IV. (p. 124 and p. 131).

NITROGEN AS NITRATES IN WHEAT-SOILS VARIOUSLY MANURED, OCTOBER 1881, IN LB. PER ACRE (Rothamsted Soils).

- Excess over Plot Manuring. 1st 9 2nd 9 3rd 9 Total 27 plots inches inches inches inches 3 and 4 - lb. lb. lb. lb. lb. 3 No manure, 38 years 9.7 5.3 2.8 17.8 4 " 30 " 9.2 4.0 1.8 15.0 16a " 17 " 10.6 5.0 2.3 17.9 1.5 5a Ash constituents, 30 years 12.6 7.1 4.6 24.3 7.9 17a " " 1 year 10.3 7.5 3.4 21.2 4.8 6a " and ammonium salts, 200 lb. 16.5 7.5 4.7 28.7 12.3 7a " " 400 " 22.8 11.3 5.7 39.8 23.4 8a " " 600 " 21.1 13.9 7.8 42.8 26.4 9a Ash and sodium nitrate, 550 " 19.7 10.0 8.2 37.9 21.5 9b Sodium nitrate, " " 16.3 20.1 17.7 54.1 37.7 10a Ammonium salts, 400 " 14.2 11.9 7.3 33.4 17.0 11a Superphosphate and ammonium salts, 400 lb. 17.9 9.3 3.6 30.8 14.4 19 Rape-cake, 1700 lb. 14.1 13.0 7.1 34.2 17.8 2 Farmyard manure, 14 tons 38 years 30.0 15.4 6.8 52.2 35.8 -

NITROGEN AS NITRATES IN BARLEY-SOILS VARIOUSLY MANURED, MARCH 1892, IN LB. PER ACRE (Rothamsted Soils).

- - - - - Total Excess 1st 9 2d 9 3d 9 27 over Plot. Manuring. inches. inches. inches. inches. plot 10. - - - - - lb. lb. lb. lb. lb. 10 No manure 5.9 4.7 5.1 15.7 - 20-40 Ash constituents (mean) 6.7 7.0 6.4 20.1 4.4 1 A Ammonium salts, 200 lb. 6.1 8.3 7.0 21.4 5.7 2A-4A Ammonium and ash constituents (mean) 7.7 7.8 7.6 23.1 7.4 1 AA Sodium nitrate, 275 lb. 9.7 6.8 9.0 25.5 9.8 2AA-4AA Sodium nitrate and ash constituents (mean) 8.3 7.4 7.5 23.2 7.5 1C Rape-cake, 1000 lb. 10.6 13.7 7.9 32.2 16.5 2C-4C Rape-cake and ash constituents (mean) 8.8 11.9 8.7 29.4 13.7 7-1 No manure, 10 years formerly dung 14.8 11.8 10.9 37.5 21.8 7-2 Farmyard manure, 14 tons 18.6 14.6 10.9 44.1 28.4 - - - - - -

NOTE V. (p. 134).

EXAMPLES OF INCREASE OF NITROGEN IN ROTHAMSTED SOILS LAID DOWN IN PASTURE.

- Age of Nitrogen in pasture. 1st 9 inches. - Years. Per cent. Arable land - 0.140 Barn-field pasture 8 0.151 Apple-tree pasture 18 0.174 Dr Gilbert's meadow 21 0.204 Dr Gilbert's meadow 30 0.241 -

NOTE VI. (p. 141).

In connection with the loss by drainage of nitrogen in the form of nitrates, it may be mentioned that the water of many of the famous rivers contains large quantities of nitrates. Thus the water of the Seine has been found to contain fifteen parts of nitrates per million of water, and the Rhine eight parts per million. Some idea of what this amounts to per annum may be obtained by the statement that "the Rhine discharges daily 220 tons of saltpetre into the ocean, the river Seine 270, and the Nile 1100 tons."—(Storer's Agric. Chem., vol. i. p. 318.)

NOTE VII. (p. 142).

EXAMPLES OF DECREASE OF NITROGEN IN ROTHAMSTED SOILS.

+ Nitrogen in 1st 9 inches. + Per cent. Old pasture 0.250 Arable land in ordinary culture 0.140 Wheat unmanured, 38 years 0.105 Wheat and fallow unmanured, 31 years 0.096 Barley unmanured, 30 years 0.093 Turnips unmanured, 25 years 0.085 +

MANURING, PRODUCE OF WHEAT, AND ALTERATION IN THE COMPOSITION OF THE SOIL IN BROADBALK FIELD, ROTHAMSTED, FROM 1865 TO 1881.

-+ + + Average Nitrogen per acre produce in 1st 9 inches Plot. Manures per acre, per acre. of soil. annually applied, + -+ + + -+ - 16 years, 1865-81. Gain or Dressed Total 1865. 1881. loss in grain. produce. 16 years. -+ + -+ + + -+ - bush. lb. lb. lb. lb. 3 Unmanured 11-7/8 1715 2507 2404 -103 5a Mixed mineral manure 12-3/4 1963 2574 2328 -246 10a Ammonium salts, 400 lb. 17-7/8 2881 2548 2471 -77 111 Ammonium salts, with superphosphate 23-1/4 3856 2693 2676 -17 7a Ammonium salts, with mixed mineral manure 28 4993 2829 2908 +79 9a Nitrate of soda, 550 lb., and mixed mineral manure 36 6949 2834 2883 +49 16a Unmanured* 13-1/2 2194 2907 2557 -350 2 Farmyard manure, 14 tons 31-1/2 5356 4329 4502 +173 -+ + -+ + + -+ - * During 1852-64 received annually ammonium salts, 800 lb., with mixed mineral manure, and yielded an average product of 39-1/2 bushels of grain and 46-5/8 cwt. of straw.

NOTE VIII. (p. 141).

$1$2

+ + -+ Nitrogen as nitrates. + -+ Amount of Per million of Per acre. Rainfall drainage. water. + + + + + + - 20-inch 60-inch 20-inch 60-inch 20-inch 60-inch gauge. gauge. gauge. gauge. gauge. gauge. + + + + + + + - Inches. Inches. Inches. lb. lb. March 1.70 0.85 0.94 7.3 8.9 1.41 1.89 April 2.25 0.72 0.79 8.3 9.0 1.35 1.61 May 2.48 0.80 0.79 8.4 9.1 1.53 1.63 June 2.59 0.78 0.78 9.2 9.1 1.62 1.60 July 2.85 0.68 0.62 13.5 11.8 2.08 1.66 August 2.69 0.84 0.76 15.1 13.3 2.87 2.28 September 2.70 0.97 0.82 17.7 13.4 3.86 2.50 October 3.12 1.86 1.68 13.8 11.9 5.83 4.53 November 3.20 2.44 2.32 11.8 11.4 6.50 5.98 December 2.34 1.88 1.88 9.5 10.6 4.06 4.51 January 2.13 1.79 1.93 7.4 8.9 2.99 3.88 February 2.16 1.84 1.74 7.7 9.1 3.19 3.57 + + + + + + + - March-June 9.02 3.15 3.30 8.3 9.0 5.91 6.73 July-September 8.24 2.49 2.20 15.6 13.0 8.81 6.44 October-Feb. 12.95 9.81 9.55 10.2 10.4 22.57 22.47 + + + + + + + - Whole year 30.21 15.45 15.05 10.7 10.5 37.29 35.64 + + + + + + + -



CHAPTER IV.

NITRIFICATION.

The most important compound of nitrogen for the plant is nitric acid. It is as nitrates that most plants absorb the nitrogen they require to build up their tissue. In nature the nitrogen, present in the soil as ammonia and different organic forms, is constantly being converted into nitric acid. This conversion of nitrogen into nitrates, known as nitrification, is a process of very great importance, and, as has been already pointed out in the Introductory Chapter, is effected through the agency of micro-organisms (ferments).[97] The process of nitrification, as well as the nature of the other changes taking place in the soil between the various compounds of nitrogen, are as yet but most imperfectly understood, but much light has been thrown on this most interesting department of agricultural research during the last few years; and it cannot be doubted that the increased attention which it is receiving from different investigators, both on the Continent and in this country, will be fraught with most important results for practical agriculture.

Occurrence of Nitrates in the Soil.

The occurrence of nitre,[98] or potassium nitrate, in soils has been long known, although it is only within the last few years that we have obtained any precise knowledge with regard to the mode of its production. While its amount in most soils, especially in this country,[99] is very minute, there are certain parts of the world where nitrates are found in large quantities. The nitrate fields of Chili and Peru are the chief natural sources of nitrates, and they are referred to in the chapter on Nitrate of Soda. We have other parts of the world, however (in China and India), where soils rich in nitre occur, and which in the past have formed a source of the commercial article.[100]

Nitre Soils of India.

The most important of these nitre soils are those found in the North-west of India, in the province of Bengal. In these districts the soil is of a light porous texture, rich in lime, and situated at a considerable height above water-level. They are the sites of old villages, and the nitre is found in the form of an efflorescence on the surface of different parts of the soil. The occurrence of nitre under such conditions is due, partly to the natural richness of the soil in nitrogen, and partly to its artificial enrichment through receiving the nitrogenous excrements of the inhabitants of the villages and their cattle. The constant process of evaporation going on in such a warm climate has the effect of inducing an upward tendency of the soil-water, the result being a concentration of all the nitre the soil contains in its surface layer. This goes on until a regular incrustation is formed, and the soil is covered by a white deposit of nitre. Whenever this becomes apparent, the surface portion of the soil is scraped off by the sorawallah, or native manufacturer, and collected and treated for the purpose of recovering, in a pure state, the saltpetre.

Saltpetre Plantations.

The large demand for saltpetre, larger than could be supplied by these nitre soils, soon gave rise to the semi-artificial method of production, formerly so largely practised in Switzerland, France, Germany, Sweden, and in many other parts of the Continent, by means of the so-called "nitre beds," "nitraries," or "saltpetre plantations." Previous to the introduction of this method of manufacture, the demand for saltpetre for gunpowder had become so great, that every source of nitre was eagerly sought for. Thus, when it was discovered that the earth from the floors of byres, stables, and farmyards were particularly rich in nitre, and when mixed with wood-ashes formed an important source of it, the right to remove these in France was vested in the Government under the Saltpetre Laws, which obtained till the French Revolution. This great scarcity soon led, however, to a careful investigation being made into the conditions under which potassium nitrate was formed in nitre soils.[101] These conditions, which included the presence of rich nitrogenous matter, warmth, free aeration of the soil, and a certain proportion of moisture, became, in the course of years, more and more thoroughly understood, and the result was the institution of numerous "saltpetre plantations." These generally consisted of heaps of mould, rich in nitrogen, mixed with decomposing animal matter, rubbish of various kinds, manurial substances, ashes, road-scrapings, and lime salts.[102] The heap was interlaid with brushwood, and was watered from time to time with liquid manure from stables, consisting chiefly of dilute urine. In forming the heap care was taken to keep the mass porous, so as to admit of the free access of air. The heap was further protected from the rain by covering it with a roof. In course of time considerable quantities of nitrates were developed, and the nitre was occasionally collected by scraping it from the surface, where it became concentrated just as in the nitre soils. In all cases, however, the heaps, when considered rich enough in nitre, were treated from time to time with water which, by subsequent evaporation, yielded the nitre in a more or less pure condition.[103]

This mode of obtaining nitre is no longer practised to any extent, since it is now more conveniently obtained from the treatment of nitrate of soda with potassium chloride.

Cause of Nitrification.

We have adverted to these nitre plantations as showing how the conditions most favourable for the development of nitrification were recognised long before anything was known as to the true nature of the process. It was only in 1877 that the formation of nitrates in the soil was proved to be due to the action of micro-organic life,[104] by the two French chemists, Schloesing and Muentz, who discovered the fact when carrying out experiments to see if the presence of humic matter was essential to the purification of sewage by soil. In these experiments sewage was made to filter slowly through a certain depth of soil (the time occupied in this filtration being eight days). It was found that nitrification of the sewage took place. By treating the soil with chloroform[105] it was found that it no longer possessed the power of inducing the nitrification of the sewage. When, however, a small portion of a nitrifying soil was added, the power was regained. From this it was naturally inferred that nitrification was effected by some kind of ferment. This conclusion was soon confirmed by subsequent experiments by Warington at Rothamsted, who showed that the power of nitrification could be communicated to media, which did not nitrify, by simply seeding them with a nitrifying substance, and that light was unfavourable to the process. Since then the question has formed the subject of a number of researches by Mr Warington at Rothamsted, as well as by Schloesing and Muentz, Munro, Deherain, P. F. Frankland, Winogradsky, Gayon and Dupetit, Kellner, Plath, Pichard, Landolt, Leone, and others. From these researches we have obtained the following information with regard to the nature of the organisms concerned in this process, and the conditions most favourable for their development.

Ferments effecting Nitrification.

The importance of isolating and studying them microscopically was recognised at an early period in these researches. Messrs Schloesing and Muentz were the first to attempt this. They reported that they had successfully accomplished this, and described the organism as consisting of very small, round, or slightly elongated corpuscles, occurring either singly or two together. According, however, to the most recent researches of Warington, Winogradsky, and P. F. Frankland, nitrification is not effected by a single micro-organism, but by two, both of which have been successfully isolated and studied.[106] The first of these to be discovered and isolated was the nitrous organism, which effects the conversion of ammonia into nitrous acid; the second, which has only been lately isolated by Warington and Winogradsky, effects the conversion of nitrous acid into nitric acid. Each of these ferments thus has its distinctive function to perform in this most important process, the nitric ferment being unable to act on ammonia, as the nitrous ferment is unable to convert nitrites into nitrates. Both ferments occur in enormous quantities in the soil, and seem to be influenced, so far as is at present known, by the same conditions. Their action will thus proceed together. Nearly all we know as yet on the subject of their nature is with regard to the nitrous ferment.

Appearance of Nitrous Organism.

Mr Warington[107] thus describes the appearance of the nitrous organism: "As found in suspension in a freshly nitrified solution, it consists largely of nearly spherical corpuscles, varying extremely in size. The largest of these corpuscles barely reaches a diameter of 1/1000th of a millimeter; and some are so minute as to be hardly discernible in photographs, although shown there with a surface one million times greater than their own. The larger ones are frequently not strictly circular. These forms are universally present in nitrifying cultures. The larger organisms are sometimes seen in the act of dividing."

Nitric Organism.

So far as at present known, the nitric organism is very similar in appearance to the nitrous organism, so much so that it is difficult to distinguish the one from the other. As the same conditions influence their development, the process may be regarded as a whole.

Difficulty in isolating them.

A great difficulty has been experienced in the attempt to isolate these micro-organisms for the purpose of studying their nature. This arises from the fact that they refuse to grow on the ordinary solid cultivating media used by bacteriologists. Winogradsky, however, has recently succeeded in cultivating them in a purely mineral medium—viz., silica-jelly.[108]

Nitrifying Organisms do not require Organic Matter.

The fact that they can develop in media destitute of organic matter, is one of very great interest and importance to Vegetable Physiology. It implies that they can derive their carbon from carbonic acid—a power which it was believed was possessed by green plants alone among living structures. For organisms destitute of chlorophyll, the source of their protoplasmic carbon, it has been hitherto commonly believed, must be organic matter of some sort. While it would appear that the nitrifying organisms can, when opportunity affords, feed upon organic matter, yet it has been proved beyond doubt that they can also freely develop in media entirely devoid of it, and are capable, under such circumstances, of deriving their carbon from a purely mineral source.[109] This fact, which is subversive of what was believed to be a fundamental law of Vegetable Physiology, is one of the most important of the many important and interesting facts which these nitrification researches have elicited.[110]

CONDITIONS FAVOURABLE FOR NITRIFICATION.

We may now proceed to discuss the conditions favourable for nitrification.

Presence of Food-constituents.

Among these conditions the first is the presence of certain food-constituents. To both animal and vegetable life alike a certain amount of mineral food is absolutely necessary. Among these phosphoric acid is one of the most important, and in the experiments on nitrification it has been found that the nitrifying organisms will not develop in any medium destitute of it. That other mineral food-constituents are necessary is highly probable, although the influence of their absence on the development of the process has not been similarly studied. Probably potash, magnesia, and lime salts are necessary. In the cultivating solutions used in the experiments on the subject, the mineral food-constituents added consisted of lime, magnesia, and potash salts and phosphoric acid.[111]

As we have seen above, the presence of organic matter is not necessary for the process. In this respect these organisms are differentiated from all other ferments hitherto discovered.

Presence of a Salifiable Base.

The presence of a sufficient quantity of a base in the soil with which the nitric acid may combine, when it is formed, is another necessary condition.[112] The process only goes on in a slightly alkaline solution. The substance which acts as this salifiable base is lime. The presence of a sufficient quantity of carbonate of lime in the soil will thus be seen to be of first-rate importance. This furnishes an explanation of one of the many benefits conferred by lime on soils. The activity of nitrification in many soils may be hindered by the absence of a sufficiency of lime salts, and in such cases most striking results may follow the application of moderate dressings of chalk. The absence of the nitrifying organisms in certain soils, such as peaty and forest soils, may be thus accounted for. In such soils humic acids are present and the requisite alkalinity is thus awanting.

Only takes place in slightly Alkaline Solutions.

But while a certain slight amount of alkalinity is necessary, this must not exceed a certain strength, otherwise the process is retarded. This is the reason why strong urine solutions do not nitrify. The amount of carbonate of ammonia generated in them by putrefaction renders the development of nitrification impossible by rendering the alkalinity of the solution too great.[113] The practical importance of this fact is considerable, as it shows the importance of diluting urine very considerably before applying it as a manure. Similarly, when large quantities of lime, especially burnt lime, are applied to soils, the result will be to arrest the action of nitrification for the time. The presence of alkaline carbonates in the soil, unless in minute quantities, is apt, therefore, to seriously interfere with the process.[114]

Action of Gypsum on Nitrification.

It has been found by Pichard that the action of certain mineral sulphates is extremely favourable to the process, and among these gypsum. Warington has carried out some experiments on the action of gypsum in promoting nitrification. The reason of its favourable action is probably because it neutralises the alkalinity of nitrifying solutions. It thus permits the process to go on in unfavourable conditions. Where, therefore, too great alkalinity exists for the maximum development of nitrification, the best specific will be found to be gypsum.[115] The practical value of gypsum as an adjunct to certain manurial substances, where nitrification is desired to be promoted as rapidly as possible, such as sewage and farmyard manure, will thus at once become apparent. So far as there is a proper degree of alkalinity maintained, the presence of large quantities of saline matter does not seem to interfere with the process.

Presence of Oxygen.

The nitrification bacteria belong, it would seem, to the aerobic[116] class of ferment—i.e., they cannot develop without a free supply of oxygen. Exclusion of the air is sufficient to kill them, and in those portions of the soil where access of air is not freely permitted, nitrification will be found to be correspondingly feeble. Thus it has been found in experiments with different portions of soils, that but little signs of nitrification occur in the lower soil layers. According to experiments by Schloesing on a moist soil, in atmospheres respectively containing no oxygen and varying quantities of it, the action of oxygen in promoting nitrification was strikingly demonstrated. In an atmosphere of pure nitrogen, entirely devoid of oxygen, the process no longer took place, but the nitrates already present in the soil were reduced and free nitrogen was evolved. In an atmosphere, on the other hand, containing 1.5 per cent of oxygen, a considerable amount of nitrification took place; while in the presence of 6 per cent, nitrification took place to double the extent. An addition of 10 to 15 per cent again doubled the quantity. When the amount of moisture added was increased, the effect of larger percentages of oxygen was found to be less marked. The reason of this is that the oxygen probably acts as dissolved oxygen; the addition of water meaning at the same time an addition of available oxygen. This condition exemplifies the value of tillage operations. The more thoroughly a soil is tilled the more thoroughly will the aeration of its particles take place, and consequently the more favourable will this necessary condition of nitrification be rendered. The benefits conferred on clayey soils by tillage will in this respect be especially great.

Temperature.

Another of the conditions determining the rate at which nitrification takes place, and one which is most important, is Temperature. According to Schloesing and Muentz the temperature at which maximum development takes place is 37 deg. C.[117] (99 deg. F.), at which temperature it is ten times as active as at 14 deg. C. (57 deg. F.) Below 5 deg. C. (40 deg. F.) the action is extremely feeble. It is clearly appreciable at 12 deg. C. (54 deg. F.), and from there up to 37 deg. C. (99 deg. F.) it rapidly increases. From 37 deg. C. (99 deg. F.) to 55 deg. C. (131 deg. F.), at which temperature no nitrification takes place, its activity decreases; at 45 deg. C. (113 deg. F.) it is less active than at 15 deg. C. (59 deg. F.), and at 50 deg. C. (122 deg. F.) it is very slight. These results by Schloesing and Muentz have not been exactly confirmed by Warington. He has found that a considerable amount of nitrification goes on at a temperature between 3 deg. and 4 deg. C. (37 deg. and 39 deg. F.), while the highest temperature at which he has found it to take place is considerably lower than 55 deg. C. (131 deg. F.) Thus he was unable to start nitrification in a solution maintained at 40 deg. C. (104 deg. F.) It would thus seem that the nitrifying ferments are able to develop at lower temperatures than most organisms; and although nitrification entirely ceases during frost, yet in a climate such as our own there must be a considerable proportion of the winter during which nitrification is moderately active.

Presence of a sufficient quantity of Moisture.

The presence of moisture in a soil is another of the necessary conditions of nitrification. It has been shown that it is at once arrested, and indeed destroyed, by desiccation. Other conditions being equal, and up to a certain extent, the more moisture a soil contains the more rapid is the process. Too much water, however, is unfavourable, as it is apt to exclude the free access of air, which, as we have just shown, is so necessary, as well as to lower the temperature. During a period of drought the rate at which nitrification takes place will, therefore, be apt to be seriously diminished.

Absence of strong Sunlight.

It has been found that the process goes on much more actively in darkness; indeed Warington has found in his experiments that nitrification could be arrested by simply exposing the vessel in which it was going on to the action of sunshine.

Nitrifying Organisms destroyed by Poisons.

It has already been pointed out that nitrification is arrested by the action of antiseptics, such as chloroform, bisulphide of carbon, and carbolic acid. Another substance which has been found to have an injurious action is ferrous sulphate or "copperas," a substance which is apt to be present in badly drained soils, or soils in which there is much actively putrefying organic matter. Maercker has found that in moor soils containing ferrous sulphate, no nitrates, or mere traces of nitrates, could be found. A substance such as gas-lime, unless submitted to the action of the atmosphere for some time, would also have a bad effect in checking nitrification, owing to the poisonous sulphur compounds it contains. Common salt, it would seem, also arrests the process; and this antiseptic property which salt exercises on nitrification throws a certain amount of light on the nature of its action when applied, as it is often done, along with artificial nitrogenous manures.

Denitrification.

In connection with the process of nitrification, it is of interest to notice that a process of an opposite nature may also take place in soils—viz., denitrification—a process which consists in reducing the nitrates to nitrites, nitrous oxide, or free nitrogen. That a reduction of nitrates takes place in the decomposition of sewage with the evolution of free nitrogen, was a fact first observed by the late Dr Angus Smith in 1867; and the reduction of nitrates to nitrites, and nitric and nitrous oxides in putrefactive changes has been subsequently noticed by different experimenters, who have further observed that such reduction takes place in the case of putrefaction going on in the presence of large quantities of water or where there is much organic matter.

Denitrification also effected by Bacteria.

This change was supposed to be of a purely chemical nature, and it has only been recently discovered that it is effected, like nitrification, by means of bacteria. It has been surmised by some that the action of denitrification may be effected by the same organisms that effect nitrification, and that it depends on merely external conditions which process goes on. There is no reason, however, to suppose that this is so, and several of the denitrifying organisms have been identified.

Conditions favourable for Denitrification.

That it is a process that goes on to any extent in properly cultivated soils is not to be supposed. The conditions which favour denitrification are exactly the opposite of those which favour nitrification. It is only when oxygen is excluded, or, which practically means the same thing, when large quantities of organic matter are in active putrefaction, and the supply of oxygen is therefore deficient, that denitrification takes place. Schloesing, as we have already seen, found that in the case of a moist soil, kept in an atmosphere devoid of oxygen, a reduction of its nitrates to free nitrogen took place.

Takes place in water-logged Soils.

The exclusion of oxygen from a soil may be effected by saturating the soil with water; and Warington has found in experiments carried out in an arable soil, by no means rich in organic matter, that complete reduction of nitrates may be effected in this way. It would thus seem that the process of denitrification will take place in water-logged soils, or in the putrefaction of sewage matter in the presence of large quantities of water. Whether this reduction will result in the production of nitrites, nitrous oxide, or free nitrogen, depends on different conditions. This process is one of great importance from an economic point of view, as it reveals to us a source of loss which may take place in the fermentation of manures. In the rotting of our farmyard manure it is possible that the denitrifying organisms may be more active than we have hitherto suspected, and that a considerable loss of nitrogen may in this way be effected.

Distribution of the Nitrifying Organisms in the Soil.

The nitrifying organisms are probably chiefly confined to the soil, and do not usually occur in rain or in the atmosphere. That, however, they are found in spots which we might be inclined to think extremely unlikely, is shown by some recent interesting researches carried out by Muentz, who discovered that the bare surfaces of felspathic, calcareous, schistose, and other rocks at the summit of mountains in the Pyrenees, Alps, and Vosges, yielded large numbers of them, and that they occurred to a considerable depth in the cracks and fissures of the rocks. The nitrifying organisms are also found in river-water, in sewage, and well-waters.

Depth down at which they occur.

In Warington's earlier experiments, the conclusion he arrived at was that the occurrence of the nitrifying organisms was almost entirely limited to the superficial layers of the soil, and that they were seldom to be met with much below a depth of 18 inches. His subsequent experiments, however, considerably modified this conclusion, and showed that nitrification may take place to a depth of at least 6 feet.[118] But although it may take place at this depth, it probably, as a general rule, is limited to the surface-soil, as it is only there the conditions for obtaining circulation of air are sufficiently favourable. A great deal, of course, will depend on the nature of the soil—i.e., as to its texture. In a clayey subsoil the principal hindrance to nitrification will be the difficulty of obtaining sufficient aeration. In clay soils it is probable, therefore, that nearly all the nitrification goes on in the surface layer; in sandy soils it may take place to a greater depth.[119]

Action of Plant-roots in promoting Nitrification.

In this connection the action of plant-roots in permitting a more abundant access of air to the lower layers of the soil, and thus promoting nitrification, is worth noticing. This has been observed in the case of different crops. Thus the action of nitrification has been found to be more marked in the lower layers of a soil on which a leguminous crop was growing than on that on which a gramineous. "The conditions which would favour nitrification in the subsoil are such as would enable air to penetrate it, as artificial drainage, a dry season, the growth of a luxuriant crop causing much evaporation of the water in the soil. Such conditions, by removing the water that fills the pores of the subsoil, will cause the air to penetrate more or less deeply and render nitrification possible. Subsoil nitrification will thus be most active in the drier periods of the year" (Warington).

Nature of Substances capable of Nitrification.

What kinds of nitrogenous substances are capable of undergoing this process of nitrification are not yet well known. The question is, of course, one of great importance, as the rapidity with which a nitrogenous body nitrifies will be an important factor in determining its value as a manure. Unfortunately, on this subject we know, as yet, very little. We are well aware that the nitrogen present in the humic matter of the soil is readily nitrifiable. In the experiments on nitrification the nitrogenous bodies used have been chiefly ammonia salts, so that it is difficult to say whether, in the case of other nitrogenous substances, micro-organic life of a different sort has not also been active and has converted the nitrogen into ammonia, and thereby prepared the way for the process of nitrification.

That various manures, such as bones, horn, wool, and rape-cake are readily nitrifiable, has been shown by experiment. Laboratory experiments have also been carried out on such different nitrogenous substances as ethylamine, thiocyanates, gelatin, urea, asparagin, and albuminoids of milk. But in all these experiments, how far these bodies have been directly acted upon by the nitrifying organisms, or how far they have first undergone a preparatory change in which their nitrogen has been first converted into ammonia, is impossible to say. It is at least quite probable that all the organic forms of nitrogen have first to be converted into ammonia ere they are nitrified.

Rate at which Nitrification takes place.

A question which is practically of no little importance is the rate at which nitrification takes place. From what has been already said as to the nature of the conditions favourable for the process, it will be at once seen that this will depend on how far these conditions are present in the soil. In point of fact the rate at which nitrification takes place will vary very much in different soils. A greater difference, however, in the rate at which it takes place, will be found even in the same soils at different periods of the year. In this country, where the most favourable temperature for its development is seldom reached, it never goes on at the same rate as in tropical climates. One of the causes of the greater fertility of tropical soils is due, doubtless, to the very much longer duration of the period of nitrification, as well as to its greater intensity. As, however, temperature is not the only condition, and the presence of moisture is quite as necessary, it may be that its development is seriously retarded in many tropical climates by the extreme dryness of the soil during long periods.