Lime acts both mechanically and chemically.
Lime may be said to act on the soil both mechanically and chemically. It alters the texture of the soil, and affects its mechanical properties, such as its absorptive, retentive, and capillary powers with regard to water. It acts upon its dormant fertility, and decomposes its mineral substances as well as its organic matter. Lastly, its influence on the micro-organic life of the soil, which plays such an important part in the preparation and elaboration of plant-food, is of the highest importance. We cannot do better, therefore, than discuss its properties under the headings mechanical, chemical, and biological.
I. Mechanical Functions of Lime.
Action on Soil's Texture.
The effect of lime upon the texture of a soil is among its most striking properties. Every farmer knows well what a transformation is effected in the texture of a stiff clay soil by the application of a dressing of lime. The adhesive property of the soil—its objectionable tendency to puddle when mixed with water—is greatly lessened, and the soil is rendered very much more friable when it becomes dry. Several reasons exist for this change. In the first place, the tendency to puddle in a clayey soil is due to the fine state of division of the soil-particles. The way in which lime counteracts this adhesive property is by causing a coagulation of the fine soil-particles. This flocculation or aggregation of the fine clay-particles, when mixed with water by lime, is strikingly demonstrated by adding to some muddy water a little lime-water. The result will be that the water will speedily be rendered clear, the fine clay-particles coming together and sinking to the bottom of the vessel. Even a very small quantity of lime will effect this change. This property possessed by lime, we may mention, is utilised in the treatment of sewage. As it is the fine clay-particles that are the chief cause of the puddling of clay soils, their flocculation does much to destroy this objectionable property. Another reason why lime renders a clay soil more friable when dry is, that lime does not undergo any shrinkage in dry weather. As clay soils shrink very much in drying, the mixture with such a substance as lime tends to minimise this tendency to cake in hard lumps. The effect of even a very small addition of lime to a clay soil, in the way of increasing its friable nature, is very striking, and can be easily illustrated by taking two portions of clay, into one of which a small percentage of lime is introduced, and working both into a plastic mass with water, and then allowing them to dry. It will be found that while the one is hard and resists disintegration, that portion to which the lime has been added crumbles away easily to a powder. This effect which lime has in "lightening" heavy soils has been known to last for years. The disintegrating effect of quicklime when applied to heavy soils is also due, it may be added, to the change undergone by the lime itself from the caustic state to the mild state.
Lime renders light Soils more cohesive.
Although it may seem somewhat paradoxical, lime, it would appear, in some cases exercises an effect upon the soil exactly the reverse of what has just been stated. That lime should act as a binding agent is only natural when we reflect on the way in which it acts when used as mortar. It is quite to be understood, therefore, that its action on light friable soils should be to increase their cohesive powers, and at the same time to increase the capillary power of the soil to absorb water from the lower layers. The extent of this action, of course, would depend on the form in which the lime is applied, and the amount. A striking example of the binding power of lime is to be found in certain soils extremely rich in lime, in which what is known as a lime-pan has been formed at some distance from the surface.
II. Chemical Action of Lime.
But more important probably than even its mechanical action is the chemical action of lime. It is a most important agent in unlocking the inert fertility of the soil. This it does by decomposing different minerals and setting free the potash they contain. The disintegrating power of lime in this respect depends, of course, on its chemical condition, the caustic form being much more potent than the other forms. Its action in decomposing vegetable matter and rendering the inert nitrogen it contains available for the plant's use, is also one of its most important properties, and accounts for its beneficial action when applied to soils, such as peaty soils, rich in organic matter. Again, its use as a corrective for sour lands has long been practically recognised. The presence of acidity in a soil is hurtful to vegetable life. Lime, by neutralising this acidity, removes the sourness of the land, and does much to restore it to a condition suitable for the growth of cultivated crops. The generation of sourness in a soil is almost sure to give rise to certain poisonous compounds. Lime, therefore, in sweetening a soil, prevents the formation of these poisonous compounds. Badly drained and sour meadow-lands, as every farmer knows, are immensely benefited by the application of this useful manure; for not merely is their sourness removed and their general condition ameliorated, but many of the coarser and lower forms of plant-life, which alone flourish on such soils, are killed out, and the more nutritive grasses are allowed to flourish instead. The action of lime in promoting the formation of a class of compounds of great importance in the soil—viz., hydrated silicates—is worthy of notice. According to the commonly accepted theory, much of the available mineral fertilising matter of the soil is retained in the form of these hydrated silicates. Hence lime, by increasing these compounds, not merely adds to the amount of the available fertility in the soil, but also increases its absorptive power for food-constituents.
III. Biological Action of Lime.
The last way in which lime acts is what we have termed biological. By this we mean the important rôle lime plays in promoting or retarding, as the case may be, the various kinds of fermentative action which go on so abundantly in all soils. The presence of carbonate of lime in the soil is a necessary condition for the process of nitrification. Lime is the base with which the nitric acid, when it is formed, combines; and as we have seen, when discussing nitrification, soils of a chalky nature are among those best suited to promote the natural formation of nitrates. This is one of the reasons for the beneficial effects produced by lime when applied to peaty soils. Not merely does it help to decompose the organic matter so abundant in such soils, but it also furnishes the base with which the nitric acid may combine when it is formed. But while the action of lime is to promote fermentation, it must not be forgotten that there may be cases in which its action is rather the reverse of this. Fermentation of organic matter goes on when there is a certain amount of alkalinity present; while, on the other hand, the presence of acidity seems to retard and check it. Too great an amount of alkalinity, however, would, in the first instance, retard fermentation as much as too great acidity. It has been claimed that the addition of caustic lime to fresh urine may act in this way; and if this were so, the addition of lime to farmyard manure might, to a certain extent, be defended. The experiment, however, would be a hazardous one and not to be recommended, as loss of ammonia would most likely ensue.
Action of Lime on Nitrogenous Organic Matter.
The action of lime on nitrogenous organic matter is of a very striking kind, and is by no means very clearly understood. As we have pointed out, it sometimes acts as an antiseptic or preservative; and this antiseptic or preservative action has been explained on the assumption that insoluble albuminates of lime are formed. Its action in such industries as calico-printing, where it has been used along with casein for fixing colouring matter; or in sugar-refining, where it is used for clarifying the sugar by precipitating the albuminous matter in solution in the saccharine liquor; or lastly, in purifying sewage,—has been cited in support of this theory. While, however, there may be circumstances in which lime, especially in its caustic form, acts as an antiseptic, its general tendency is to promote these fermentative changes, such as nitrification, so important to plant-life.
An important use of lime in agriculture is in preventing the action of certain fungoid diseases, such as "rust," "smut," "finger-and-toe," &c., as well as in killing, as every horticulturist and farmer knows, slugs, &c.
Recapitulation.
We may, in conclusion, sum up in a single paragraph the different ways in which lime acts. Its action is mechanical, chemical, and biological. It acts on the texture of the soil, rendering clay soils more friable, and exerting a certain binding effect on loose soils. It decomposes the minerals containing potash and other food-constituents, and renders them available for the plant's needs. It further decomposes organic matter, and promotes the important process of nitrification. It increases the power of a soil to fix such valuable food-constituents as ammonia and potash. It neutralises sourness, and prevents the formation of poisonous compounds in the soil. It increases the capillary condition of the soil, prevents fungoid diseases, and promotes the growth of the more nutritive herbage in pasture-land.
Gypsum.
In the previous chapter mention was made of gypsum as a compound of lime, but no reference to its action as a manure was made. In the past, gypsum was used extensively and highly valued. It was found to be of especial value for clover; and there is a story told of Benjamin Franklin which illustrates the very striking nature of its action on this crop. It is related that he once printed with gypsum the words "This has been plastered" on a field of clover, and that for a long time afterwards the legend was plainly discernible on account of the luxuriance of the clover on the parts of the field which had been thus treated.
Mode in which gypsum acts.
Despite the fact that gypsum is a most ancient manure, it is only of late years that we have come to understand the true nature of its action. For long it was believed that the reason of its striking effect in promoting clover was due to the fact that, as clover was a lime-loving plant, the action of gypsum was owing to the lime it contained. That, however, the action of gypsum is not due to the fact that it supplies lime to the plant, seems evident when it is stated that were this so, any other form of lime would have the same beneficial effect. It is well known, however, that this is not so. Besides, as we have already pointed out, lime is not a constituent which most soils lack, so far as the needs of the crop are concerned. There is a certain amount of truth in the old belief that gypsum enriches the soil in ammonia by fixing it from the air. The power that gypsum has as a fixer of ammonia has already been referred to in the chapter on Farmyard Manure; but in this case the gypsum is brought in contact with the ammonia. The origin of this old belief was due to a misconception as to the amount of ammonia in the atmosphere. No doubt gypsum greatly increases the power of a soil to absorb ammonia from the air; but the quantity of ammonia in the air is so very trifling, that its action in this respect is hardly worth considering. The true explanation of the action of gypsum is to be found in its effect on the double silicates, which it decomposes, the potash being set free. Its action is similar to that of other lime compounds, only more characteristic. As a manure, therefore, its action is indirect, and its true function is to oust the potash from its compounds. Its peculiarly favourable action on clover is due to the fact that clover specially benefits by potash, and that adding gypsum practically amounts to adding potash. Of course it should be borne in mind that the soil must contain potash compounds if gypsum is to have its full effect. Now, however, that potash salts suitable for manuring purposes are abundant, it may well be doubted whether it is not better to apply potash directly. Further, it must be borne in mind that gypsum is applied to the soil whenever it receives a dressing of superphosphate of lime, as gypsum is one of the products formed by treating insoluble phosphate of lime with sulphuric acid.
It is possible that gypsum may act as an oxidising agent in the soil, just as iron in the ferric condition does. It has a large quantity of oxygen in its composition, and under certain conditions may act as a carrier of oxygen to the lower layers of the soil. When it is used, it should be applied some months before the crop is sown.
Gypsum, therefore, although it contains two necessary plant-constituents, lime and sulphuric acid, cannot be regarded as a direct manure; and as its action comes to be more fully understood, its use, which was never very abundant in this country, will probably decrease. We have already, in the chapter on Nitrification, referred to the action of gypsum in promoting nitrification.
The action of salt as a manure presents a problem which is at once of the highest interest and surrounded with the greatest difficulties. In view of the large quantities now used for agricultural purposes, a somewhat detailed examination of the nature of its action is not out of place in a work such as the present.
Antiquity of the Use of Salt.
The recognition of the manurial functions of salt dates back to the very earliest times. Its use among the ancients is testified by numerous allusions in the Old Testament; while, according to Pliny, it was a well-known manure in Italy. The Persians and the Chinese seem also to have used it from time immemorial, the former more especially for date-trees.
Nature of its Action.
Despite, however, the great antiquity of its use, much difference of opinion seems always to have existed as to the exact method of its action, and as to its merits as a manure in promoting vegetable growth. It furnishes, in fact, a good example of the difficulty which exists in the case of many manures, whose action is chiefly indirect, of fully understanding their influence on the soil and on the crop. In fact, the action of salt is probably more complicated than that of any other manurial substance.
Salt not a necessary Plant-food.
We have already seen that neither sodium nor chlorine—the two constituent elements of salt—are in all probability absolutely necessary plant-foods. If they are necessary, the plant only requires them in minute quantities. Despite this fact, soda is an ash-constituent of nearly every plant, and in many cases one of the most abundant. In amount it is one of the most variable of all the ash-constituents, being present in some plants only in minute quantities, while in others it occurs in large quantities. Mangel and plants of the cabbage tribe may be cited as examples of plants containing large amounts of soda in their composition. But the plants which contain it in largest quantity are those which thrive on the sea-coast, and it has been thought that for them at least salt is a necessary manure. This, however, does not seem to be the case. In fact, the amount of soda in a plant seems to be largely a matter of accident. It may be added that the succulent portions of a plant are generally richest in soda.
Can Soda replace Potash?
Again, it has been believed that soda is capable of replacing potash in the plant; but this does not seem to be the case to any extent. The view that soda is able to replace potash, it has been thought, is supported by the variation which exists in the proportion of soda and potash in different plants. It must be remembered, however, that it is highly probable that most plants contain a larger quantity of ash-constituents than is absolutely necessary for their healthy growth. Especially is this the case with such a necessary plant-food as potash, of which there is generally present, in all likelihood, an excess. The variation in the quantity of potash and soda present in many plants under different circumstances can scarcely, therefore, be regarded as furnishing a proof of the replacement of potash by soda. Incidentally we may mention, as a fact worthy of notice, that cultivated plants have more potash and less soda in their composition than wild plants. What has been said of soda may be held to apply equally to chlorine, as it seems to be chiefly in the form of common salt that soda enters the plant. The amount of salt, therefore, present in plants must be regarded as largely accidental and dependent on external circumstances, such as the nature of the soil, &c.
Salt of universal Occurrence.
But even were salt a necessary plant-food, its occurrence in the soil is already of sufficient abundance to obviate any necessity for its application. It may be said to be of almost universal occurrence. Even the air contains it in traces. That this is the case in the neighbourhood of the sea-coast is well known; but even in air far inland, accurate analysis of the air would probably demonstrate its presence in greater quantity than is commonly believed. It is a wise provision that plants absorb salt, for it increases their efficiency as food,—the function of salt as a constituent of animal food being of the very highest importance. It is an indispensable food-ingredient for animal life. With regard to ordinary farm-stock, the amount of salt which naturally occurs in their food is quite sufficient. In the case, however, of pastures in countries far removed from the sea, the custom of specially supplying stock with salt is common. This is done by placing a piece of rock-salt in the fields.
Special Sources of Salt.
The salt of commerce is obtained from various sources. Besides the sea, we have ample sources of salt in the large saline deposits found in many parts of Europe, especially in Austria, and in England in Cheshire.
The Action of Salt indirect.
From what has been said above, it is clear that the action of salt as a manure is indirect and not direct. What the nature of that indirect action is we shall now proceed to discuss.
In considering the evidence of the manurial value of salt, we are at once brought face to face with the fact that the experience of its action in the past has as often been unfavourable as favourable. Salt, it is well known, is both an antiseptic and a germicide. It is, indeed, one of the most commonly used of preservatives. When applied in large quantities to the soil, it has a most deleterious action on vegetation. This hurtful action of salt has long been known; and it is as often mentioned in the writings of antiquity on account of its unfavourable as on account of its favourable action. Thus, for example, among the ancient Jews it was customary, after the conquest of a hostile town, to strew salt on the enemy's fields, for the purpose of rendering them barren and unfertile. And again, among the Romans, for the same purpose, salt was often spread on a spot where some great crime had been committed.
While, therefore, its unfavourable action has long been known, the fact that there are circumstances under which its action is, on the contrary, favourable for promoting vegetable growth has also been long recognised. The difficulty for the agricultural student is to reconcile these two seemingly contradictory experiences. For the English agriculturist the subject possesses especial interest, since in England it has been in the past most generally used and its action most discussed since the time of Lord Bacon, who discusses in his writings the action of solutions of it on different plants.
The true explanation of salt being so different in its action is to be found in the quantity applied, the nature of the soil, the crop to which it is applied, and the conditions under which it is applied—i.e., whether it is applied alone or along with other manures.
In the first place, it must be noted that salt exerts a mechanical action on the soil of a very similar kind to that exercised by lime. When applied to clay soils it causes a flocculation or coagulation of the fine clay-particles, and thus prevents the soil from puddling to the same extent as would otherwise be the case. In fact, an example of this action of salt when in solution causing the precipitation of fine suspended clayey matter, is afforded by the formation of deltas at the mouths of rivers. The power of clarifying muddy water is common indeed to saline solutions. Schloesing attributes the clarifying power of a soil to the presence of the saline matters it contains; and from this point of view it would appear that manures containing any saline substance may exert an important mechanical influence on the soil.
Solvent Action.
But a much more important property of salt is its solvent action on the plant-food present in the soil. Its action in decomposing the minerals containing lime, magnesia, potash, &c., is similar to the action of gypsum. By acting upon the double silicates it liberates these necessary plant-foods. It is not only on the basic substances upon which it acts, but also on the phosphoric and silicic acids, which it sets free. Its power of dissolving ammonia from the soil is considerable. Experiments with a weak solution of salt on a soil by Peters and Eichhorn to test its solvent power, showed that the salt solution dissolved more than twice as much potash and nearly thirty times as much ammonia as an equal quantity of pure water did. When applied to the soil, it seems chiefly to liberate lime and magnesia. The exact nature of the chemical action taking place is a point of some dubiety. According to some, it is changed into nitrate of soda; according to others, into carbonate of soda. The latter theory seems to be the more probable one. Its action on the lime and magnesia compounds is to convert them into chlorides; and this chemical reaction explains the action that salt has in increasing the water-retaining and water-absorbing power of the soil; for the chlorides of magnesia and lime are salts which have a great power of attracting water from the air.
Again, the very fact that salt acts as an antiseptic may serve to explain its beneficial action in certain cases where it prevents rankness of growth. No doubt this was its function when applied along with Peruvian guano. This it might do by preventing too rapid fermentation (nitrification) of the manure, or by actually weakening the plant. Its action when applied with farmyard manure may also be similar. But while its effect in many cases may be towards retarding fermentation, on the other hand its action, when applied along with lime to compost-heaps, is towards promoting more rapid decomposition. Probably a reaction takes place between the lime and the salt, the result of which is the formation of caustic soda.
Such are some of the ways in which salt may act. It must at once be seen how its action in one case will be favourable and in another case unfavourable. There must be fertilising matter present in the soil if it is to act favourably. Again, it will only be under such circumstances, where rankness of growth is likely to ensue, that its antiseptic properties will act favourably and not unfavourably.
Best used in small Quantities along with Manures.
Probably it is for these reasons that its action has been found to be most favourable when applied along with other manures and not alone. Applied along with nitrate of soda, as is commonly done, it doubtless increases the efficiency of the nitrate. Some plants seem to be undoubtedly benefited by salt: of these flax may be mentioned. The application of salt to plants of the cabbage tribe seems also to be highly beneficial. On mangels, along with other manures, it has also been found to have a very favourable effect. But with many crops its action has been proved to be less favourable.
Affects Quality of Crop.
Although salt has often been found to increase the quantity of a crop, the quality of the crop has been made to suffer. Its action on beetroot has been more especially studied. The effect of its application is to lessen the total quantity of dry matter and sugar in the plant. This has been found to be the case both when the salt was applied alone and along with nitrate of soda and other manures. On potatoes, again, its action has been found to be deleterious, lessening their percentage of starch. The deleterious action of chlorides on the quality of potatoes is also seen when potassium chloride is applied. It is for this reason that potash should never be applied to the potato crop in the form of chloride.
In the late Dr Voelcker's opinion, the conditions under which salt had the most favourable action on the mangel crop was in the case of a light sandy soil, and applied at the rate of 4 to 5 cwt. per acre. Its action when applied to clay soils was not so favourable.
Rate of Application.
Lastly, the rate at which it may be applied will naturally vary. From 1 cwt. and even less, up to 6 cwt. or even more, has been the rate at which it has been commonly applied in the past. From what has been said, it will be seen that it is more likely to exert a favourable influence when applied only in small quantities.
The conditions which regulate the application of manures are many and varied, and the subject, it must be admitted, despite the large amount of investigation already carried out, is most imperfectly understood. For these reasons it is impossible to do little more than lay down certain general principles which may be of service to the agriculturist in guiding him in carrying out the manuring of his crops.
Influence of Manures in increasing Soil-fertility.
In the first place it may be asked, How far can what we may call the permanent fertility of a field be influenced by the application of manures? And to this question the answer must be made, that the influence of manuring in increasing soil-fertility is very slight and only very gradually felt. This is illustrated by the difficulty experienced in attempting to restore to a fertile condition a soil which has long been treated by an exhaustive system of cultivation. In such a case it will be found impossible to restore the fertility of the soil, except very gradually. Farmers who farm in new countries, and in rich virgin soils, little realise sometimes how quickly they may impoverish the fertility of their soils by exhaustive treatment, and how slow the process of restoration is. Nor is this strange when we reflect on the relatively small quantities of fertilising ingredients we are in the habit of adding to the soil by the application of manures, and the nature of their action. The small rate at which they are applied, and the impossibility of distributing them equally in the soil, explain how comparatively limited their action must necessarily be. Some manures, it is true—viz., those which are soluble—are more equally distributed; but then such manures, from their very nature, are little likely to affect the permanent fertility of the soil.
Influence of Farmyard Manure on the Soil.
Of manures which have the best effect in improving a soil's permanent fertility, farmyard manure is undoubtedly the most important. This is owing partly to the fact that it is applied in such large quantities, and partly on account of its composition. Liberal manuring with farmyard manure, systematically carried out, will in time do much to build up a soil's fertility. But liberal manuring with artificial manures will also effect the same end. This it does in an indirect manner by means of the increased crop residues obtained under such treatment. Indeed one of the speediest methods of bringing a soil into good condition is by heavily manuring certain green crops, and then ploughing them in.
Farmyard Manure v. Artificials.
The question how far farmyard manure may be supplanted by artificials is one often discussed. We have already referred to this question in the chapter on Farmyard Manure. It is possible that, with our increasing knowledge of agricultural science, we may in the future be able to dispense with farmyard manure, and make shift to do with artificials alone. At present, however, all our experience points to the fact that the most satisfactory results are obtained from manures by using artificials in conjunction with farmyard manure. It is better both for farmyard manure and artificial manures to be applied together,[241] so that they may mutually act as supplementary the one to the other. While this is so, there may be circumstances in which it will be best to use artificials alone. Where, for example, fields, owing to their situation, are inaccessible, and where the expense of conveying the bulky farmyard manure would be very considerable, it may be found more economical to apply the more concentrated artificial manures. With few exceptions, however, it will be found most desirable to use artificial manures as supplementary to farmyard manure, and not as substitutes for it.
Farmyard Manure not favourable to certain Crops.
While the above is true, it may be well to point out one or two facts regarding the nature of the influence of farmyard manure on certain crops. For instance, it has long been recognised as inadvisable in strong rich soils to apply it directly to certain grain crops, such as barley and wheat, since such a practice is apt to encourage rankness of growth—an undue development of straw at the expense of the grain. It is consequently customary to apply farmyard manure to the preceding crop. The direct application of farmyard manure to wheat, however, according to Sir J. B. Lawes, is not fraught with unfavourable results where the soil is a light one; it is only when the soil is of a heavy nature that it is best to apply it to the preceding crop. Potatoes are another crop to which it is best not to apply it directly. On the other hand, many are of the opinion that mangels seem to be able to benefit from large applications of farmyard manure.
Conditions determining the Application of Artificial Manures.
In the application of artificial manures a large number of considerations have to be taken into account. Among these may be mentioned the nature of the manure itself, and its mechanical and chemical condition; the nature of the soil and its previous treatment with manures, as well as the nature of the climate, the nature of the crop, and the previous cropping. It may be well, therefore, to examine somewhat in detail some of these considerations.
Nature of the Manure.
Nitrogen, phosphoric acid, and potash exist in the common manures, as has already been pointed out, in different states of availability. Nitrogen, for example, may exist in a soluble or insoluble condition, as nitrates, as ammonia, or in various organic forms. Phosphoric acid, similarly, may exist in a soluble form, as it does in superphosphate of lime, or in an insoluble form, as it does in bones or basic slag. Potash, on the other hand, exists—or should exist—in artificial manures only in a soluble form. Now a correct knowledge of the behaviour of these different forms of the common manurial ingredients when applied to the soil is, in the first place, necessary for their successful and economical use.
Nitrogenous Manures.
Thus our knowledge of the inability of the soil-particles to retain nitrogen in the form of nitric acid, as well as our knowledge of the fact that nitrogen is in this form immediately available for the plant's needs, teaches us that nitrate of soda should never be applied before the plant is ready to utilise it—in short, that it should only be applied as a top-dressing; and further, that the use of such a fertiliser in a damp season is less likely to be economical than in a dry one. Again, with regard to nitrogen in the form of ammonia salts, our knowledge of the fact that ammonia is retained by the soil-particles, and that before it becomes available for the plant's needs it has to undergo the process of nitrification, teaches us the desirability of applying it a short time before it is likely to be used. While, lastly, with regard to the nitrogen in the various organic forms in which it occurs, our knowledge of the rate at which these are converted into an available form in the soil will determine when they are best applied. Some forms of organic nitrogen are in a soluble condition, and are quite as speedy in their action as sulphate of ammonia. This is the case with a considerable proportion of the different organic forms of nitrogen present in guano. Other forms of organic nitrogen are only slightly less so—as, for example, dried blood, which ferments very speedily. With regard, therefore, to nitrates and ammonia salts, as well as the more quickly available organic forms of nitrogen, they should either be applied as a top-dressing after the plant has started growth, or only shortly before seed-time. Bones, shoddy, and the various so-called native guanos, should be applied a considerable period before they are likely to be required—not later than the previous autumn.
Phosphatic Manures.
With regard to phosphatic manures the same considerations hold good. Inasmuch as phosphoric acid, whether applied in the soluble condition, as in superphosphate, or the insoluble form, as in bones, basic slag, &c., is not liable to be washed out of the soil, the risk of loss is very slight, and need not be taken into account. As we have pointed out in considering the action of superphosphate, phosphoric acid in this latter form is more speedily available to the crop, and the necessity of applying it much before it is likely to be used does not exist. Hence superphosphate and manures which contain any appreciable amount of soluble phosphoric acid, such as guano, should only be applied shortly before seed-time. Bones, basic slag, or mineral phosphate ought to be applied, on the other hand, a long time before they are likely to be used. Hence an autumn application is to be recommended in the case of such manures.
Potash Manures.
Lastly, with regard to potash manures, as these are soluble, there is no necessity for applying them much before they are likely to be absorbed by the plant. Some are of the opinion that potash is, except in the case of sandy soils, best applied some little time before it is likely to be used, so as to permit of its being washed down into the soil—a process which takes place only comparatively slowly. As potash manures have often been found to give a better result on pastures during the second year than during the first, they are best applied in the autumn.
The above statement as to the behaviour of the different fertilisers when applied to the soil, has a not unimportant bearing on the quantities in which they may safely be respectively applied. The rate at which manures may be applied depends, as we shall immediately see, on other conditions; but what it is here desirable to point out is, that it is not safe to apply such manures as nitrate of soda, or, for that matter, sulphate of ammonia, in large quantities at a time. In fact these manures, especially the former, will best be applied in very small quantities, and rather in several doses. With regard to other manures, more especially phosphatic manures, the same reasons for small application do not exist.
The truth of the above statements is so obvious that it may be regarded as superfluous to make them. As, however, their clear apprehension is essential to understanding the conditions of successful manuring, no apology need be made for making them.
Nature of Soil.
Another condition which has to be taken into account in considering the application of manures is the nature of the soil, as well as its previous treatment. Soils poor in organic matter are those which are most likely to be benefited by the application of nitrogenous manures. Soils of a dry light character require less phosphoric acid than they do of nitrogen and potash; while on a damp and heavy soil phosphatic manures are more likely to be beneficial than nitrogenous or potassic manures. Lastly, a soil rich in organic matter generally requires phosphates, and possibly potash. A point of considerable importance to notice is, that a soil rich in lime can stand a larger application of phosphoric acid than one poor in lime. As a rule, it will be found that the best results with potash will be obtained when applied to a sandy soil. The nature of the soil is an important consideration in determining how far it is advisable to apply readily soluble manures. To a very light and non-retentive soil the risk of loss in applying an easily soluble manure is considerably increased. The nature of the climate is also of importance. Thus, in a dry climate, manures of a soluble nature will have a better effect than in a wet climate, while the opposite will be the case with the more slowly acting manures.
Nature of previous Manuring.
A consideration of equal importance is the previous treatment of the soil with manure. For example, where a soil has been liberally treated with farmyard manure, it has been found that mineral manures have a very inferior effect to that obtained by nitrogenous manure. Lawes and Gilbert have found this to be strikingly the case in their experiments on the growth of wheat. In these experiments it was found that the application of mineral manures was accompanied with little or no benefit to the crop, whereas very striking results followed the application of nitrogen. This they attributed to the fact that the supply of mineral fertilisers in the straw of the farmyard manure is largely in excess of the supply of nitrogen. The nature of the action of the manure previously applied is also to be taken into account in determining how long its influence may probably last. Where, for example, the manure has been nitrate of soda or sulphate of ammonia, it may be safely concluded that its direct influence is no longer felt a year after application. The influence of superphosphate of lime, while scarcely so temporary, may be said to last only for a comparatively short time.[242] On the other hand, when the manure applied is of a slow-acting nature, such as bones or basic slag, its influence will probably be felt for a number of years.
Nature of the Crop.
But more important than any of the above-mentioned conditions is the nature of the crop itself. Our knowledge of the requirements of the different farm crops is still very imperfect. A very wide experience, however, of the effect of different manures on different crops, has conclusively proved that their manurial requirements differ very considerably. The subject is complicated by other considerations, such as the nature of the soil, &c.; but notwithstanding this fact, certain points seem to be pretty well established.
In seeking to understand the respective requirements of the different crops for different fertilisers, two important considerations must be borne in mind. These are—(1) the quantities of the three fertilising ingredients—nitrogen, phosphoric acid, and potash—which different crops remove from the soil; and (2) the different power crops possess of assimilating these ingredients.