Chapter 26

FERTILITY OF THE SOIL.

It is necessary to clearly understand to what the fertility of a soil is due ere we can hope to master the theory of manuring.

_What constitutes Fertility in a Soil._

The question, What constitutes fertility in a soil? is by no means an easy one to answer. If we say, The presence of a plentiful supply of the constituents which form the plant's food, our answer will be incomplete. Similarly, if we reply, A certain physical condition of the soil--here, again, it will be found equally unsatisfactory; for fertility of a soil depends both on its physical condition and on its chemical composition, and indeed even on other circumstances. It may be well, then, before proceeding to treat of the nature and action of the different manures, to offer a brief statement of the conditions of fertility so far, at any rate, as we at present know them. For it may be well to warn the reader that, despite the great amount of work carried out on this subject by experimenters, we still have much to learn before we shall be in a position fully and clearly to understand the subject of soil-fertility in all its bearings.

Apart altogether from the influence exerted by climate, latitude, altitude, and exposure, the fertility of a soil may be said to depend on the following properties. These we may divide into three groups or classes:--

1. Physical or mechanical. 2. Chemical. 3. Biological.

=I. Physical Properties of a Soil.=--The physical properties of a soil are generally admitted to have a very important bearing on its fertility. This has been long practically recognised, and perhaps has in the past been unduly exalted in importance, at the expense of the no less important functions of the chemical.[33] The reason of this is doubtless to be ascribed to the fact that it is much easier to study the physical properties of a soil than it is to study the chemical; and that, while we are in possession of a very large amount of useful information with regard to the former, we are at present only on the threshold of our knowledge of the latter.

_Variety of Soils._

It is a matter of common observation that soils differ widely in their mechanical nature. The early recognition of this fact is evidenced by the large number of technical terms which have been long in vogue among farmers descriptive of these differences. Thus soils are in the habit of being described as "heavy," "light," "stiff," "strong," "warm," "cold," "wet," "damp," "peaty," "clayey," "sandy," "loamy," &c., &c.

_Absorptive Power for Water._

One of the most important of the physical properties of a soil is its power to absorb water.

Water to the plant economy is just as important and necessary as it is to the animal economy. Consequently it is of primary importance to examine into the conditions which regulate the absorption of this important plant-food by the soil.

By the absorptive power of a soil is meant its capacity for drinking in any water with which its particles may come in contact. This power depends, first, on the predominance of its proximate constituents--viz., _sand_, _clay_, _carbonate of lime_, and _humus_; and secondly on the fineness of the soil-particles.

_Absorptive Power of Sand, Clay, Humus._

First, then, with regard to the absorptive power of sand, clay, and humus. Of these, sand possesses this power to the least extent, clay to a greater extent, while humus possesses it most of all.[34]

The extent, therefore, of the absorptive power of a soil depends very much on the proportions in which it possesses these three ingredients. The more sandy a soil is, the less will its power be of absorbing water; and this, there is little doubt, is one of the reasons why a sandy soil is, as a rule, an unfertile soil. Of course there are other and even more important reasons; but that this absorptive power has an important bearing on the question is conclusively proved by the fact that sandy soils are more fertile in a climate where rain is frequent than in one where much dry weather prevails. The incapacity of a sandy soil to absorb a large quantity of moisture is not fraught with such evil effects to the crops in the former case, because it is counteracted by the climatic conditions, which obviate the necessity, in a soil, of possessing great absorptive powers.

The converse, of course, we may mention in passing, holds good of clayey soils.

_Fineness of Soil-particles._

The second quality in a soil on which its absorptive power depends is the fineness of its particles. The great benefit which a soil derives from a good tilth, in this respect, was one of the reasons why Tull's system of horse-hoeing husbandry was so successful in its results.[35] The finer the soil-particles, it may be said generally, the greater is the absorptive power of the soil.

_Limit to Fineness._

There is, however, a limit to the fineness to which the particles of a soil ought to be reduced; for it has been found by experiment that when a certain degree of fineness is reached, the absorptive power decreases with any further pulverisation. A German experimenter found, for example, that a garden loam, capable of absorbing 114 per cent of water in its natural state, when pulverised very fine was able to absorb only 62 per cent of water. Here, clearly, the limit to which it is advisable to pulverise a soil had been exceeded.

_Reason of the above._

It is not difficult to see why this should be so. The amount of water that a soil can soak up is due to the number of pores, or air-spaces, it contains of a certain size. If these pores are large and few in number, the amount of water absorbed will be naturally less than when they are numerous and smaller in size. Up to a certain extent, the more a soil is broken the greater will be the number of pores created, of a size to permit the water to soak in. Beyond that point the pores become too minute, and the soil becomes too compact, each particle clinging together too closely.

_Retentive Power of Soils for Water._

Now closely connected with this absorptive power of soils, which we have just been considering, is the power soils possess of holding or retaining the water they absorb. This power, it will be seen at a glance, must have an important bearing on the fertility of a soil.

_Importance of Retentive Power._

As a considerable interval often elapses between the periods of rainfall, soils, if they are to support vegetable growth, must be able to store up their water-supply against periods of drought. This is all the more necessary when we remember that, in the case of heavy crops, the rainfall would often be inadequate to supply the water necessary for their growth. In fact, it has been estimated that the average evaporation from soils bare of any cultivation is equal to the rainfall. That the evaporation from soils covered with vegetation is very much greater, has been strikingly shown by a calculation made by the late eminent American botanist, Professor Asa Gray, who calculated that a certain elm-tree offered a leaf-surface, from which active transpiration constantly went on, of some five acres in extent; while it has further been calculated that a certain oak-tree, within a period of six months, transpired during the daytime eight and a half times more water than fell as rain on an area equal in circumference to the tree-top.[36] Just as the state of the fineness of the soil-particles has an important influence on the absorptive power of soils, so, too, it is found, it has an important bearing on the rate at which evaporation takes place. Evaporation goes on to the greatest extent in soils whose particles are compacted together, capillary action in this case taking place more freely, and effecting evaporation from a greater depth of soil. The stirring of the surface portion of the soil, as for example by hoeing or harrowing, has for this reason an important influence in lessening the amount of evaporation, and minimising the risks of drought, by breaking the capillary attraction. The amount of evaporation which takes place from a soil covered with a crop, depends largely on the nature of the crop; a deep-rooted crop, since it draws its moisture from a wider area of soil, being more effective in drying a soil than a shallow-rooted crop. The difference in the amounts evaporated from a cropped and a bare fallow soil has been shown at Rothamsted to equal a rainfall of nine inches, the crop being barley. The increase, of course, is due to the water which the crop transpires.[37]

It may be generally said that the greater the absorptive power of a soil, the greater is its retentive power; for soils that most largely absorb water are the most reluctant to part with it.

While these properties are undoubtedly necessary for fertile soils, it is needless to add that they may be possessed by a soil to too great an extent. The soil that is unable to throw off any excess of water becomes cold and damp, and does not admit of proper tillage. Its pores become entirely choked up, and the circulation of air, which, as we shall see, is of so much importance, is rendered impossible. Plants in such a soil are apt to sicken and die, the water becomes stagnant, and certain chemical actions are caused which give rise to poisonous gases, such as sulphuretted hydrogen, &c. A stiff clayey soil offers a good example of the disadvantage of over-retentiveness. Owing to the difficulty such soils experience in throwing off their excessive water, they are extremely difficult to till; and sowing operations are on that account apt to be delayed.

_Power Plants have of absorbing Water from a Soil._

It is a strange fact, and one worth noticing in this connection, that the power plant-roots have of drawing their moisture from a soil, seems to depend on the retentive power of the soil. By this is meant that plants have not the means of exhausting the water in a retentive soil to such an extent as in a non-retentive soil.

In some extremely interesting experiments, carried out by the well-known German botanist Sachs, it was found that plants wilted in a loamy soil, whose water-holding capacity was 52 per cent, when its moisture reached 8 per cent; while in a sandy soil--water-holding capacity 21 per cent--the same species of plant did not wilt until its moisture reached 1-1/2 per cent. Here, then, we see that on one kind of soil the plant was able to live, and obtain sufficient water for its needs, while it died of thirst in another soil, although that soil contained quite as much moisture.

Speaking generally, we may say that Hellriegel's experiments have shown that any soil can supply plants with all the water they need so long as its moisture is not reduced below one-third of the whole amount it can hold.[38]

_How to increase Absorptive Power of Soils._

The absence or presence, in excess, of the above properties, suggests a word or two on how these natural defects may, to a certain extent, be remedied artificially. It stands to reason, that if organic matter in a soil renders its absorptive power greater, a simple method of improving a soil defective in this property is by the addition of organic matter. One of the benefits of ploughing-in green crops on sandy soils is undoubtedly due to this fact; the addition of farmyard manure having also a similar effect. The absence of a sufficient amount of retentiveness, such as is found in sandy soils, in the same way suggests, as a remedy, the addition of clay; and, _vice versa_, where the soil is too clayey, the natural method of improvement will be the addition of sand.[39]

_Shrinkage of Soils._

In drying, soils shrink. Those which shrink least are sandy and chalky soils. Humus soils, on the other hand, shrink most.

_Most favourable Amount of Water in a Soil._

The amount of water in a soil most favourable for plant-growth is a question of considerable difficulty. Too great an amount of moisture renders the land cold; air cannot obtain access to the soil-particles, and the plants sicken and die. Hellriegel has found that as much as 80 per cent of what the soil can hold is hurtful to plants, and that from 50 to 60 per cent is the best amount.[40]

_Hygroscopic Power._

A property possessed by soils in relation to water, which is quite distinct from absorptive power, is their hygroscopic power. By this is meant their power of absorbing water from the air where it is present in the gaseous form. This property is identical with the property which will be adverted to immediately--viz., capacity for absorbing gases. The extent to which soils possess this hygroscopic property seems to be regulated very much by the same conditions as regulate their ordinary absorptive power.[41] This property is considered to be of great importance in the case of soils in hot climates, where their agricultural value may be said to depend to a large extent upon it. The amount of water, however, absorbed in this way is, comparatively speaking, insignificant. Lastly, it may be observed that there are certain methods of drying soils afflicted with too much moisture. These consist in making open ditches, and thus relieving them of their superabundance of water, or in planting certain kinds of trees, such as willows and poplars. The amount of green surface presented by the large number of leaves of trees, from which the constant evaporation of water goes on, is very great. The consequence is that trees may be regarded as pumping-engines. It is from this cause that foresters have noticed that clay lands are apt to become wetter after the trees growing upon them have been cut down.[42]

_Capacity for Heat in Soils._

A property which depends largely on those we have just been considering is the capacity soils possess of absorbing and retaining heat.[43] The temperature of a soil, of course, largely depends on the temperature of the air; but this, we must not forget, depends also on the soil itself. The heat given forth by the sun's rays strikes the soil, with the result that, while so much of its heat is absorbed, a certain portion--and this will vary according to the nature of the soil--of its heat is radiated into the air.

The changes in the temperature of the soil naturally take place more slowly than the changes in the temperature of the air; the depth of soil thus affected by those changes varies also in different climes. It has been calculated that in temperate climes the changes of temperature occurring from day to night are not felt much below three feet down.

_The Explanation of Dew._

We have, it may be stated, generally two processes going on. During the day the soil is engaged in absorbing its heat from the sun's rays; when night comes, and the sun goes below the horizon, the air is chilled below the temperature of the soil, which radiates out its stored-up heat into the air. The result is that the temperature of the soil is soon reduced below the temperature of the air, and the moisture, present in the air in the form of vapour, coming in contact with the cold surface of the earth, is condensed into dew, which is deposited, and is seen best early in the morning before the sun has had time to evaporate it again. Dew is most abundant in summer-time, for the reason that the difference in temperature of the day and night is then greatest. In winter-time it is seen as hoar-frost.

_Heat of Soils._

The temperature of a soil, however, is due to other sources than the sun's rays. Whenever vegetable matter decays, there is always a certain amount of heat generated. Soils, therefore, in which there is a large amount of decaying vegetable matter, are certain to receive more heat from this source than soils of more purely mineral nature.

_Heat in Farmyard Manure._

A good example of the amount of heat that accompanies fermentation, or decay of vegetable matter, is seen in the case of rotting farmyard manure. The danger of loss of the volatile ammonia from this cause is often great, and care must be taken to prevent fermentation going on too quickly, and the temperature from becoming too high.[44] The actual increase in the temperature of a soil effected by the addition of certain bulky organic manures, such as farmyard manure, may thus be considerable. In some experiments carried out at Tokio, Japan, it was found that the application of 20 tons of farmyard manure per acre increased the temperature of the soil to a depth of five inches, for a period of nearly a month, on an average, one and a half degrees Fahrenheit. The amount of water present in a soil, it may be noticed in passing, will have a considerable effect in regulating its temperature, a damp soil being, as a rule, a cold soil.

_The Cause of the Heat of Fermentation._

It may be asked, How is the decay, or fermentation, of vegetable matter, such as farmyard manure, caused? or rather, To what is it due? Decay of any substance is just its slow combustion or burning. When a substance unites with the active chemical element in air--the oxygen gas--it is said to be oxidised. Now, this union of a substance with oxygen is the explanation of burning, and the phenomena of burning and decay are explained by the same chemical operation. When bodies decay, or when they burn, they unite with oxygen: when this union of a body and oxygen takes place very quickly, and the result is a flame and very great heat, then we call it burning; when, however, it takes place slowly, it is not called burning, but simply oxidation or decay. The ultimate products are the same, however, whether the body burns or decays; and the process of decay is always accompanied by heat, as well as the process of burning.[45] It is not, of course, only the vegetable or organic matter in a soil that decays, but also the mineral matter. The oxidation, however, of the mineral matter in the soil takes place so slowly, and the amount of heat generated by this oxidation is so slight, that the temperature of the soil can scarcely be said to be much affected by it.

_Influence of Colour of a Soil._

There is still another quality of a soil on which its temperature depends, and that is its colour. This may seem at first sight to be scarcely worth taking into account, and yet it has been shown to have a very striking influence on the temperature of a soil. This naturally is best seen in climates where there is a good deal of sun. Dark-coloured soils have a greater heat-absorbing capacity than light-coloured soils; and experiments carried out for the purpose of determining the extent of this influence have shown that under certain conditions the difference between a soil covered with a black substance, and one covered with a white substance, amounted to from 13 deg. to 14 deg. Fahr. Other things being equal, a crop on a dark-coloured soil will be sooner ripened than one on a light-coloured soil. A soil covered by a crop is cooler than one without any crop.

_The Power Soils have for absorbing Gases._

We have just seen that one cause of the heat of soils is the oxidation which is constantly going on in all soils, but more rapidly in soils containing a large quantity of vegetable matter. This suggests a word or two on the power soils have of absorbing gases.

The chief gases in the atmosphere are oxygen and nitrogen. Both these gases are absorbed by soils, although not in similar proportions.[46] With regard to the former, it is well known that a plentiful supply of oxygen in the pores of the soil is a necessary condition of fertility. This was long ago experimentally proved by de Saussure, who showed that plants absorbed oxygen through their roots. At certain periods of their growth this demand for oxygen on the part of the plant is greater than at other times. For example, seeds in the process of germination require to have free access to a plentiful supply of oxygen. This fact emphasises the enormous importance of providing a good seed-bed, and of seeing that the seed is not buried too deeply.

_Carbonic Acid and Ammonia._

In addition to oxygen and nitrogen, the air contains other gases which are absorbed by the soil. Of these, carbonic acid is the most abundant. By far the largest portion of the carbonic acid which the soil obtains from the air, is washed down in solution in the rain.[47] Of the other constituents of the atmosphere, the combined forms of nitrogen--viz., _ammonia_, _nitric_, and _nitrous acids_--are the most important. These are all absorbed by the soil, but, like carbonic acid, they are chiefly washed down by the rain. The amount of ammonia which may be absorbed by a soil from the air, is very much greater than was formerly supposed. Some recent experiments by Schloesing, referred to in a following chapter,[48] show this. A damp soil may in the course of a year absorb far more ammonia than that washed down in rain.

_Gas-absorbing Power of Soils varies._

The power of different soils to absorb these gases varies. This variation depends not only on their physical properties, but also on their chemical as well. Soils containing much organic matter have a greater capacity for absorbing gases than the more purely mineral ones.

_Absorption of Nitrogen._

The absorption of nitrogen by the soil is a question of considerable importance. It will be referred to later on under the heading of the biological properties of soils, as it is fixed by the agency of micro-organisms.[49]

To recapitulate, the chief physical or mechanical properties of a soil are its absorptive and retentive powers for water; its capacity for heat; and its power of absorbing gases. It will be easily seen how tillage operations are calculated to influence these physical properties of a soil. Thus, in the case of a stiff soil, tillage increases its power for absorbing the atmospheric gases, chiefly oxygen, which are so necessary for rendering its fertilising matters available. On the other hand, in a light and too open soil it may exert quite a contrary effect.

It may be also well to refer here to the important influence these physical properties exercise on the growth of the plant.

_Plant-roots require a certain Openness in the Soil._

One of the functions of the soil is to support the plant in an upright position, and this is a function which requires in the soil a certain amount of compactness or firmness. On the other hand, however, a soil must not possess too great compactness, otherwise the plant-roots will experience a difficulty in pushing their way downwards. This is especially the case during the earlier periods of growth, when the plant-roots are as yet extremely tender, and experience great difficulty in overcoming much resistance. The importance of preparing a mellow seed-bed will be thus at once seen to be based on sound scientific principles; and this for a double reason. Not only does the young plant require every facility for developing its roots, but also, as has just been pointed out, an abundant supply of oxygen is of paramount importance during the process of germination.

_Soil and Plant-roots._

The whole question of the influence of the mechanical condition of the soil on the development of plant-roots is one of the highest importance and interest, and is not so generally recognised as it ought to be.

_Natural tendency of Plant-roots to grow downwards._

It may be taken as certain that the tangled condition of plant-roots is due to the resistance offered by the soil-particles, and that the natural tendency of the plant-root is to grow downwards. The roots, in short, would probably grow in as symmetrical a form as do the stalks or branches, were it not that they are hindered from so doing by the soil-particles. Where, then, the soil is such as to offer much hindrance, the growth of the plant cannot but be retarded. Some extremely interesting experiments have been performed by the eminent German chemist Hellriegel on the influence which the closeness of the soil-particles has on root-development. In these experiments peas and beans were grown in moistened sawdust, more or less compactly compressed. It was found that when the sawdust Was compressed to any extent, plant-growth took place very slowly, or entirely ceased.

The importance of having plant-roots as widely developed in the soil as possible, will be at once seen when we reflect that this means that the area of soil from which the plant derives its soil-food is thereby greatly increased. Another important consideration is, that the deeper plant-roots can penetrate in a soil, the more able--other conditions being equal--is the plant to withstand the action of drought, as it can draw water for its needs from the deeper layers of the soil, long after a plant, whose roots do not penetrate so deeply, has wilted.

_Plants require Room._

Another important bearing tillage has on plant-growth may here be discussed. A problem of considerable difficulty is presented in the question, How many individual plants will a certain piece of soil support in a healthy way? For as plants require room, it is imperative that they be not too closely crowded together.

The question resolves itself pretty much into one of quality against quantity.

Experiments on this subject have shown that a certain area of soil is only able to support the healthy growth of a certain number of plants. If the limit be exceeded, the result is imperfect development.

_Number of Plants on certain Area increased by Tillage._

It is obvious, however, that the more thoroughly tilled a soil is, the greater will be the number of plants it will be possible to grow on it. The roots, instead of being forced to spread themselves along the surface-soil, and thus take up a large amount of room, will find no difficulty in striking downwards. Two or three plants may thus be enabled to grow in a thoroughly tilled soil in the same space as only one could before tillage.

_American and English Farming._

The above considerations throw considerable light on what seems to many farmers a strange anomaly--viz., the fact that the return of farm produce per acre on American farms is, as a rule, very much less than that from our own impoverished soils in this country. To many, at first sight, this seems to be in direct contradiction to our common belief, and to point to the conclusion that the virgin soils of America are, after all, actually inferior in fertility to the soils of Britain.

It is not, however, necessary to draw this conclusion, as the facts of the case admit of another explanation. The inferior returns obtained from American farms are due, not to the fact that the American soil is less fertile than the British--for this is not true--but to the fact that it is less _intensively_ cultivated.

In America land is cheap and labour is dear; it is consequently found to be more economical to cultivate a large tract of land less thoroughly than a small area more thoroughly. In Britain the reverse is the case, labour being cheap and land being dear. It is thus necessary to make the land go as far as possible, and produce as heavy a crop as it is possible to produce. There can be little doubt, that were American farming to be carried on as intensively as is British farming, the present yield would be at least probably doubled.

We have now to consider the second class of properties which influence the fertility of a soil. These are _chemical_.

=II. Chemical Composition of a Soil.=--Chemically considered, the soil is a body of great complexity. It is made up of a great variety of substances. The relations existing between these substances and the plant are not all of equal importance; some--and these form by far the largest proportion of the soil-substance--are concerned in acting simply as a mechanical support for the plant, and in helping to maintain those physical properties in the soil which, as we have just seen, exercise such important functions in the plant's development.

_Fertilising Ingredients._

A small portion of the soil-substance, however, takes a very much more active part in promoting plant-growth, by acting as direct food of the plant. As we have already seen in the Introductory Chapter,[50] the substances which have been found in the ash of plants are the following: _potash_, _lime_, _magnesia_, _oxide of iron_, _phosphoric acid_, _sulphuric acid_, soda, silica, chlorine, oxide of manganese, lithia, rubidia, alumina, oxide of copper, bromine, and iodine. The general presence of some of these substances is doubtful; the presence of others, again, probably purely accidental; while some are only found in plants of a special nature, as, for instance, iodine and bromine, which are only found in the ash of marine plants.

Of these ash constituents, only the first six substances--those marked in italics--are absolutely necessary to plant-growth. In addition to these six ash constituents, the plant also derives its _nitrogen_, which is a necessary plant-food, chiefly from the soil.[51]

_Importance of Nitrogen, Phosphoric Acid, and Potash._

But of these seven constituents of the soil which are necessary to plant-growth, some have come to be regarded by the agriculturist with very much greater interest than others. This is due to the fact that they are normally present in the soil in very much smaller quantities than is the case with the other equally necessary food ingredients; that, in short, they are nearly invariably present in the soil, in a readily available form, in lesser quantities than the plant is able to avail itself of, and often, as in impoverished or barren soils, in quantities too small for even normal growth. These ingredients are _nitrogen_, _phosphoric acid_, and _potash_.[52]

The importance of seeing that all the necessary plant ingredients are present in a soil in proper quantities will be at once properly estimated when it is stated that the absence or insufficiency in amount of one single ingredient is capable of preventing the growth of the plant, although the other necessary ingredients may be even abundantly present.

With lime, magnesia, iron, and sulphuric acid, most soils are abundantly supplied. The substances with which the farmer has to concern himself, then, are nitrogen, phosphates, and potash. It is these substances therefore, that, as a rule, are alone added as manures.

_Chemical Condition of Fertilising Ingredients in Soil._

But in considering the chemical properties of a soil, a simple consideration of the quantity of the different ingredients present is not enough. A very important consideration is their chemical condition. Ere any plant-food can be assimilated by the plant's roots, it must first be rendered soluble. The quantity of soluble, or, as it is known, _available_, plant-food in a soil is very small. It is, of course, being steadily added to each day by the process of disintegration constantly going on in soils.

_Amount of Soluble Fertilising Ingredients._

The exact nature and dissolving capacity of the soil-water, charged as it is, to a greater or less extent, with different acids and salts, as well as the dissolving power of the sap of the rootlets of the plant itself, render the exact estimation of the available fertilising constituents wellnigh impossible. An approximate estimate, however, may be obtained by treating the soil with pure water and dilute acid solutions. The treatment of the soil with dilute acid solutions is for the purpose of simulating, as nearly as may be done, the conditions it is submitted to in the soil. By treating a soil with water, we obtain a certain amount of plant-food dissolved in the water. This can only be regarded as indicating approximately the amount available at that moment to the plant. But every day, thanks to the numberless complicated reactions going on in the soil, this soluble plant-food is constantly being added to. Considerations such as the above, together with our ignorance as to the exact combinations in which the necessary minerals enter the plant, will serve to indicate the great difficulty of this part of the subject.[53]

_Value of Chemical Analysis of Soils._

It is largely for these reasons that a chemical analysis of a soil is from one point of view of little value in giving evidence of its actual fertility. What it demonstrates more satisfactorily is its potential fertility. It is useful in revealing what there is present in it, not necessarily, however, in an available condition. Under certain circumstances it may be made of great value, as, for example, when we are anxious to know what will be the result of certain kinds of treatment, such as the application of lime, &c.

It is hardly advisable, therefore, to place before the reader a number of soil analyses. That he may obtain an approximate idea of the composition of a soil, one or two representative analyses will be found in the Appendix,[54] along with a short account of the chief minerals out of which soils are formed.

A point of considerable interest is the quantity per acre different soils contain of nitrogen, phosphoric acid, and potash. Although the amount of these ingredients when stated in percentage seems very trifling, yet when calculated in lb. per acre, it is seen to be in large excess of the amount removed by the different crops. This question will be dealt with in succeeding chapters.

A point of further interest is the chemical form in which the necessary plant constituents are present in the soil. For information on this point the reader is referred to the Appendix.[55]

The third class of properties which affect the fertility of a soil are those which have been termed the _biological_.

=III. Biological Properties of a Soil.=--The important functions which modern discoveries have shown to be discharged by minute organic life in the terrestrial economy are nowhere more strikingly exemplified than in the important _role_ they perform in the soil.

_Bacteria of the Soil._

The soil of every cultivated field is teeming with bacteria whose function is to aid in supplying plants with their necessary food. The nature of, and the functions performed by, these organisms differ very widely. Regarding many of them we know very little; every day, however, our knowledge is being extended by the laborious researches of investigators in all parts of the world, and it is to be anticipated that ere long we shall be in possession of many facts regarding the nature and the method of the development of these most interesting agents in terrestrial economy. That they are present, however, in enormous numbers in all soils we have every reason to believe, one class of organism connected with the oxidation of carbonic acid gas being estimated to be present to the extent of over half a million in one gramme of soil[56] (Wollny and Adametz). One class--and their importance is very great in agriculture--prepare the food of plants by decomposing the organic matter in the soil into such simple substances as are easily assimilated by the plant. The so-called "ripening" of various organic fertilisers is effected, we now know, entirely through the agency of bacteria of this class. Plant-life is unable to live upon the complex nitrogenous compounds of the organic matter of the soil, and were it not for bacteria these substances would remain unavailable. Attention will be drawn in the Chapter on Farmyard Manure to this question more in detail. Of these bacteria, among the most important are those which are the active agents in the process known as "nitrification"--_i.e._, the process whereby organic nitrogen and ammonia salts are converted into nitrites and nitrates. The presence of these organisms, it would appear, is indispensable to the fertility of any soil. There are organisms, on the other hand, which have the power of reversing the work of the nitrification bacteria by converting nitrates into other forms of nitrogen. The reduction of nitrates in the soil is often the source of much loss of valuable nitrogen, which escapes in the free state, so that the action of bacteria is not altogether of a beneficial nature.

_Three Classes of Organisms in the Soil._

So far as the subject has been at present studied, the micro-organisms in the soil may be divided into three classes.[57]

_First Class of Organisms._

We have, first of all, those whose function it is to oxidise the soil ingredients. Organisms of this class may act in different ways. They may assimilate the organic matter of the soil and convert it into carbonic acid gas and water; or, on the other hand, they may oxidise it by giving off oxygen. Some of these organisms, whose action is of the first kind, choose most remarkable materials for assimilation. One has been found to require ferrous carbonate for its development, which it oxidises into the oxide (Winogradsky); while another,[58] the so-called sulphur organism, converts sulphur into sulphuretted hydrogen according to some, and according to others into sulphates. To this class of organism the nitrifying organisms belong. As will be seen more fully in a subsequent chapter, two distinct organisms connected with this process have already been isolated and studied--one of these effecting the formation of nitrites from organic nitrogen or ammonia salts, and the other the conversion of nitrites into nitrates. The second method in which these oxidising organisms act is by giving off oxygen. There is much interest attaching to this fact, as it was supposed till quite recently that all evolution of oxygen in vegetable physiology was dependent on the presence of light, and also intimately connected with chlorophyll, or the green colouring matter of plants. It would seem, however, that among the soil organisms these conditions are not necessary, and the evolution of oxygen may be carried on in the case of colourless organisms as well as in the case of light. With organisms of this kind every soil is probably teeming. A typical example is the organism which is the active agent in the oxidation of carbonic acid gas, and which has already been referred to as existing in the soil in such numbers.[59]

_The Second Class of Organisms in the Soil._

The second class of organisms are those which reduce or destroy the soil constituents. The most important of these, from the agricultural point of view, are those which effect the liberation of nitrogen from its compounds. In the putrefaction of organic matter the organisms chiefly act, it is probable, in the entire absence of atmospheric oxygen; but it would seem, however, that they may also act in the presence of oxygen. It is through their agency that the soil may lose some of its nitrogen in the "free" form. To this class belong the denitrifying organisms already referred to which reduce the nitrates and nitrites in the soil.[60]

_Third Class of Organisms._

The third class of organisms are those by whose agency the soil is enriched. Of this class those fixing the free nitrogen from the air are the most important. The nature of these organisms is still somewhat obscure, but that leguminous plants have the power of drawing upon this source of nitrogen is now a firmly established fact. Further reference to these interesting organisms may be delayed to another chapter.

The important point to be emphasised is, that for the healthy development of these organisms, which are so necessary in every fertile soil, certain conditions must exist. These necessary conditions will be treated more in detail later on. It is sufficient to notice that they have to do with the physical properties as well as the chemical composition of the soil. This furnishes a further reason for the necessity of having the mechanical condition of a soil satisfactory.

_Recapitulation._

From what we have said, it will be seen that the question of soil-fertility is a very complicated one, and depends on numerous and varied conditions; that the properties which constitute fertility, while seemingly very widely different in their nature, in reality influence one another to a very great extent; that not merely is the presence in a soil of the necessary plant constituents necessary to fertility, but that the possession by the soil of certain physical or mechanical properties is equally necessary; while, lastly, we have seen that the presence of certain micro-organic life is bound up with the problem of fertility in a very direct and practical manner.

The importance of the conditions, other than those of a purely chemical nature, have been thus far somewhat prominently emphasised, for the reason that in what follows attention will be almost exclusively devoted to the purely chemical conditions of fertility. It is well, then, to realise that, while the latter conditions are by far the most important, so far as the farmer is practically concerned, inasmuch as they are most under his control, they are not the only conditions, and are not by themselves able to control fertility.

FOOTNOTES:

[33] This statement perhaps needs qualification. While the important _role_ played by the physical qualities of the soil were in the early years of the science recognised, of more recent years the chemical composition of the soil has been engaging almost exclusive investigation. Physical properties of the soil have recently acquired a further importance in the eyes of the agricultural chemist, from the important influence they exert on what we have here called the biological properties of a soil--viz., the development of those fermentative processes whereby plant-food is prepared to a large extent.

[34] A good example of the absorptive capacity of a soil containing a large quantity of vegetable matter is furnished by peat-bogs, which, sponge-like, can absorb enormous quantities of water. (See Appendix, Note I., p. 98.)

[35] Jethro Tull, an early well-known agricultural writer, who lived about the middle of last century, propounded the theory, that as the food of plants consisted of the minute earthy particles of the soil, all that was required by the skilful farmer was to see that his soil was properly tilled. He accordingly published a work entitled 'Horse-hoeing Husbandry,' in which he advocated a system of thorough tillage. (See Historical Introduction, p. 10.)

[36] See Introduction, p. 55.

[37] See Introductory Chapter, p. 55.

[38] It is not exactly known why excess of water should prevent normal growth in the plant. Probably it is on account of the fact that free access of oxygen is hindered in such a case. The roots are thus not freely enough exposed to this necessary gas, and fermentative processes of the nature of nitrification are not promoted. It may be also due to the fact that the solution of plant-food is too dilute when such excess of water prevails.

[39] See Appendix, Note II., p. 98.

[40] Some experiments by E. Wollny show this. He found, when experimenting with _summer rape_, that the best results were obtained when the soil contained only 40 per cent of its total water-holding power; when the amount was either lessened or increased the results obtained fell off. The effect of either too little or too much water is seen in the development of the different organs of the plant as well as on its period of growth, much water seeming to retard the growth. The quality of the plant seems also to be influenced by this condition. Experiments on cereal grains by Wollny show that not merely is the texture of the grain influenced, but that much moisture lessens the percentage of nitrogen. Wollny is of the opinion that for crops generally, the best amount is from 40 to 75 per cent of the total water-holding capacity of the soil.

[41] See Appendix, Note III., p. 99.

[42] See p. 55.

[43] The effect of the temperature of the soil on the development of the plant is most important. This is especially marked at the period of germination, but is felt at subsequent periods of growth. Up to a certain temperature the warmer the soil the more rapid the plant's development. In this country the temperature most favourable to growth is rarely exceeded, or indeed reached.

[44] See Chapter on Farmyard Manure.

[45] As will be seen further on, the fermentation of organic substances is caused by the action of micro-organic life.

[46] See Appendix, Note IV., p. 100.

[47] Of course it must be remembered that a large amount of carbonic acid in soils comes from the decay of vegetable matter. Soils are twenty to one hundred times richer in carbonic acid than the air.

[48] See Chapter III., p. 119.

[49] See Introduction, p. 40.

[50] See Introductory Chapter, p. 54.

[51] See pp. 44 and 135.

[52] Occasionally also _lime_.

[53] See Appendix, Notes V. and VI., pp. 100, 101.

[54] Note VI., p. 101.

[55] Note VII., p. 107.

[56] Even larger estimates of the number of germs in a gramme of soil have been made--from three-quarters to one million (Koch, Fuelles, and others).

[57] These organisms consist of molds, yeast, and bacteria, the last-named being most abundant. In the surface-soil, among the bacteria, bacilli are most abundant. Micrococei are not abundant.

[58] Investigated by Winogradsky, Olivier, De Rey Pailhade, and others.

[59] Organisms of this kind have been investigated among others by Heraues, Hueppe, and E. Wollny. According to the two first-mentioned investigators, certain colourless bacteria effect the formation in the absence of light from humus and carbonates a body resembling in its nature cellulose.

[60] Investigated by Springer, Gayon and Dupetit, Deherain, and Marguenne.

APPENDIX TO CHAPTER I.

NOTE I. (p. 68).

The following determinations by Schuebler show the absorptive power of different kinds of soil-substances. These were obtained by soaking weighed quantities of the soil in water, and allowing the excess of liquid to drain away, and weighing the wet earth.

Per cent of water absorbed by 100 parts of earth.

Siliceous sand 25 Gypsum 27 Calcareous sand 29 Sandy clay 40 Strong clay 50 Arable soil 52 Fine calcareous 85 Garden-earth 89 Humus 190

It has been calculated that the absorptive power of a mixture of different substances is not simply equal to the sum of their separate ingredients.

NOTE II. (p. 74).

EVAPORATION.

The retentive property of a soil for water tends to retard evaporation. The following table by Schuebler shows the rate at which evaporation proceeds in different soils. The experiment was conducted in the following way. The soil experimented upon was saturated with water and spread over a disc, and allowed to evaporate for four hours, when it was weighed. The amount of time required for the evaporation of 90 per cent of the water was also estimated. Of 100 parts of water in the wet soil there evaporated, at 60 deg. Fahr.--

Time required to In four hours-- evaporate 90 per cent. From-- per cent. Hours. Minutes.

Quartz 88 4 4 Limestone 76 4 44 Sandy clay 52 5 1 Stiffish clay 46 6 55 Loamy clay 46 7 52 Pure grey clay 32 11 17 Loam 32 11 15 Fine calcium carbonate 28 12 51 Humus 21 17 33 Magnesium carbonate 11 33 20

NOTE III. (p. 76).

HYGROSCOPIC POWER OF SOILS.

Davy found the hygroscopic power of soils to be as follows. He found that 100 parts by weight of three samples of different sands absorbed 3, 8, and 11 parts of water, respectively, in one hour; while three loams absorbed similarly 1.3, 1.6, and 1.8 parts.

The following samples of soil were dried at 212 deg. Fahr., and exposed to an atmosphere saturated with water and a temperature of 62 deg. Fahr., when it was found they absorbed the following amounts in twelve hours' time:--

Quartz sand 0.0 Limestone sand 0.3 Lean clay 2.1 Fat clay 2.5 Clay soil. 3.0 Pure clay. 3.7 Garden-loam 3.5 Humus 8.0

NOTE IV. (p. 81).

GASES PRESENT IN SOILS.

The air which we find enclosed in the pores of the soil is distinctly _poorer_ in oxygen than ordinary air. Boussingault found the percentage of oxygen in a sandy soil, freshly manured and wet with rain, to be as low as 10.35 per cent; while the air in forest-soil contained 19.5 per cent of oxygen, and .93 per cent of carbonic acid. The percentage of oxygen in soils depends on the rate of decay of the organic portions. The depth of the soil-layer also determines the quantity. This is owing to the fact that diffusion takes place more slowly deep down than near the surface.

NOTE V. (p. 90).

AMOUNT OF SOLUBLE PLANT-FOOD IN THE SOIL.

Two of the most reliable methods of ascertaining an approximation of the quantity of soluble soil constituents are (1) by treating the soil with distilled water, and (2) by analysing the drainage-water. With regard to the former of these two methods, it has been found that even the amount of fertilising matter dissolved out by pure distilled water varies. This variation depends on the amount of distilled water used, as well as the length of time the soil is left in contact with the solvent. By washing the soil with different quantities of water, different amounts of soluble soil ingredients will be found to have been washed out; for although the first washings contain by far the greater portion of the soluble matter, each subsequent washing will be found to contain further quantities.

A number of experiments have shown that 1000 parts of distilled water dissolved out from different soils from one half to one and a half parts of soluble constituents; or from .05 to .15 per cent. Of this soluble matter from 30 to 67 per cent is mineral in its nature, and from 33 to 70 per cent organic. Poor sandy soils yield the minimum quantity, while peaty soils yield the maximum. The quantity of soluble matter in a regular peaty soil may vary from .4 to 1.4 per cent; this consists chiefly, however, of organic matter. (See Johnson's 'How Crops Feed,' p. 312.)

Perhaps a more satisfactory method is by analysing the drainage-water of a soil. This has been found to vary very considerably in composition. The average of a large number of analyses are .04 to .05 per cent of dissolved matter. Of this dissolved matter the largest proportion is made up of organic matter, nitric acid, lime, and soda salts. It must be borne in mind, however, that even the drainage-water does not furnish an exact indication of the amount of dissolved matter in a soil. Much, perhaps the largest proportion of dissolved matter, never finds its way into the drainage-water. That contained by the drainage-water really represents the surplus quantity of dissolved matter which the soil is unable to retain, and which is thus washed by the rain into the drains. The composition of drainage-water is interesting, as it shows that, practically speaking, all the necessary plant ingredients are in a state of solution in the soil.

NOTE VI. (p. 90).

CHEMICAL COMPOSITION OF THE SOIL.

The most important substances present in soils are as follows: silica, alumina, lime, magnesia, potash, soda, ferric oxide, manganese oxide, sulphuric acid, phosphoric acid, and chlorine. Of these substances the presence of alumina, silica, lime, and, in certain cases, magnesia, along with the organic portion of the soil--the humus--has the chief influence in determining the nature and the physical properties of a soil.

In order to clearly understand to what it is soils owe the nature of their chemical composition, it is necessary to consider the composition of some of the chief minerals out of the disintegration of which soils are formed.

While we know of some seventy elements present in the earth's crust, it is practically made up of only some sixteen. These sixteen are--oxygen, silicon, carbon, sulphur, hydrogen, chlorine, phosphorus, iron, aluminium, calcium, magnesium, sodium, potassium, fluorine, manganese, and barium.[61] Of these, oxygen is by far the largest constituent, forming, roughly speaking, about 50 per cent.

The main mass of the rocks consists of silica, and this is generally combined with alumina, as in clay, forming aluminium silicate, and with the commoner alkalies and alkaline earths. Another extremely abundant compound is carbonate of lime, which, as limestone, chalk, and marl, forms one-sixth of the earth's total rocks.

The word "mineral" means a definite chemical compound of natural occurrence. The number of minerals is very great, and it is impossible to go into the subject here. Reference can only be made to a few of the more prominent ones, which are chiefly concerned in the formation of soils.

Those formed out of silicates are, from the agricultural point of view, the most important, as they form a very large group; and it is by their disintegration that soils are chiefly formed. They consist of silica and alumina, along with various other substances, chiefly alkalies and alkaline earths. It is important to note one peculiarity about the solubility of silicates. We have two classes of silicates: the one, which is called "acid," and contains an excess of silica; the other, "basic," and which contains an excess of base. Now, while the former of these is more or less insoluble, the second is soluble. This fact has an important signification in the process of the disintegration of the silicate minerals we are about to consider.

The first and most important class are the _Felspars_. Felspar is not really a definite mineral, with a definite chemical composition, but rather the name of a class of minerals of which there are several different kinds. The felspars are composed of silica and alumina, along with potash, soda, and lime, with traces of iron and magnesia. Their principal constituents, however, are silica and alumina, along with either potash, soda, or lime. According as the base potash, soda, or lime predominates, the felspar is known as Orthoclase, Albite, and Oligoclase, respectively.

The following are the analyses of the three minerals (by the late Dr Anderson):--

+-----------------+-----------------+----------------- | Orthoclase. | Albite. | Oligoclase. +--------+--------+--------+--------+--------+-------- | 1. | 2. | 1. | 2. | 1. | 2. --------------------+--------+--------+--------+--------+--------+-------- Silica | 65.72 | 65.00 | 67.99 | 68.23 | 62.70 | 63.51 Alumina | 18.57 | 18.64 | 19.61 | 18.30 | 23.80 | 23.09 Peroxide of iron | traces | 0.83 | 0.70 | 1.01 | 0.62 | none Oxide of manganese | traces | 0.13 | none | none | none | none Lime | 0.34 | 1.23 | 0.66 | 1.26 | 4.60 | 2.44 Magnesia | 0.10 | 1.03 | none | 0.51 | 0.02 | 0.77 Potash | 14.02 | 9.12 | none | 2.53 | 1.05 | 2.19 Soda. | 1.25 | 3.49 | 11.12 | 7.99 | 8.00 | 9.37 --------------------+--------+--------+--------+--------+--------+-------- | 100.00 | 99.47 | 100.08 | 99.83 | 100.79 | 101.37 --------------------+--------+--------+--------+--------+--------+--------

According as these various felspars are present in a soil, so will the quality of the soil be. It stands to reason that as the presence of potash in a soil is one of the distinguishing features of its fertility, much will depend on the extent to which the orthoclase felspar is present; and also, not only on the extent, but on the state and degree of its disintegration. It is important to note the method of this disintegration. It is effected by the absorption of water. This water is not merely absorbed mechanically, but actually enters into the composition of the mineral. It is not present as moisture merely, capable of being expelled at ordinary boiling temperature, but it forms what is known as water of composition. In this process of hydration, the mineral loses its lustre and crystalline appearance, crumbles away into a more or less--according to its state of disintegration--powdery mass. A very great change is also effected in its chemical composition; it loses nearly all its base. This is effected in the following way. As water enters into the mineral's composition, it sets free a certain portion of the base; there is thus formed a basic silicate, which, being soluble in water, is washed away in solution. This change may be illustrated by quoting the analysis of a kaolin clay formed by the disintegration of orthoclase felspar.

_Kaolin Clay formed by disintegration of Orthoclase._

Silica 46.80 Alumina 36.83 Peroxide of iron 3.11 Carbonate of lime 0.55 Potash 0.27 Water 12.44 ------ 100.00 ------

The chief difference here is the almost total loss of potash and a portion of the silica, and the gain of water. The other constituents practically remain insoluble.

Another important mineral is _Mica_. Its composition is not unlike felspar. It contains silica, alumina, and iron, in considerable quantities, also magnesia and potash. There are two kinds of mica--that containing potash, and that containing magnesia, in excess. The analyses of these two kinds are as follows (by the late Dr Anderson):--

MICAS. (_a_) Potash. (_b_) Magnesia. Silica 46.36 42.65 Alumina 36.80 12.96 Peroxide of iron 4.53 none Protoxide of iron none 7.11 Oxide of manganese 0.02 1.06 Magnesia none 25.75 Potash 9.22 6.03 Hydrofluoric acid 0.70 0.62 Water 1.84 3.17 ----- ----- 99.47 99.35 ----- -----

The decomposition of mica is very slow, however, as it is a peculiarly hard mineral.

Other important minerals are _Hornblende_ and _Augite_. These are composed of silica, alumina, iron oxide, manganese oxide, lime and magnesia. These are the chief minerals out of which soils are formed. It is scarcely necessary to say that few soils are made up out of any of these three minerals alone. Nearly all rocks are formed out of a mixture of these minerals. Where, however, any one mineral predominates over the rest, the nature of the soil will be thereby affected. In order to illustrate this, it may be well to mention the composition of one or two of the commoner rocks.

1. _Granite_, which is so abundant in certain parts of the north of Scotland, and which gives rise to the soils in the neighbourhood of Aberdeen, is made up of a mixture of quartz, felspar, and mica. It depends on the felspar present--_i.e._, whether it is orthoclase, oligoclase, or albite--whether the soil will be rich in potash or not. Granite containing orthoclase felspar produces a fairly fertile soil. An important consideration, which is apt to complicate this question, is the situation of such soils. They are generally so high above sea-level, that their fertility is seriously impaired on these grounds.

2. _Gneiss_, another common rock, is similar in composition, only that it contains very little felspar, and a correspondingly greater amount of mica.

3. _Syenite_ contains quartz, felspar, and hornblende.

The rocks of which greenstone and trap are types, are found very largely scattered over the country. They are of two kinds, diorite and dolorite.

4. _Limestone_ is of two great classes. We have (1) Common, (2) Magnesian. The following are the analyses of these two classes by Dr Anderson:--

+---------------------------+------------------------ | Common. | Magnesian. +-------------+-------------+-------------+---------- | Mid-Lothian | Sutherland. | Sutherland. | Dumfries. ----------------------+-------------+-------------+-------------+---------- Silica | 2.00 | 7.43 | 6.00 | 2.31 Iron oxide and alumina| 0.45 | 0.76 | 1.57 | 2.00 Carbonate of lime | 93.61 | 84.11 | 50.21 | 58.81 Carbonate of magnesia | 1.62 | 7.45 | 41.22 | 36.41 Phosphate of lime | 0.56 | - | - | - Sulphate of lime | 0.92 | - | - | - Organic matter | 0.20 | - | - | - Water | 0.50 | - | - | - | ----- | ----- | ----- | ----- | 99.86 | 99.75 | 99.00 | 99.53 ----------------------+-------------+-------------+-------------+----------

Clays are formed by the disintegration of any of the crystalline rocks; the purest clays being formed from felspar. A pure clay consists simply of silica and alumina, all the other constituents having been washed out. Disintegration, however, seldom reaches such an extent; otherwise clay soils would be completely barren, which they are notably not. The impurities present in clay, which consist of alkalies, especially potash and other mineral ingredients of the plant, are what confer on clay soils their fertility. Clays differ, however, very considerably in their composition. The following is an analysis of a clay soil by Dr Anderson:--

Silica 60.03 Alumina 14.91 Peroxide of iron 8.94 Lime 2.08 Magnesia 4.22 Potash 3.87 Soda 0.06 Water and carbonic acid 5.67 ----- 99.72 -----

NOTE VII. (p. 91).

FORMS IN WHICH PLANT-FOODS ARE PRESENT IN SOIL.

The forms in which the bases necessary for plant-food are present in the soil, are chiefly as _hydrated silicates_, and in combination with organic acids, forming humates, &c., as well as in the form of sulphates and chlorides.

Phosphoric acid is present in combination with iron, alumina, or lime, or possibly also as magnesium-ammonium-phosphate. Sulphuric acid is generally present in a more or less insoluble condition, in combination with iron and lime; whereas chlorine is combined with the alkali bases in an easily soluble form. An important point is as to the form in which the plant absorbs these food constituents. In this connection reference may be made to a theory put forward by a very distinguished French agricultural chemist, Professor Grandeau. His theory is that the necessary ingredients of plant-food are absorbed into the plant as humates, or, at any rate, that the medium of this transference is humic acid, and organic acids of a similar nature. This theory, however, while ingenious, has not yet been supported by sufficient evidence to make its acceptance advisable. It is probable that it is only in the form of soluble salts that the plant can absorb its food. It is quite probable, however, at the same time, that the exact form in which the different food substances enter the plant may be largely determined by circumstances. According to Nobbe, chloride of potassium is the most suitable form of potassium salts, although the plant may absorb its potassium as sulphate, phosphate, or even silicate.

FOOTNOTES:

[61] Composition of the earth's solid crust in 100 parts by weight:--

Oxygen 44.0 to 48.7 | Calcium 6.6 to 0.9 Silicon 22.8 to 36.2 | Magnesium 2.7 to 0.1 Aluminium 9.9 to 6.1 | Sodium 2.4 to 2.5 Iron 9.9 to 2.4 | Potassium 1.7 to 3.1

(Roscoe's 'Lessons in Elementary Chemistry,' p. 8.)