Manures and the principles of manuring

by Charles Morton Aikman

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]