THE CHEMISTRY OF THE SOIL. »>!• 



Relation of Texture to Moisture. 



The ideal condition of the soil as regards moisture is obtained when the 

 soil contains about half the amount of water which it is capable of holding. 



For example, a fairly heavy clay soil will have on the average a capacity 

 for water of about 40 per cent. — that is to say, 100 lb. of such soil 

 will contain, when fully saturated with moisture, about 40 lb. of water. It 

 is generally agreed that the amount of moisture most favourable to plant 

 growth is something under half this amount, namely, about 18 per cent. If 

 a much larger proportion of water exists than this, the interstices of the soil 

 are filled with water instead of air, consequently there is a deficiency of 

 oxygen, which we have seen is one of the prime agents in promoting chemical 

 action within the soil. The soil becomes what is called water-logged, and 

 the chemical action which we have recognised as essential to fertility is at 

 a standstill. The roots of the plants, moreover, are immersed in water. 



The condition of things in a soil containing the proper amount of water, 

 and in good tilth, is pretty much as follows: — The minute grains of which 

 the soil is composed do not form a compact mass, but the intervening spaces 

 are so small that they act as capillary tubes, of the same nature as the pores 

 of a sponge or the little tubes of an ordinary wick, and their effect upon the 

 water present is exactly the same as the bundles of hollow tubes in the wick 

 — that is to say, they draw the water up by the attraction of the sides of the 

 tubes. Each grain of the soil will be then surrounded by a thin film of 

 water, which in its turn encloses and surrounds small bubbles of air. (In 

 the case of a waterlogged soil, these bubbles are driven out, and there is 

 little or no air in the soil.) In and out amongst the grains of soil the plant 

 pushes its roots and its root-hairs in search of food and moisture. If the 

 above rough description is at all clear it is obvious that the water in the soil 

 is continuous — that is to say, a sufficiently minute organism could pass 

 through the entire soil in the water, without ever having to touch a particle 

 of soil, or pass through a bubble of air. The result of this is that, supposing 

 a particle of water to be removed at any point, either by evaporation at the 

 surface or by absorption by means of the root-hairs, its place is at once 

 taken by adjacent water, and the whole of the water in the soil is at once 

 set in motion until equilibrium is again restored. It follows from this that 

 a crop with well-developed roots is itself an important factor in retaining 

 moisture within the soil, for, as the water is absorbed by the root-hairs at 

 any point and circulates through the plant, its place is taken by adjacent 

 particles of water, so that a steady flow of water is set up towards that 

 point. 



The evil effects of too much moisture, which cannot get away, have been 

 already mentioned. In addition, it is to be noted that this is one of the 

 most common causes of sourness in the soil. Sourness — that is, the forma- 

 tion of certain acids within the soil — is directly due to the absence of air 

 and oxygen, and can be remedied by the free admission of air, as by turning 

 over and exposing to the atmosphere. 



To prevent this accumulation of water, the remedy is to drain. In many 

 cases where it is due to the presence of a stiff clay subsoil, through which the 

 water cannot penetrate, subsoiling is resorted to; but it is also possible to 

 have too little water in the soil, with the result that the crops wither and die, 

 and this takes place when the evaporation equals or exceeds the absorption 

 by the roots. Plants absorb water only through their roots and root-hairs. 



