THE CHEMISTRY OF THE SOIL. 67 



amount of fertilising material in the soil, even if we could say how much 

 was actually available for any particular crop, is not all that is wanted if 

 we are to judge of the soil's fertility. 



The fertility of a soil depends, then, in the first place, upon the presence 

 of a sufficiency of plant-food, and, secondly, upon certain physical charac- 

 teristics, possessed more or less by all soils, which effect the splitting up of 

 the mineral ingredients and nitrogenous matter in such a way as to render 

 them available to plants, as well as regulating the supply of water, warmth, 

 A:c. 



Of these physical characteristics the most important are — 

 The texture or porosity of the soil. — On this characteristic depends a large 

 number of the properties conducive to fertility. 



By the porosity of the soil is meant the fineness and the number of its 

 pores. We must distinguish between this and permeability to water; a 

 coarse sand, for example, is permeable to water, but possesses properties 

 exactly opposed to those of a porous soil. Humus soils are especially porous. 

 On the fineness of texture depend the following characteristics: — 



The capillary power, by which is understood the power of imbibing water. 

 This property maintains a continual circulation of water within the soil, and 

 consequent aeration. It is, moreover, largely through the agency of this 

 circulating water, which is charged with carbonic acid and different salts, 

 that the mineral and, in a less degree, the organic matter of the soil is 

 rendered available for plant-food, and presented in solution to the plant. 



The capillary power of a soil depends very largely upon the fineness of its 

 texture. The nearer the texture approaches that of a sponge the greater will 

 be its capillarity. 



Humus has a very high capillary power, which is not possessed to any 

 extent by either coarse sand or clay. 



The capacity of the soil for water is also of special interest, and depends 

 partly upon its porosity and partly on its content of organic matter. Peaty 

 and humus soils, otht?r things being equal, have the highest capacity for 

 water, followed in order by marls, clays, loams, and sand. 



The hygroscopic power — that is, the power of attracting water vapour — is 

 of practical importance, in that it prevents undue evaporation, and prevents 

 the soil from becoming parched up. It also serves as a guide to the absorp- 

 tive power for other gases. This property, like capillarity, is due entirely to 

 the fineness of texture, and the order is the same — humus, clay, loam, marl, 

 sand, and coarse sand. 



The absorptive power of the soil for salts is another factor of very great 

 importance in determining the fertility of a soil. This power which soils 

 ' possess of removing saline matter from solution, and retaining it within 

 their pores, is due partly to the chemical nature of the soil, resulting in a 

 chemical interchange of basic constituents, and partly to its mechanical 

 structure, the fineness of its texture, substances such as humus and clay 

 possessing the power in a remarkable degree. 



Nitrification. 



"We now come to the most important property possessed by soils as affecting 

 their fertility, and, at the same time, the most obscure, namely, their power 

 of nitrification. This property depends upon a number of points on some 

 of which our information is not very clear. 



