46 ELEMENTARY AGRICULTURAL CHEMISTRY 



into nitrates by oxidation brought about under the influence 

 of the micro-organisms in the soil. 



The physical properties of humus are also of importance. It 

 is a dark -coloured, bulky, porous substance, possessing con- 

 siderable tenacity or cementing power and a great capacity for 

 holding water, also for absorbing and retaining many import- 

 ant items of plant food — e.g., ammonia, potash and lime. 

 The dark colour causes soils rich in humus to become heated 

 by the sun's rays more rapidly than light-coloured ones. The 

 bulkiness and porosity permit of freer admission of air, and 

 60 promote oxidation. In clay soils these properties confer 

 lightness and permeability. The cementing power is particu- 

 larly important in sandy soils, to which it imparts coherence 

 and water-retaining power, while its absorptive power is of 

 great value in retaining for the use of the plant many soluble 

 substances which might otherwise be removed in the drainage. 



By the decay of humus, which is always going on, the 

 proportion of carbon dioxide in the soil water is increased, 

 and thus the solvent powers of the latter for plant food existing 

 in the mineral portions of the soil are enhanced. 



Chemical Changes occurring" in Soils.— The reactions 



taking place in a soil are very numerous, and of a type not 

 familiar to a student of elementary chemistry, the direction 

 of the change being largely dependent upon the conditions of 

 temperature, concentration, relative quantities of the reacting 

 substances, and other ciicumstances. 



The inorganic matter is subjected to the same influences as 

 lead to its breaking down in the formation of the soil from the 

 rock, but the changes go on at an accelerated rate, because of 

 the greater quantity of carbon dioxide produced by the decay 

 of the organic matter. Fragments of felspar, for example, are 

 probably decomposed in accordance with the equation 



A\fi,.-Kfi.QSiO, + CO, + lOH.O = Al,03.2SiO,.2H,0 

 + -KfiO, + 4H,SiO„ 



