CONSTITUENTS OF SOIL. 19 



Clay is a hydratecl silicate of aluminium, produced by 

 the decomposition of felspar and other silicates. Pure clay 

 is a colloid body ; the amount present in any soil is extreme- 

 ly small. The purest natural clays are mainly composed of 

 a very line sand, which has the same general composition 

 as the true clay associated with it. Pure clay is a powerful 

 cement, causing the coarser particles of the soil to cohere. 

 A pure silicate of aluminium would furnish no food to a 

 plant ; clay always, however, contains some potash, and 

 frequently a notable quantity. It has also the important 

 property of absorbing and retaining phosphoric acid, am- 

 monia, potash, lime, and other substances necessary for 

 plant nutrition. 



Carbonate of calcium is beneficial to the soil in many 

 ways. It preserves the clay in a coagulated condition, 

 thus making heavy soils friable and pervious to water. It 

 enables clay to exercise its absorbent power on various salts, 

 which would otherwise escape its action. It also promotes 

 the decomposition of vegetable matter, and the formation 

 of nitrates in the soil. The presence of some salifiable 

 base is essential for the performance of the chemical 

 operations belonging to a fertile soil ; the salifiable bases 

 usually present are either carbonate of calcium, or the 

 alkalis derived from the decomposition of silicates. The 

 calcareous matter of soil supplies lime to the plant; lime- 

 stone also generally contains phosphoric acid. 



The humus, or decayed vegetable matter of soils, has 

 its origin in the dead roots and leaves of a previous 

 vegetation, or of a previous organic manuring. It acts 

 effectively as a cement in sandy soils, holding the particles 

 together ; in a clay soil it increases the porosity. Humus 

 is the principal nitrogenous ingredient of soils. A black 



