CLAY. 51 



It should be noted that the amount of water held by any 

 powdered substance varies with the fineness of its particles ; 

 very fine sand, for example, will hold more than twice as much 

 water as a coarser sand. 



It is thus apparent that sand, \\hich in most soils con- 

 stitutes by far the largest ingredient, profoundly affects their 

 relations to water and heat and thus their fertility. 



Clay has characteristic physical properties which greatly 

 affect any soil in which it forms a large constituent. As has 

 been already stated, clay usually contains undecomposed or 

 partially decomposed silicates, e.g., felspar, containing potash, 

 iron, and lime. It thus serves as a source of plant food. 

 When carefully examined, it is found to consist of exceedingly 

 fine particles, which in typical clay are kaolin (Al. 2 O 8 .2Si0 2 . 

 2H. 2 0), but in common clays often partly quartz, felspar, or 

 even (as in marls) calcium carbonate, cemented together 

 with a colloidal form of kaolin, whose particles are so small 

 as to escape detection even under the microscope. This 

 colloidal or jelly-like form of clay only constitutes about one 

 and a half per cent, of the whole, even in stiff clays, and it is 

 probably more hydrated than the rest of the kaolin (Schloes- 

 ing). It is upon the condition of this colloidal constituent 

 that the peculiar properties of clay mainly depend. If it be in 

 a fully swollen condition the clay is sticky and impervious, 

 whereas if it be coagulated and shrunken the clay loses its 

 stickiness and becomes quite workable. 



If pure clay be mixed with a large quantity of distilled water 

 a muddy liquid is obtained which shows practically no sign of 

 depositing its suspended matter. By the addition of a small 

 quantity of a mineral acid, of many salts, or, best of all, of 

 lime-water, coagulation of the colloidal clay occurs with com- 

 plete separation as a flocculent precipitate. Salts of calcium 

 are nearly as good as lime-water in causing this coagulation, 

 while alkaline solutions tend to favour the diffusion of the 

 colloidal clay and will even destroy the flocculation produced by 

 acids or salts. 



It is this action of lime or salts upon colloidal clay which 

 causes the improvement in the texture of clay soils which is 



E 2 



