III. THE SOIL 45 



The conductivity, however, becomes greater with an increase in 

 the coarseness and compactness of the material, as well as with an 

 increasing amount of moisture, since the bad conductivity of all powders 

 is due mainly to air spaces. 



The amount of water held by the constituents varies greatly ; thus, 

 100 parts by weight 



of sand were found to hold 25 parts of water 



cla Y . ,r 70 



,, fine calcium carbonate ,, ,, ,, ,, 85 ,, 

 humus 181 



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, which in most soils constitutes by far 

 the largest ingredient, profoundly affects their relations to water and 

 heat and thus their fertility. 



Clay. This term is loosely used. Sometimes, and by the agricul- 

 turist generally, it is employed to denote any earthy deposit which is 

 free from granular matter and which possesses plasticity. In the 

 mechanical analysis of soils, it is usually given to the smallest particles 

 present, all with a diameter less than -002 millimetre being considered 

 as clay, without reference to their chemical nature. Chemically, " clay," 

 if it is given a definite meaning, refers to hydrated aluminium silicate, 

 Al 2 O 8 .2SiO a .2H 2 O. 



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 (A1 3 . 

 2SiO 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 1-J- per cent of the whole, even in stiff clays, and it 

 is probably more hydrated than the rest of the kaolin (Schloesing). It 

 is upon the condition of this colloidal constituent that the peculiar 

 properties of clay mainly depend. If it be in a fully swollen condi- 

 tion, the clay is sticky and impervious, whereas, if it be coagulated 

 and shrunken, the clay loses its stickiness and becomes quite work- 

 able. 



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

 muddy liquid is obtained which shows practically no sign of deposit- 

 ing its suspended matter. By the addition of a small quantity of a 

 mineral acid, of many salts, or of lime-water, coagulation of the colloidal 

 clay occurs with complete separation as a flocculent precipitate. Salts 

 of calcium are as good as lime-water in causing this coagulation, while 



