KINDS OF SOIL 191 



(c) Soil and Subsoil. — In humid regions there is a con- 

 siderable difference between the soil and the subsoil. The 

 most striking contrast is in the amount of organic matter. 

 In fact a common way of distinguishing between them is to 

 note the dividing line between the dark soil and the light 

 subsoil. It is frequently very sharply defined. This means 

 of course that there is more nitrogen in the soil than in the 

 subsoil. In addition there is found ordinarily more phos- 

 phoric acid and total lime in the soil than in the subsoil. 

 There is, however, less potash, ferric and aluminium oxides, 

 and calcium carbonate. The finer clay particles are washed 

 down into the subsoil, which results in finer textured sub- 

 soils than soils. This fact also accounts for the greater 

 amount of potash, ferric and aluminium oxides, and calcium 

 carbonate in the subsoil, since these constituents are more 

 rapidly weathered on the surface and washed into the 

 subsoil, where they are fixed. In the soil there are more 

 bacteria and bacterial food — organic matter — consequently 

 there are more organic acids and carbon dioxide. The latter 

 makes a stronger reagent of soil moisture and as a result 

 greater availability of plant food. Being more open and 

 porous and of better texture than subsoil, the soil is in better 

 physical condition for crop growth. Aeration being better, 

 all compounds are in a higher state of oxidation, iron, for 

 example, is all in the ferric form. This is not always the case 

 in subsoils. Iron mav be in the ferrous form, and such 

 compounds as ferrous sulphate which may be derived from 

 the imperfect oxidation of iron sulphide or pyrites, are 

 poisonous to plants. It very often happens that when too 

 much subsoil has been turned up in plowing the crop is 

 very poor. This may be due to ferrous salts, to poor texture 

 — too compact and badly aerated — to lack of organic matter. 

 This latter lack reduces the rate of availability of plant 

 foods. 



In arid regions there is very little difference between 

 soil and subsoil. Organic matter extends to considerable 

 depths, due to deep root penetration. There is no sharp, 

 dividing line. The texture is the same throughout, lack 

 of water causing little clay formation, and the soils are 



