REFERENCES 197 



lies in the fact that lack of water in arid regions results in 

 rock decomposition products which do not contain kaolinite 

 or the basis of clay; kaolinite being essentially a hydrated 

 mineral. Soluble plant food is not leached, and the soils are 

 of good texture, needing only water for maximum production. 

 Humid soils on the other hand are produced largely by the 

 action of water and may contain considerable quantities of 

 clay, with not so great an amount of soluble plant food 

 material. Sand soils are composed of coarse particles of 

 rock, usually of silica, but not necessarily so. Thev are not 

 very fertile and do not contain very much organic matter. 

 Clay soils consist of very fine particles of rock, largely of 

 kaolinite. Their composition ordinarily is much better than 

 that of sand. Loam soils are popularly soils which are of 

 good texture and well supplied with organic matter. Peat 

 soils consist of the undecomposed remains of organic matter 

 and but little mineral matter. Muck soils contain less organic 

 matter and are generally somewhat more fertile. In humid 

 regions there is considerable difference between the soil and 

 the subsoil. There is more organic matter, nitrogen, phos- 

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

 The other common constituents occur to a greater extent in 

 subsoils. In arid regions there is but little difference between 

 the soil and the subsoil. Soil is more open and porous, 

 organic matter is distributed to a greater depth, aeration is 

 more uniform, and the compounds are in a higher state of 

 oxidation in the subsoils of this region than in those of the 

 humid region. Alkali soils are those which contain excessive 

 amounts of soluble salts. "White alkali" consists largely 

 of sodium chloride, sodium sulphate, with some chlorides 

 and sulphates of calcium and magnesium. "Black alkali" 

 is partly sodium carbonate, a solution of which dissolves 

 humus from the soil and colors the salts black. White alkali 

 is not so injurious to vegetation as is black alkali. 



REFERENCES 

 See references at end of Chapter VI. 



