THE SOIL AND SOIL FORMATION 53 



which allows water to enter, holds to the water as 

 against either seepage or evaporation, conserves fer- 

 tility, and yet allows the roots of crops to penetrate 

 easily and supplies them with that combination of water, 

 air and plant food which makes them thrive. Any rea- 

 sonable proportion of the coarser and finer particles, as 

 one-fourth clay and three-fourths sand, or three-fourths 

 clay and one-fourth sand, provides good physical condi- 

 tions for crops. Soils arising from many kinds of rocks, 

 the particles mixed by various agencies in varying pro- 

 portions and reasserted into layers, are of many types. 

 The various sizes of particles, the substances of which 

 the particles from different rocks are composed, the 

 proportion of organic matter and other characters, en- 

 able soils to be classified in rather definite groups. 



Whitney of the Bureau of Soils of the United States 

 Department of Agriculture, under whom soil surveys 

 of many counties have been made, has classified the 

 soils of the country under several hundred types, and 

 expects to add still more types. This is an artificial 

 classification made for convenience in mapping soils and 

 in studying farm management, and is necessarily less 

 definite than the classifications that botanists and 

 zoologists have made of plants and animals. Living 

 organisms have had the living force of heredity plus the 

 inorganic forces to develop natural grouping, while soils 

 have been grouped by the common physical forces 

 alone. 



Areas of types of soils. The elevation and depression 

 of the earth's crust, causing the shifting of seas and 

 changes in the direction of the flow of streams ; the flow- 

 ing of surface waters; the action of winds, glaciers and 

 other agencies, have in one place made mixed soils and 

 in another soils of only one kind or size of materials. 

 Some soils are thus very simple and others very 

 complex. 



