CLASSIFICATION OF SOILS 73 



2. Lithological. In many cases soils have been classified 

 according to the rocks from which they have been derived or upon a 

 lithological basis. Kocks of the same name are so different in com- 

 position and are exposed to so many and such varying conditions 

 and agencies of change that they may give rise to very different 

 soils. A soil derived from a granite may be very fertile under one 

 set of conditions or almost absolutely sterile under another. 



3. Temperature. Besides breaking down rocks into soil mate- 

 rial and aiding solution slightly, heat does not play such an impor- 

 tant part directly in the formation of soil, but indirectly through 

 its effect and influence upon other agencies, temperature is of the 

 greatest importance. Moderately high temperatures influence the 

 growth of plants and bacterial action as manifested in oxidation 

 and humification of organic matter. This brings about most impor- 

 tant physical, chemical, and biological differences. On the basis 

 of temperature soils may be divided into (a) tropk-, (b) subtropic, 

 (c) temperate, (d) subarctic, and (e) arctic. These are only very 

 general and have but little significance in a system of classification. 



4. Moisture. Moisture is not only a very important factor in 

 breaking down rocks into soil material, but it brings about very 

 fundamental changes in the soils themselves, both chemically and 

 physically. The presence of moisture is necessary for all chemical 

 changes, hence decomposition of minerals can take place only when 

 water is present. It is usually accompanied by the formation of 

 soluble compounds that are leached out and carried away, and fre- 

 quently to such an extent as to leave the soil deficient in plant food. 



Soils are sometimes divided into arid, where the annual precipi- 

 tation is less than 10 inches; semi-arid, 10 to 20 inche-*; sub-humid, 

 20 to 30 inches; humid, more than 30 inches, and super-humid, 

 including swamps. There can be no distinct line of difference 

 between the soils of such groups. In some parts of India, with 

 a rainfall of 28 inches, the conditions are extremely arid because the 

 rainfall comes in a very few niontbs and as torrential showers, 

 resulting in much loss by run-off. The evaporation is very great 

 during the rest of the year. In parts of Texas, with a rainfall of 

 30 inches, it is much more arid than in North Dakota with the same 

 rainfall, because of the character of the rainfall and the greater 

 evaporation in the former. 



Great differences exist between soils of arid and humid regions, 

 primarily due to the amount of rainfall. The moisture as well as 

 the tempejature influences the amount and character of organic 



