84 BACTERIA AND SOIL FERTILITY 



ging down to varying depths, depending upon the soil, one may 

 iind native rock similar to that from which they were formed. 

 However, these soils often consist of only a small fraction of the 

 native rock from which they were derived. For example, a soil 

 derived from a limestone composed of 6o per cent of calcium 

 carbonate and 40 per cent of impurities may on complete weath- 

 ering consist almost entirely of the impurities plus varying quan- 

 tities of organic matter. 



(2) Transported Soils — These are soils the material of which 

 has been moved from its native origin by means of gravity, water, 

 ice, or wind. These soils are known as colluvial or gravity-laid 

 soils, sedimentary or water-laid soils, glacial or ice-formed soils, 

 and eolian or wind-formed soils, depending upon the means used 

 in their transportation. They vary in composition, depending 

 upon the native rock from which they were derived as well as 

 upon the material added to or subtracted from them in transpor- 

 tation. About 90 per cent of the soils so far mapped in the 

 United States are transported soils. 



Soils may also be classed as to physical composition as (i) 

 sand, (2) sandy loam, (3) loam, (4) clay loam, (5) clay, and 

 (6) peat. This classification is based upon the size and com- 

 position of the ultimate particles which compose the soil. 



Soil Texture. — Soil texture refers to the size of the particles 

 which compose a soil. We may have soils of fine or coarse tex- 

 ture just as we have cloth of fine or coarse texture, depending 

 upon the size of the particles which compose it. It has been 

 estimated that a sandy soil containing only 4.77 per cent of clay 

 contains 2,000,000,000 particles in one gram, whereas a soil 

 containing 32.45 per cent of clay contains 15,000,000,000 per 

 gram. Soils adapted to the growth of various crops contain vary- 

 ing numbers of soil particles, as is shown in the following. 



Early truck 1,955,000,000 per gram 



Truck and small fruit 3,955,000,000 per gram 



Tobacco 6,786,000,000 per gram 



Wheat 10,228,000,000 per gram 



Grass . 14,735,000,000 per gram 



