CHAPTER VI 

 THE SOIL PARTICLE 



The soil formed by the grind ing-up of rocks and the 

 intermixing therewith of small quantities of organic 

 matter must be studied physically from the standpoint 

 of its particles. These particles, varying in size from 

 coarse gravel easily discernible by the naked eye to 

 particles so fine as to be invisible under the ultramicro- 

 scope, determine very largely the different relationships 

 of the soil to the plant. The movement of air in the soil, 

 the circulation of water, the rate of oxidation and hydra- 

 tion, and the presence and virility of various organisms, 

 are determined very largely by the size of the particles 

 making up the soil. Texture is the term used to express 

 this size of particle. Thus a soil texture may be coarse, 

 medium, or fine, indicating that the particles making up 

 that soil conform in general to such description. Tex- 

 ture is of great importance in soil study and utilization. 



There is hardly any condition exhibited by the soil 

 that is not influenced, if not directly determined, by the 

 size of the soil particles. A study of plant conditions, 

 whether physical or chemical, ultimately leads either 

 directly or indirectly to a consideration of soil texture. 

 Texture, however, is an element which can be but little 

 modified under normal conditions. We have seen how 

 a rock can be disintegrated and decomposed into a soil. 

 A change in texture has been wrought, but such a process 

 demands geologic ages for its fulfillment. In the time 



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