CHAPTER VI. 



PHYSICAL COMPOSITION OF THE SOILS. 



As has already been stated (chapt. i, p. 10), the general 

 physical constituents of soils are rock powder or sand and silt, 

 more or less decomposed according to the nature of the orig- 

 inal rocks; cla\, the product of the decomposition of feldspars 

 and some other silicates; luinnis, the complex product of the 

 decomposition of vegetable and animal matters on and in the 

 soil mass; as well as vegetable matter not yet humified. Each 

 of these several constituents must now be considered more in 

 detail. Since clay is the substance whose functions and 

 quantitative proportions influence most strikingly the agri- 

 cultural qualities of land, it should be first discussed. 



Clay as a Soil Ingredient. 



The plasticity and adhesiveness of clay, together with the 

 extreme fineness of its ultimate particles (said to reach the 

 1-25000 of an inch), explains its great importance as a 

 physical soil ingredient. It serves to hold together and im- 

 part stability to the tlocculent aggregates of soil particles that 

 compose a well-tilled soil; for without clay the sand would 

 collapse into close-packed single grains so soon as dried, and 

 loose tilth would be impossible. Sand drifts illustrate this 

 condition. 



On the other hand, the fineness of the particles serves to 

 render clay very retentive of moisture as well as of gases and 

 of solids dissolved in water, imparting these important prop- 

 erties to soils containing it; while coarse sandy soils are often- 

 times so deficient in them as to render them unadapted to any 

 useful culture, despite the presence of an adequate supply of 

 plant-food. 



\Yhen to these essential physical properties of clay, there is 

 added the fact that usually the clay-substance as it exi.-ts in 



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