CHAPTER III 

 WHAT THE SOIL SUPPLIES 



17. Chemistry of the soil. The soil is a mixture of many 

 substances that have come from various sources. In the 

 first place, its original material and relatively permanent 

 part consists of material derived from rocks in various ways, 

 such as particles of sand, clay, etc. If during its history it 

 was submerged in sea water or fresh water, it became mixed 

 with shells of water animals, which contributed calcium salts 

 (carbonates and phosphates). When it began to be covered 

 with vegetation and the plants contributed their bodies to 

 the mixture, there was a slow accumulation of this organic 

 material (humus) which gave a dark color to the soil mix- 

 ture. To all of these substances the water of the soil must 

 be added as of great importance, containing in dilute solution 

 the substances of the soil that are soluble. 



The differences among soils are brought about by the 

 various proportions in which these substances occur in the 

 soil mixture, and it is evident that the combinations are 

 numerous. Soils are spoken of as sandy soils, clay soils, 

 lime (calcareous) soils, alkali soils, acid soils, humus soils, 

 etc., because they are rich in sand, clay, lime, salts, acids, 

 humus, etc. Humus soils, rich in organic material, may be 

 too acid for plants of ordinary cultivation because there is 

 not enough lime to neutralize the acids from decomposition ; 

 but when there is sufficient lime, the humus is rendered 

 neutral and is very favorable for plants. 



The effects of these numerous substances upon on'e another 

 are intricate and poorly understood, but enough is known 

 to assure us that chemical changes are always in progress, 



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