16 



THE SOIL AND THE PLANT 



industry. Ground bone may also be utilized as a source of this 

 element. Potassium is used as the sulfate and chloride which are 

 obtained from natural salt deposits. 



Continuous cultivation of soils may lead to a depletion of the 

 basic substances and to the development of an acid reaction, 

 particularly where artificial fertilizers are injudiciously used. 

 Both of these conditions are unfavorable to the development of 

 many of the important agricultural plants. To offset these 

 effects, calcium is applied in various forms such as the oxide, 

 hydroxide, or carbonate. Its addition to the soil is not so much 

 for the purpose of overcoming a deficiency of an element 

 required for plant growth as to correct certain other unfavor- 

 able soil conditions. 



An idea of the abundance of some of the most important 

 constituents in soils may be derived from Table 6. Although 



TABLE 6 



Concentration of Important Constituents in Mineral Soils 

 (from Lyon and Buckman) 



these data suggest that a soil may have sufficient nutrients to 

 supply the requirements of large crops for many decades, this 

 conclusion is not justified. Very small amounts of these sub- 

 stances are available to plants at any one time. Most of the 

 nitrogen is locked up in organic compounds which are slowly 

 decomposed by microbes. The phosphorus may exist in organic 

 combination, in mineral materials such as apatite, and as other 



