CHAPTER XXIV 

 THE FERTILITY OF THE SOIL 



The Exhaustion of the Soil by Plants would occur 

 in a comparatively short time unless there were some 

 way to replace the substances which they remove. In an 

 earlier chapter it was shown by chemical analysis that the 

 bodies of plants contain the elements carbon, hydrogen, 

 oxygen, nitrogen, phosphorus, sulphur, potassium, mag- 

 nesium, calcium, iron, silicon, sodium, and chlorine. It 

 was further demonstrated by the water culture method 

 that all of these except the last three are indispensable to 

 green plants. All of them except carbon and oxygen are 

 derived entirely from the soil in water solution. Suitable 

 soluble salts of nitrogen (nitrates)'; phosphorus (phos- 

 phates), and potassium are scarce in most soils. Hence 

 their prompt return to it in a form suitable for use is a 

 prime requisite in maintaining the fertility of the soil. 



Natural Fertility. — In a state of nature the com- 

 pounds removed from the soil are, after a longer or shorter 

 sojourn in the bodies of plants or animals, usually re- 

 turned to it in the same vicinity, where they are acted on 

 by a series of microorganisms and fitted for use again. 

 Under such conditions there is no exhaustion of fertility, 

 but there may actually be an increase of it, by means to 

 be explained presently. 



Fertilizers. — Under cultivation it usually happens 

 that the crops are removed from the fields and little or 

 none of their substance returned. The loss thus sustained 

 must be made good by the application of fertilizers. By 

 this term the agriculturalist means compounds or mix- 

 tures which contain the elements in which the soil is 



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