CHAPTER VIII 



MAINTAINING SOIL FERTILITY 



Thus, when a field is poor, it is a custom, instead of manuring it, to plow in 

 a crop of lupines before the pods appear sometimes a crop of beans before 

 the pods are so far advanced as to render the fruit fit for being gathered. 

 VARRO 



89. How the soil is wasted. Soil productivity may be de- 

 creased through tillage, by the removal of plant food through 

 cropping or by leaching or by washing away of the surface soil. 

 Under the systems of agriculture common to the greater part of 

 the country, all three of these means of wasting the soil are 

 important. It is impossible entirely to exhaust the plant food of 

 a soil under any system of agriculture, since profitable culture 

 ceases long before a complete state of exhaustion is reached. 



90. Tillage wastes the soil. Soils covered with natural vege- 

 tation slowly increase in organic matter and in nitrogen. There- 

 fore soils in their virgin state contain the maximum quantity 

 of organic matter, and this in turn sets free abundant supplies 

 of nitrates and makes the other plant foods available. This is 

 why new soils are usually very productive. In the rapid decay 

 of organic matter, however, large quantities of nitrogen are lost, 

 partly through the leaching 1 of nitrates (especially from porous 

 soils), but principally through the escape of nitrogen into the 



1 As water passes down through the soil, it carries with it soluble plant 

 food, a part of which may be permanently lodged in the subsoil, and a part 

 leached out in the drainage water. 



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