276 ROCKS AND SOIL 



can be guarded against. One of them is the too rapid 

 removal of soluble compounds, by drainage. This is 

 often due to improper cultivation of sloping ground. 

 Nitrogen compounds, especially, disappear in this way; 

 for they are easily converted into nitrates of metals (c/. 

 220), which are very soluble. 



Nitrogen leaves a soil not only because its compounds are dissolved, 

 but because they are decomposed, so that the nitrogen escapes back into 

 the atmosphere (c/. 55). It was found, in Minnesota, that a long- 

 continued cultivation of wheat upon a piece of land caused 6 times as 

 much nitrogen to leave the soil as was removed by the crops harvested. 



If certain crops are grown too long upon a piece of land, its soil 

 becomes acid, or "sour." This is one of the common causes of loss of 

 fertility (c/. 302). Attempts to grow clover, alfalfa, etc., upon acid 

 soil are almost sure to fail. 



300. Preserving and Restoring Fertility. While we 

 cannot help removing fertility when we remove a crop, 

 yet the loss is often much greater than it should be. 

 Farmers are learning that, as far as is possible, they must 

 put back upon the land everything not actually sold from 

 it. Hence they need to preserve carefully, and to use, 

 manure, the most common of farm fertilizers, so as to 

 restore humus to the soil. 



Another great aid in the preserving of fertility is good 

 management. A farm cannot pay a profit if it sells the 

 fertility of its soil at a low price. The crops that are sold 

 should be those that take the least possible fertility from 

 the farm. The crops that take much fertility, and them- 

 selves bring a low price, should not be sold from the farm, 

 but should be converted into products that have a greater 

 market value. Thus, when grains are cheap, they should 



