MAINTAINING THE FERTILITY OF THE LAND 111 



older sections of the United States are thus abandoned 

 for a few years to recuperate to such an extent that a 

 small crop may be grown. A wiser way of farming would 

 be to begin to raise animals for manure production before 

 the soils become so exhausted. 



109. Causes of Decreased Productivity. (1) The fer- 

 tile surface soil may be carried away by erosion by wind 

 or water. Probably more soil fertility is lost in this way 

 than by cropping. This may be prevented by keeping the 

 soil in sod, by keeping cover crops on it during the win- 

 ter and by terracing the land as is done in the South. 



(2) The soil may cease to hold the proper moisture 

 supply. This may be remedied by drainage and tillage, 

 and by additions of humus. 



(3) The soil may cease to be favorable for the develop- 

 ment of soil organisms. This may be remedied as No. 2 

 and by the application of lime. 



(4) The nitrogen of the soil may be carried away in 

 drainage water or may escape to the air by denitrification. 

 Many conditions favor the activity of soil organisms that 

 decompose the nitrogen compounds and allow the nitro- 

 gen to escape as a gas. 



(5) The constant cropping may exhaust the available 

 supply of some plant-food. Each crop removes a certain 

 amount of nitrogen, phosphoric acid, or potash. In time 

 this may limit the available supply. Usually it is not a 

 shortage of the absolute amount of such food in the soil,, 

 but a shortage of that which the plant can secure in solu- 

 ble form. This may be remedied by drainage, tillage, 

 additions of humus, lime, fertilizer and manure. 



(6) The exhaustion of the humus supply is usually 



