Climate, Soils and Rotation 105 



produce are now high enough so that this can be done. 

 The great danger which the history of American farming 

 has shown is that the temptation to sell off cash crops 

 too closely is so great that attention to maintaining fer- 

 tility is put off too long. Animal husbandry has the 

 advantage in practice that the return of the manure to the 

 soil is a necessary part of farm work and soil robbery is 

 less likely to result. 



The third way to return organic matter to the soil is 

 to return the entire product of any crop. Any crop may 

 be used for this purpose, although, in practice, those used 

 are usually of low cash value. Great need of soil organic 

 matter and increase in yield incident to such a valuable 

 cash crop as the potato may make it advisable to plow 

 under crops like clover which would be inadvisable in 

 other crop rotations of less value. The term "green- 

 manure" refers to crops plowed under before maturity. 

 The term "catch-crop" refers to one grown between 

 crops raised for sale or feed or for other uses. The term 

 "cover-crop" is usually restricted to those grown to 

 occupy the soil during the winter. In addition to supply- 

 ing organic matter, any of these may cover and protect 

 the soil and so prevent losses by erosion and leaching. 

 A crop plowed under while still green and immature, and 

 which contains such a large proportion of water that it 

 decays quickly, may give only a small net gain over the 

 cost of seed and planting and the amount of organic 

 matter destroyed by tillage. Green-manures must be 

 used with caution wherever their use is likely to reduce 

 the soil moisture for the following crop. Rye plowed 

 under in spring is often injurious in this way. The 

 amount of water needed for large crops of potatoes is so 

 great that early spring plowing or surface tillage is usually 



