CHAPTER XXII 
CROP ROTATION 
682. The necessity. — Crop rotation has long- been 
recognized as a necessity. Its value has been more evi- 
dent, perhaps, in the production of cereals and the gen- 
eral farm crops than in vegetable growing, but it is 
scarcely less important. Numerous examples might be 
cited of decline in yields and quality, due primarily to 
the failure to rotate. In some instances entire communi- 
ties have been forced practically to abandon the culture 
of a certain crop mainly because of its continuous or too 
frequent production. 
683. Relation to food supply. — Plants differ greatly in 
their food requirements. Some utilize large amounts of 
nitrogen, while others must have liberal supplies of the 
mineral elements. Crops which have about the same 
food requirements should not be planted consecutively, 
if it can be avoided. When cabbage follows cabbage, or 
lettuce follows lettuce, a heavy draft is made upon the 
supply of nitrogen. If, instead, these crops were followed 
by tomatoes the food supply would be utilized more 
economically, successful crop production would be more 
certain and the yielding power of the soil would be con- 
served to better advantage. Many examples might be 
given to illustrate this point. While this principle should 
always be considered, certain conditions, as the use of 
muck soils or the accessibility of cheap supplies of 
manure, may justify a system of cropping which would 
not be permissible under other circumstances. 
684. Relation to humus. — Crops differ widely in their 
ability to supply humus to the soil, and therefore to 
change the physical as well as the chemical properties 
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