506 



FIELD CROPS 



bulkier and less valuable products of the farm into more con- 

 centrated and more readily salable products. With proper 

 care given to the manure, a large part of this vegetable matter 

 may be returned to the soil. While grain crops and cultivated 

 crops are' exhaustive of vegetable matter, grass crops, 

 because they have extensive root systems, materially increase 

 the vegetable matter in the soil. 



Fig. 150. Samples of soil from (1) a grass plot, and (2) from one which has 

 been in corn continuously for a number of years. Note the absence of vege- 

 table matter in the sample from the corn field. 



662. Rotations Lessen Damage from Insects and Dis- 

 eases. Most of the r plant diseases and injurious insects are 

 decidedly limited in the number of plants on which they can 

 live. Many of them are destructive to only one of the crops 

 commonly grown. They are not generally capable of move- 

 ment for any considerable distance during a season, but 

 increase very rapidly from year to year if a single crop is 



