28O SOILS AND FERTILIZERS 



products which remove the least fertility, or if those 

 are sold which remove large amounts, to return in 

 cheaper forms the fertility sold. In a good rotation 

 it is the plan to have at least one salable crop each 

 year. The whole farm need not undergo the same 

 rotation at the same time, and the rotation may be 

 subject to minor changes, as circumstances require. 

 To illustrate, wheat and flax occupy about the same 

 position in a rotation. If at seeding time the indica- 

 tions are that wheat will be a poor paying crop and 

 flax command a high price, flax should be sown. The 

 rotation should be such that one of two or three crops 

 may be grown as circumstances require. A rotation 

 should be reasonably flexible. 



339. Rotation Advantageous in Other Ways. A good 

 rotation will be found advantageous in other ways than 

 those mentioned. With one line of cropping, land be- 

 comes foul with weeds and insects which do not thrive 

 when crops are rotated. Frequently the rotation must be 

 planned so as to reclaim the land from weeds and rav- 

 ages caused by insect pests. Many insects are capable 

 of living only on a special crop ; when this crop is grown 

 continuously on the same land the best conditions for in- 

 sect ravages exist, and relief is secured only by rotation 

 of crops. Fungous diseases also are most liable to occur 

 on soils which produce annually the same crop, as the 

 conditions are favorable for the propagation and hiber- 

 nating of the disease-producing spores. 



