THE ROTATION OF CROPS. 115 



eases that cause rot and blight. Crops lose vigor by 

 being grown year after year, and thus are less able to 

 withstand these attacks; besides, a change of crops de- 

 prives the pests of their particular kind of food, causing 

 them to disappear or perish. 



4. Certain crops derive their nitrogen, phosphoric acid, 

 and potash entirely from the soil ; the cereals grown 

 for their grain, which is usually sold, belong to this 

 class. Certain others, the clovers, peas, and beans, de- 

 rive their nitrogen from the atmosphere ; their removal 

 does not decrease the store of nitrogen in the soil. A 

 rotation of crops, including the latter, therefore, lessens 

 the necessity for nitrogenous manuring. 



5. A rotation of crops distributes labor throughout 

 the season, thus giving continuous work for men and 

 horses. In farming districts it is difficult to procure 

 labor for short periods, while horses have to be kept 

 throughout the year. 



6. A variety of crops marketed at different periods, 

 permits a steady and regular income to the farmer, which 

 enables him to do business on a smaller capital ; wages 

 can be paid when due, and his supplies of seeds, fer- 

 tilizers, implements, aud tools can be bought in the 

 lowest market for cash. 



It has already been stated that the adoption of what 

 are now " systems of crop rotation " was largely a matter 

 of growth, due to circumstances, and was not in the be- 

 ginning based upon scientific principles. Science, how- 

 ever, furnishes the reasons why rotations are useful, and 

 why certain rotations are more useful than others. 



The Need of Rotations. The need of rotation as 



