UNDERLYING PRINCIPLES 



139 



tion, money is saved which would otherwise need to be used in the 

 purchase of nitrogen in the form of manure or commercial fertilizer. 

 The growth of legumes may be so planned as to actually increase 

 the store of nitrogen in the soil. The growth of a legume with a 

 non-legume is shown in figures 91 and 92. 



2. Crops differ widely in their root systems. Some have very 

 shallow roots, as timothy and other grasses; while others have 

 deep roots, as alfalfa and clover. The growth of crops varying in 



FIQ. 91. Peanuts may be grown between the rows of corn as far north as Nebraska and 

 New Jersey. (Fights of the Farmer.) 



this respect on a certain field will make better use of the fertility, 

 and the plant food will not be exhausted so soon. 



3. Cover crops should be grown in the winter and usually in 

 other seasons after the main crops have been harvested. These 

 cover crops prevent the loss of soil and plant food by washing and 

 leaching. In a well-planned rotation a growing crop can be kept 

 on the field practically all the time. When but one crop is grown 

 the soil is often poorly protected. This does not apply, however, 

 where the single crop is grass or alfalfa. 



