124 FOOD FOR PLANTS 



legumes is one-half of one per cent. In regard to the 

 amount of Nitrogen supplied from the nodules 

 formed on the roots it is practically impossible to 

 figure this correctly as it depends upon the root struc- 

 ture of the plant and the quantity of nodules formed. 

 Also when figuring the Nitrogen value of legumes it 

 must be taken into consideration that legumes take 

 Nitrogen from both the soil and the air. 



As soil Nitrogen becomes available it is taken up 

 by the legume and is turned back to the soil in the 

 form of organic Nitrogen which requires to be de- 

 composed before it can again be used as plant food. 

 In this transaction 63 per cent of original soil Nitro- 

 gen is lost. Legumes, however, take Nitrogen from 

 the air and by so doing offset this loss of soil Nitrogen 

 and supply a small amount of organic Nitrogen in 

 addition. 



It must be remembered that the main value of cover 

 crops is the organic matter they supply. The Nitro- 

 gen can be supplied and in a more available and eco- 

 nomical form. 



There are two main divisions of cover crops: 

 Summer cover crops, which should be of the shallow 

 root variety, and winter cover crops which are of the 

 deep rooting variety. 



Summer cover crops should be plowed in before 

 the second week in July. After that time they will 

 compete with the fall growth of the trees — a danger- 

 ous thing when quality and maximum production is 

 desired. This is also true of winter cover crops, 

 which should be plowed under by the first of 

 March. 



