86 Pecan-Growing 



form nodules and live on the roots of leguminous plants, such 

 as clovers, peas, beans, beggarweed, vetches, and the like, 

 and have the power of feeding directly on the free nitrogen 

 of the atmosphere and thus fixing it in solid form. Each 

 group or class of legumes requires a specific strain or type of 

 nitrogen-gathering bacteria for most satisfactory growth, and 

 unless the soil is already inoculated the bacteria should be 

 introduced from commercial cultures or by adding soil from 

 fields producing good growths of such legumes. 



The amounts of plant-food in cover-crops vary with the kind 

 of plant and the amount of growth produced. The relative 

 quantities of the three most commonly deficient elements in 

 plants used for cover-crops have been determined by a large 

 number of investigators. 



It is decidedly difficult to give an accurate valuation of 

 cover-crops, since their infiuence on the texture and the water- 

 holding capacity of the soil may exceed the actual value of 

 the mineral elements of the plant-food which they contain. 

 However, for the sake of comparison they are valued on the 

 basis of their content of phosphoric acid, nitrogen, and potash. 



Since nitrogen has more than double the value of potash 

 and four times that of phosphoric acid, it is the chief element 

 of plant-food to be considered in growing a cover-crop to be 

 turned under. This is especially true as the average analysis 

 shows a much higher percentage of nitrogen than of either 

 phosphoric acid or potash. The grower, by knowing the com- 

 position of his cover-crop, can easily determine the fertilizing 

 value by calculation on the basis of his yields. 



Just what cover-crop the grower should use will depend 

 very largely on local conditions. Whether he shall select a 

 legume or a non-legume as a winter or a summer cover- 



