10 



sufficient increase in yield of vines to pay the cost. At the Delaware 

 Station IGO pounds of muriate of potash per acre doubled the yield of 

 vines, and superphosphate produced no effect. At the Georgia Station 

 combinations of superphosphate and potash gave the best results, but 

 later experiments there indicated that large amounts of potash are 

 unprofitable, and that superphosphate at the rate of from 200 to 400 

 pounds per acre gave better results. Superphosphates are very much 

 preferable to untreated rock phosphate. The latter can be sold at 

 much lower rates, and it remains to be seen whether it would not be 

 a profitable method to apply the soft phosphate to the cowpeas for the 

 benefit of the succeeding crop in the rotation, for it has been found 

 that the insoluble phosphoric acid of the untreated rock becomes 

 changed to forms available as plant food in the presence of large 

 amounts of decaying vegetable matter in the soil. If it is found that 

 this process can be relied upon, then the cowpea will have another 

 valuable quality added to it, namely, that of being able to change 

 into high-grade and more costly superphosphate the low-grade and 

 cheap but unavailable phosphoric acid of the untreated rock. 



The chief functions of this crop, then, are to furnish large amounts 

 of nitrogen abstracted from the air and fixed in the roots and stubble 

 in a conveniently available form for the use of succeeding crops; sec- 

 ond, to produce a large yield of vines and peas rich in digestible pro- 

 tein, which, either as haj^ or for soiling purposes, will take the place of 

 concentrated nitrogenous foods; and, third, to supply humus, which 

 acts directly and indirectly to produce fertility by breaking down and 

 rendering available the basic minerals of the soil. The fertilizing 

 value of the nitrogen in the vines is entirely dissipated or greatly di- 

 minished by weathering when they are left on the surface of the field 

 during the winter. Hence, to secure the full value, the cowpeas should 

 be fed and the stable manure returned to the field. If the vines are 

 plowed under in autumn, a winter forage crop, such as winter oats, 

 -crimson clover, rye, or vetches, should be planted to prevent the 

 leaching and washing action of the winter rains. 



Jared G. Smith, 



Assistant A{/rostolo</ist. 

 Approved : 



James Wilson, Secretary. 



Washington, D. C., November 26, 1897. 



