FOOD REQUIREMENTS OF CROPS 267 



Buckwheat is ( usually sown upon the poorest soil of the 

 farm. Because it is a strong feeder it is frequently 

 used as a manurial crop, being plowed under while 

 green to serve as food for weaker feeding crops. On 

 poor soils a moderate use of mineral fertilizers and a 

 small amount of nitrogen are beneficial. 



320. Cotton. On average soils cotton stands in need 

 first of phosphoric acid and second of nitrogen. 81 It is 

 most able to obtain potash. Organic nitrogen as cot- 

 tonseed meal and stable manure appear equally as 

 effective as nitric nitrogen. Phosphoric acid must be 

 applied in the most available forms, although the crop 

 uses but little. The fertilizers should be drilled in at the 

 time of planting. The use of green manuring crops as 

 cowpeas, with an application of marl, gives beneficial re- 

 sults. Marl, which is composed mainly of calcium car- 

 bonate, combines with the acids formed from the decay 

 of the vegetable matter and as a result the plant food of 

 the soil is made more available, which is beneficial to 

 both soil and crop. There are but few crops which 

 respond so readily to fertilizers as cotton. It does 

 not remove a large amount of fertility, but when not 

 systematically grown in a rotation exhausts the soil 

 in the same way as when grain is grown continuously. 



321. Hops. - - The hop plant is exacting in its food 

 requirements. An excess of easily soluble plant food is 

 injurious, while a lack is equally so. An abundance of 



