56 KNAPP METHOD OF GROWING COTTON 



under. The humus in vegetable matter has a 

 value in soil renovation frequently greater 

 than its value as a plant food. The most 

 important sources of nitrogen used in com- 

 mercial fertilizers are as follows: 



TABLE III. NITROGEN CONTENT OF IMPORTANT FERTIL- 

 IZER MATERIALS 



Phosphoric Acid. Next in importance as a 

 plant food is phosphoric acid. It is largely 

 required by the plant for growth, but it is 

 absolutely essential in promoting fruitage and 

 is a great factor in hastening the maturity of 

 the crops. The principal commercial sources 

 of phosphoric acid are as follows : 



Table IV. PHOSPHORIC ACID CONTENT OF IMPORTANT 



FERTILIZER MATERIALS 



