MAKING THE SOIL PRODUCE CROPS 



strictly dependent upon their presence in avail- 

 able form. Of these elements, those most 

 likely to be deficient either in total amount or 

 in availability are nitrogen, phosphorus, potas- 

 sium and calcium. It is entirely feasible and eco- 

 nomical to apply concentrated chemical fertilizers 

 containing the first three elements so that their 

 lack will not constitute a limit to size of crops 

 harvested. In many cases it is necessary to 

 apply chemical fertilizers to get satisfactory 

 yields, even where natural manures are available 

 and can be applied as well. 



In addition to supplying essential plant food, 

 nitrogen, phosphorus and potassium perform 

 specific functions in plant growth. The applica- 

 tion of nitrogen in one of its readily available 

 forms (e.g., nitrate of soda and sulphate of 

 ammonia) will stimulate vegetative growth. If 

 too much of this one element is applied, leaf and 

 branch development may occur at the expense 

 of the crop. Good results follow the use of 

 nitrogen on grass sods and on leafy vegetables 

 like spinach. On the other hand, corn, peas, beans 

 and other seed-forming crops need to have the 

 nitrogen balanced with phosphorus. Potatoes, 

 in common with other tuber and root crops, 

 will utilize plenty of potassium in the develop- 

 ment of starch. 



Sources of Plant Food. Chemical fertilizers 

 can be purchased at supply stores in ready 



77 



