384 The Land and Its Treatment 



(from superphosphates) and a part insoluble (from 

 ground bone, tankage or natural phosphates). The soluble 

 portions of both nitrogen and phosphoric acid contribute 

 to the immediate needs of the plant, and the less soluble 

 to its continuous and steady growth, and to the potential 

 fertility of the soil." 



All of this formula, or part of it, may be used on an acre. 

 Commonly, 1,000 to 1,500 pounds are recommended. More 

 specific advice may be found, as recommended by Yoor- 

 hees or others, under the different vegetables. It is to 

 be said, however, that the figures given for any vegetable 

 are only by way of suggestion, for there is no invariable 

 rule to follow. The grower learns by experience how to 

 vary the indications for his land and for his method of 

 handling the crop. 



3. THE IRRIGATIOX OF THE LAXD 



In many regions the crop is determined by the amount 

 of rainfall rather than by the plant-food. The crop often 

 requires more water than is supplied by the normal rain- 

 fall of the growing season. Tillage can save much of the 

 water that fell in the early rains and the winter snows, 

 but there may still be insufficient moisture for a good crop. 

 Irrigation may be necessary to supply the deficiency. 



In the arid j^arts, irrigation is a necessity. It is a 

 general practice. In the humid parts of the country — 

 east of the plains — irrigation is often helpful and it re- 

 duces the risk of a poor crop. It is an exceptional or 

 special practice. Evidently, in all regions in which crops 

 yield abundantly without irrigation, the main reliance is 

 to be placed on good tillage. 



