146 FIELD CROPS 



have great water-holding capacity, which enables them to 

 hold the moisture till it is needed by the crop. Wheat is 

 also grown under irrigation with very satisfactory results. 



184. Manures and Fertilizers. Grain is the chief 

 product of the wheat crop; it removes from the farm con- 

 siderable amounts of nitrogen, phosphoric acid, and potash. 

 Most of the soils in the wheat belt have a much larger supply 

 of potash than of nitrogen and phosphoric acid, and as the 

 potash is used more largely in the production of straw, which 

 as a rule remains on the farm, nitrogen and phosphorus are 

 first depleted. Where live stock is kept, clover grown, and 

 the land manured frequently, satisfactory yields of wheat 

 may be obtained on naturally fertile soils for generations 

 without the .addition of commercial fertilizers. But where 

 wheat is the main crop or where wheat and other exhaustive 

 crops are grown and land is seldom if ever manured, it ulti- 

 mately becomes necessary to add to the soil some kind of 

 commercial fertilizer that will supply the needed elements as 

 they become deficient. Clover may be grown to add nitro- 

 gen, and clover, green manure, and stable manure will main- 

 tain the supply of vegetable matter. Where this is done, 

 about the only element that must be supplied by means of 

 commercial fertilizer is phosphorus. 



In the East and South, where the soils have become worn 

 by long continued cropping, commercial fertilizers are usually 

 applied for each crop. Likewise, in some of the Central 

 states the application of commercial fertilizers is becoming 

 necessary in many localities to insure profitable yields. 

 The kind and amount of fertilizer most profitable to apply 

 can be determined only by careful trials in each locality and 

 on each type of soil. A very common practice is to apply at 

 seeding time from 200 to 300 pounds of fertilizer to the acre, 

 containing about 2 per cent available nitrogen, 8 per cent 



