SOIL MANAGEMENT 



21 



purposes, (1) those that add nothing except what they get direct 

 from the soil and, (2) those that increase the nitrogen supply. 

 To the first class belong such crops as buckwheat, rye, rape, 

 mustard, etc., and grass sods that are ploAved down. Although 

 these crops add no element of plant food to the soil, neverthe- 

 less they are bene- 

 ficial because they 

 gather food from 

 the soil and on 

 tlieir decay leave it 

 in forms suitable 

 for the use of the 

 succeeding c r o 13. 

 To the second class 

 belong the clovers 

 and other legumes 

 that in addition to 

 being humus form- 

 ers, have the power 

 of extracting nitro- 

 gen from the air 

 and upon their de- 

 cay making this ni- 

 trogen available to 



non-legumes. Of the legumes, the crojos most extensively used 

 are red clover, cowpeas and crimson clover. 



In the East and South crimson clover and vetch are frequently 

 seeded in the fall and serve as a Avinter cover croj) to be plowed 

 down for the benefit of the spring crop. In the Middle West 

 rye is used in the same manner. If these cover crops are allowed 

 to make a rank growth and are turned under late in the spring 

 they may work injury to the succeeding crop by cutting off the 

 moisture supply from the deeper layers of soil. 



Commercial fertilizers. — As a rule the manure produced on 

 the farm is not sufficient to maintain fertility and the purchase 

 of lolant food materials in the form of commercial fertilizers is 

 necessary to keep up fertility. The constantly growing demand 

 for something that Avill increase crop production and help to 

 maintain fertility has given rise to a commercial fertilizer 

 industry of gigantic proportions. 



Soils and crops differ in their fertilizer requirements. — 

 Nitrogen, phosphoric acid, and potash are the plant food ele- 

 ments most likely to be deficient in soils or most quickly 



Fig. 9.- 



Plowing under a green manuring crop restores 

 the organic matter. 



