28 BULLETIN 721, U. S. DEPARTMENT OF AGRICULTURE. 



in the soil to bring about the desired change in the crop that was 

 plowed under. In the case of such crops as clover and alfalfa usually 

 one or more cuttings are made for hay, and the later growth is 

 plowed under when it has attained the height of 1 to 1^ feet, 

 as shown in Plate VI, figure 1. If cowpeas are used for green 

 manure, they are usually planted in midsummer and plowed under 

 when they have attained their normal growth in the fall. All crops, 

 when used for this purpose, should be plowed under while they are 

 still green. 



If for any reason a leguminous crop can not be grown for green 

 manure, other crops, such as oats, rye, barley, or even sorghum, may 

 be used. Maintaining the humus in the soil is more difficult in the 

 irrigated than in the humid sections of the country. The climatic 

 conditions, especially the hot winds, seem to have a decidedly reduc- 

 ing effect upon the quantity of humus in the soil. Humus in irri- 

 gated sections is doubly important, since it is necessary not only in 

 maintaining and improving soil fertility, but it also has a decided 

 advantage in increasing the water-holding capacity of the soil. 



The green crops will add little material that was not already in 

 the soil. They may, however, bring up the elements required for 

 plant growth from considerable depths and, when plowed under, 

 deposit them in soluble form in the surface soil. For this reason 

 deep-rooted crops are to be preferred to the shallow-rooted ones. At 

 any rate, some deep-rooted crops should be included in each rotation 

 system. The green crops, especially the legumes, may increase to 

 some extent the nitrogen content of the soil. Stable manure, if ap- 

 plied in sufficient quantity, will supply at least a part of the neces- 

 sary plant foods besides adding some humus to the soil. 



Commercial fertilizers. — If the required elements are not present in 

 the soil, or if those present can not be made readily available, they 

 should be supplied in the form of so-called commercial fertilizers. 

 The composition of the fertilizer used will depend upon the require- 

 ments of the crop to be grown and upon the condition of the soil 

 which is to be used for crop production. From the figures given 

 above it is apparent that a sugar-beet or clover crop should have an 

 abundant supply of potash, while for a wheat or corn crop special 

 attention should be given to the nitrogen supply. A complete fer- 

 tilizer consists of nitrogen, phosphoric acid, and potash. Compounds 

 containing these elements are mixed in different proportions for dif- 

 ferent crops and for different soil requirements. It frequently ap- 

 pears that a complete fertilizer is not required. For example, there 

 may be present in the soil an abundant supply of available potash, 

 while the supply of nitrogen and phosphoric acid is deficient. In 

 that case a fertilizer containing the required amount of nitrogen and 

 phosphoric acid only should be applied. Under certain conditions 



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