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 1J 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 
