THE BEET-SUGAR INDUSTRY IX 1920. 29 
the ground should be irrigated thoroughly, if necessary, before plow- 
ing, so that there will be sufficient moisture in the soil to bring about 
the desired change in the crop 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 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 for soil improvement to those more shallow 
rooted. At any rate some deep-rooted crops should be included in 
each rotation system. The legumes may increase to some extent the 
nitrogen content of the soil, and stable manure, if applied in suffi- 
cient quantity, will supply at least a part of the necessary plant 
foods besides adding some humus to the soil. 
Commercial fertilizers. — If the required elements are not present 
in the soil, or if present are not readily available, they should be sup- 
plied in the form of so-called commercial fertilizers. The composition 
of the fertilizer used will depend upon the requirements 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 fertilizer consists of nitrogen, phos- 
phoric acid, and potash. Compounds containing these elements are 
mixed in different proportions for different crops and for different 
soil requirements. It frequently appears that a complete fertilizer 
is not required. For example, there may be present in the soil an 
abundant supply of available potash, but the supply of nitrogen 
and phosphoric acid may be deficient. In that case a fertilizer con- 
taining the required amount of nitrogen and phosphoric acid only 
