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 



