26 BULLETIN 721, U. S. DEPARTMENT OF AGRICULTURE. 
potash; a corn crop, yielding 40 bushels of ears to the acre, will 
require 56 pounds of nitrogen, 21 pounds of phosphoric acid, and 23 
pounds of potash ; clover yielding 2 tons of hay per acre requires 83 
pounds of nitrogen, 18 pounds of phosphoric acid, and 88 pounds of 
potash. These elements are required by all field crops, and, in addi- 
tion to these, seven other elements are required in much smaller quan- 
tities. These elements are always present in agricultural soils in 
larger or smaller quantities. The two questions, therefore, with 
regard to soil fertility, so far as the composition of the soil is con- 
cerned, are whether the required elements are present in sufficient 
quantity to produce the desired crop and whether the elements are 
available or soluble in such quantity and at such time during the 
growing season as the plant requires. In addition to the presence 
of these elements, as indicated above, the soil must be in proper 
physical condition to promote plant growth in order to be fertile. 
In the production of sugar beets a moderately fertile soil is required. 
If the soil is lacking in fertility the roots may be too small to produce 
sufficient tonnage to make the crop profitable to the grower. Under 
ordinary farm conditions there is little danger of the soil being too 
fertile for satisfactory beet growing. Occasionally spots are so fer- 
tile that large roots low in sugar are produced, as, for example, an old 
feed lot, a barnyard which has been turned into a portion of the field, 
or a spot where an old straw stack has been left to decay. These areas 
are small and insignificant when compared with the total sugar-beet 
acreage in the United States, but they sometimes have an important 
bearing upon the results on an individual farm, especially where the 
sugar-beet acreage on that particular farm is small. The greatest 
danger from the standpoint of fertility arises from the lack of those 
physical conditions or the absence of available plant foods to produce 
large yields. The principal problem, therefore, in this connection 
lies in the improvement of the fertility of the soil. Soils may be 
rendered infertile through natural causes, such as leaching, and 
through artificial causes, such as single cropping, improper crop rota- 
tion, and the improper proportion of live stock to crop production. 
One of the principal methods that may be employed to increase soil 
fertility is the addition of humus to the soil, either in the form of 
stable manure or of green crops plowed under. 
Stable manure. — One of the most satisfactory methods of supply- 
ing humus to the soil is the proper use of stable manure. A close 
relation should exist between the number of live stock on the sugar- 
beet farm and the acreage under cultivation. Studies in practically 
all parts of the sugar-beet area indicate that the number of live stock 
on most farms is too small for the most profitable production of 
crops and is usually below the possibilities in both live-stock and 
crop production when we consider the number of acres under culti- 
