EXTENSION COURSE IN SOILS. 59 
of the cover crop to be given up again from decay in the soil to the 
succeeding regular crops. Where no cover crop is used, there may 
be considerable leaching during the fall and winter, especially of the 
nitrogen compounds. (2) When a cover crop is on land during the 
heavy rains of fall and winter, the covering and the roots in the soil 
are very effective in preventing erosion. This is especially true in the 
case of clay soils. (3) When legumes are grown for green manures, 
the nitrogen content of the soil is much increased. This important 
fact should not be overlooked. 
The crops most commonly grown for green manuring are: Non- 
leguminous — rye, wheat, oats, and barley; leguminous — cowpeas, soy 
beans, crimson clover, red clover, sweet clover, bur clover, Japanese 
clover, and vetch. 
COMMERCIAL FERTILIZERS. 
(Ref. No. 7, pp. 449-475.) 
Since the time when it became generally accepted that one or 
more of three essential elements of plant growth, viz, nitrogen, phos- 
phorus, and potassium, are most apt to be found in soils in such 
small quantities as to limit crop production, there have gradually 
sprung up commercial enterprises organized for the purpose of manu- 
facturing and distributing materials containing one or more of these 
so-called essential elements. Usually substances containing two or 
three of these elements are mixed in different proportions and put 
upon the market under different names, such as corn grower, cotton 
grower, potato grower, etc. When materials thus manufactured and 
sold contain the three essential elements, they are called complete 
commercial fertilizers. There are hundreds of brands of complete 
fertilizers upon the market, and the number is fast increasing. 
Materials commonly used for complete fertilizers are sodium nitrate 
for nitrogen, acid phosphate for phosphorus, and potassium chlorid 
(muriate) or potassium sulphate for potassium. In the higher grade, 
and consequently higher priced, complete fertilizers the materials 
used are comparatively pure; in the lower and cheaper grade some 
material called a filler is often used in the mixture. At the present 
time ground limestone and peat are used to some extent as fillers. 
Use of fertilizers in the United States. — The largest part of the 
fertilizers used is applied to those soils which have been longest under 
cultivation. This statement is particularly applicable, to the South- 
ern States where the sandy soils, the long hot seasons, and especially 
the single-crop system (culture of cotton, a nonleguminous crop) 
have very much depleted the humus content and general fertility of 
the soils. Quoting from the United States Census Report: 
In 1909 the farmers of the United States reported the expenditure of $114,882,541 
for fertilizers, of which $75,752,296, or 65.9 per cent, was spent by the farmers of the 
