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 



