SOIL FERTILITY 399 



From the preceding table it will be seen that of the legumes 

 given, alfalfa adds the largest amount of nitrogen to the soil, forty- 

 seven per cent in its roots, and red clover second with thirty-two 

 per cent. When the two crops of red clover are removed from the 

 land, the nitrogen left in the soil in roots and stubble is on an 

 average probably no more than equal to that taken from the soil by 

 the crop, so there is no addition of this element under such a prac- 

 tice. The table shows that the roots of cowpeas, soybeans, and 

 crimson clover contain a very low per cent of the total nitrogen. 

 These crops if harvested from the land probably not only add no 

 nitrogen but actually remove some from the soil. 



Fresh farmyard manure contains about ten pounds of nitrogen 

 per ton, and the futility of trying to maintain this element with 

 manure on the average grain farm is readily seen. All manure 

 should be used to the best advantage, but where fifty bushels of 

 corn per acre, and other crops that remove equivalent amounts of 

 nitrogen, are grown it would require about twenty tons of average 

 farmyard manure per acre every four years to maintain it, even if 

 there were no other source of loss. 



Commercial Forms of Nitrogen. The forms in which 

 nitrogen may be obtained commercially for use as a fertilizer are 

 as follows: 



1. Sodium nitrate constitutes the principal form in which the 

 element nitrogen is obtained for use in commercial fertilizers. The 

 salt occurs in northern Chili and after being purified by crystalliza- 

 tion contains 15 to 1C per cent of the element. Chlorides and sul- 

 fates are present in small quantities. It is very readily soluble and 

 should be applied only when the crop is growing to prevent loss 

 by leaching, since it is not absorbed to a very great extent by the 

 soil. It is used by market gardeners and may be applied to timothy 

 meadow and small grains. Its continued use deflocculates the soil, 

 producing a puddled condition. 



2. Ammonium Sulphate. Ammonia is a by-product in the 

 distillation of coal and the sulfate is produced by passing the am- 

 monia through sulfuric acid. It contains about v() per cent of 

 nitrogen. This salt is readily absorbed and because of this is not 

 so readily leached from the soil. It should not be applied in the 

 fall, because it will l>e changed to nitrates and leached out and lost 

 Its continued use tends to deflocculate the soil somewhat as sodium 

 nitrate docs. 



