DECOMPOSITION OF ORGANIC MATTER IN THE SOIL 381 



wool waste and dried blood (all used more or less extensively as sources 

 of nitrogen to crops) are made up in a large measure of protein 

 compounds. 



Being derived from plant residues, from microorganic, insect and 

 animal remains, and from fertilizers and manures applied, the nitrogen 

 in the soil humus exists, for the most part, in the form of protein com- 

 pounds. Hilgard reports the following humus and nitrogen content, 

 as based on the analyses of a large number of samples of humid, 

 semi-arid and arid soils. 



Taking the weight of an acre-foot of dry soil at 2,000,000 kg. 

 (4,000,000 pounds) and multiplying the nitrogen by 6.25 (the factor 

 usually employed for converting nitrogen into protein) we find the 

 protein content of these soils to range from about 11,339 kg. (25,000 

 pounds) per acre to nearly three times as much. Similarly, the 

 Illinois Experiment Station reports quantities of nitrogen equivalent 

 to 3^73 to 4,989 kg. (7,000 to 11,000 pounds) per acre to a depth of 

 1 01. 6 cm. (40 inches) in gray silt loams, of the lower Illinoisan glacia- 

 tion. In the brown silt loams the amount of nitrogen to the same depth 

 is usually more than 4,535 kg. (10,000 pounds) per acre; occasionally 

 it is more than 9,071 kg. (20,000 pounds) per acre. In one instance a 

 black clay loam of the late Wisconsin glaciation is reported to have 

 about 13, 1 54 kg. (29,000 pounds) of nitrogen per acre, to a depth of 

 101.6 cm. (40 inches). This would be equivalent to more than 81,646 

 kg. (180,000 pounds) of protein; of course, not all of the nitrogen in the 

 soil exists in the form of protein, some of it occurring as amino-com- 

 pounds, and a small portion as ammonia and nitrates. Nevertheless, 

 by far the greatest part of it occurs as protein compounds. 



The protein compounds of the soil humus must be considered from 

 the standpoint of quality as well as from the standpoint of quantity. 

 It is well known that fresh plant residues are attacked more readily by 



