DECOMPOSITION OF ORGANIC MATTER IN THE ^OIL 321 



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

 Illinois Experiment Station reports quantities of nitrogen equivalent 

 to 3,17s to 4,9^9 kg. (7,000 to 11,000 pounds) per acre to a depth of 

 101.6 cm. (40 inches) in gray silt loams, of the lower Ilhnoisan 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 13,154 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 sinall portion as ammonia and nitrates. Nevertheless, 

 by Jar 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 

 microorganisms than older plant substances. For this reason soils 

 frequently supplied with fresh organic material supply greater amounts 

 of available food to crops than similar soils whose organic matter con- 

 sists, largely of older residues. 



Carbon-nitrogen Ratio. — The decomposition of organic matter is 

 readily influenced by the relative content of nitrogenous and non-ni- 

 trogenous compounds. Substances of animal origin yield relatively and 

 absolutely more available nitrogen in a given length of time than sub- 

 stances of plant origin. The difference noted is due largely to the 

 greater proportion of protein in the animal materials; in other words, 

 to the narrower carbon-nitrogen ratio. On this basis Hilgard attempts 

 to explain the adequacy of the small proportion of humus in arid 

 and semi-arid soils. Because of the narrower carbon-nitrogen ratio 

 the humus compounds in these soils are decomposed with greater 

 rapidity and yield a sufficient amount of ammonia and nitrate to supply 

 the needs of the crop. 



But when plant substances alone are considered the statement just 

 made requires qualification. It is true that cotton-seed meal or linseed 

 meal, having a narrower carbon-nitrogen ratio, will decay more readily 

 than corn-meal or wheat flour. It is also true that any given plant sub- 

 stance, as it undergoes decay, will lose in proportion more carbon than 

 21 



