252 MICROBIOLOGY OF SOIL. 



per acre to nearly three times as much. Similarly, the Illinois Ex- 

 periment Station reports quantities of nitrogen equivalent to 3,175 to 

 4,989 kg. (7,000 to 11,000 pounds) per acre to a depth of 101.6 cm. 

 (40 in.) in gray silt loams, of the lower Illinoisan glaciation. 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 in.). 

 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-compounds, and a small por- 

 tion 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 micro- 

 organisms 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 consists, 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 



