382 MICROBIOLOGY OF SOIL 



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 

 nitrogen. Older humus has, therefore, a narrower carbon-nitrogen 

 ratio than humus of recent origin. The former is more resistant to 

 decay, however, than new humus. In a concrete way, on the other 

 hand, it may be stated that fresh vegetable material of a narrow car- 

 bon-nitrogen ratio will decay more rapidly than fresh vegetable material 

 of a wide carbon-nitrogen ratio. The reverse, nevertheless, is true of 

 vegetable materials in advanced stages of decay. Under any given 

 climatic conditions and in any given soil type, the carbon-nitrogen 

 ratio may give important indications only as to the availability of the 

 humus nitrogen. Lawes and Gilbert, as quoted by Hall, found the 

 following carbon-nitrogen ratio in the organic matter of different soils : 



Cereal roots and stubble 43 .o 



Leguminous stubble 23 . o 



Dung 18.0 



Very old grass land *3 7 



Manitoba prairie soils 13 .o 



Pasture recently laid down n-7 



Arable soil 10. i 



Clay subsoil.. 6.0 



