TRANSFORMATION OF ORGANIC MATTER 689 



This is due to the fact that different organic substances are attacked at 

 different rates even by the same groups of microorganisms. The addi- 

 tion of these different organic substances to the soil may stimulate the 

 development of different groups of organisms, with the result that there 

 is no basis for comparison between the decomposition of two different 

 substances, even in the same soil, when two different indices of decom- 

 position of organic matter (C0 2 and NH 3 evolution) are employed. 

 It was found 49 that, for a certain amount of organic nitrogen, in the 

 form of soy bean cake and herring cake, changed to ammonia, twice as 

 much organic matter is changed to C0 2 in an acid soil as in a loam soil. 

 By comparing two sources of organic matter, it was found that 1.5 

 times as much is changed to C0 2 in herring cake as in soy bean cake. 

 The mechanical soil conditions influence markedly the relation between 

 ammonia, nitrate and carbon dioxide production. 50 A detailed study 

 of the influence of straw upon utilization of nitrogen by plants is dis- 

 cussed in detail elsewhere (p. 516). 51 



Formation of "humus" as an index of decomposition of organic matter in 

 the soil. The term "humus" has commonly been applied to a hetero- 

 geneous mass of organic matter occurring in soils in different pro- 

 portions. Not only did the methods of its determination vary, but the 

 mere conception of it remained unclear. An alkaline solution, usually 

 4 per cent NH 4 OH or 2 to 4 per cent NaOH is commonly used to 

 extract the soil. In some cases, an aliquot portion of the ammoniacal 

 solution is evaporated to dryness and weighed, then ignited and weighed 

 again; the loss in weight is calculated as "humus." In other instances, 

 the alkaline solution is precipitated with hydrochloric acid and this 

 precipitate is taken to be "humus;" this preparation is often referred to as 

 "humic acid." Grandeau's 52 original method consisted in leaching the 

 soil first with a dilute acid to set the "humus" free from its combination 



49 Miyake, K., and Nakamura, K. On the effect of calcium oxide and cal- 

 cium carbonate upon the decomposition of soybean cake and herring cake in 

 two different soils. Jour. Biochem. Tokyo., 3: 27-54. 1923. 



50 Carpenter, P. H., and Rose, A. K. Ammonia, nitrate and C0 2 produc- 

 tion in relation to the best mechanical soil conditions. Indian Tea Assoc. Sci. 

 Dept. Quart. Jour. 1921, p. 103. 



5 > See also May, F. V. tJber den Einflusz von Stroh auf die Ausnutzung organ- 

 ischgebundenen Diingerstickstoffes. Mitt. Wien Hochschule f. Bodenkultur., 

 2: 433-454. 1913-14. 



62 Grandeau, L. Recherches sur le role des maticres organiques du sol dans 

 les phenomenes de la nutrition des vegetaux. Compt. Rend. Acad. Sci., 74: 

 988-991. 1872. 



