ENERGY AND NITROGEN TRANSFORMATION 517 



grew readily in soil in the presence of materials rich in cellulose, since 

 they are independent of the soil nitrogen, and the fact that no ill effects 

 were observed in partially sterilized soil served to prove the above 

 theory. Pfeiffer and Lemmermann 14 pointed out that the injurious 

 effect of straw upon plant growth was due not to denitrification but 

 to the development of organisms which assimilated the nitrates and 

 used the nitrogen for the synthesis of microbial protoplasm. It was 

 later found 15 that substances like hay and sugar caused a harmful effect 

 the first year and a beneficial effect the second year; i.e. the applica- 

 tion of materials rich in carbohydrate depressed crop growth when 

 applied just previous to planting but stimulated crop growth when a 

 considerable period elapsed. 16 According to Rahn, 17 easily assimilable 

 carbon compounds are present in the soil only in minute quantities; 

 ammonia and nitrate can, therefore, accumulate as a result of a gradual 

 decomposition of the soil organic matter. The addition of straw and 

 other plant substances rich in available carbon (energy) brings about 

 an increase in the development of microorganisms as a result of which 

 a nitrogen minimum may occur. This will last as long as easily de- 

 composable carbon compounds are still left. The nitrogen minimum 

 appears more quickly and lasts longer in nitrogen-poor than in 

 nitrogen-rich soils. During this condition, the plants cannot obtain any 

 nitrogen from the soil. The addition of available nitrogen overcomes 

 the harmful results. The depressing effects upon plant growth follow- 

 ing the application of straw has been reported by a number of other 

 investigators. 18 The results of Kellerman and Wright 19 on the nutri- 



14 Pfeiffer, Th., and Lemmermann, O. Denitrifikation und Stallmistwirkung. 

 Landw. Ver. Stat., 54: 386-462. 1900. 



15 Bredemann, G. Untersuchungen iiber das Bakterien-Impfpraparat "Heyl's 

 Concentrated Nitrogen Producer." Landw. Jahrb., 43: 669-694. 1912. 



16 Hutchinson, H. B. The influence of plant residues on nitrogen fixation and 

 on losses of nitrate in the soil. Jour. Agr. Sci., 9: 92-111. 1918. 



17 Rahn, O. Die schadliche Wirkung der Strohdiingung und desen Verhutung. 

 Zeitschr. Tech. Biol., 7: 172-186. 1919. 



18 Albrecht, W. A. Nitrate accumulation under straw mulch. Soil Sci., 14: 

 299-305. 1922; 20: 253-265; Hill, H. H. The effect of green manuring on soil 

 nitrates under greenhouse conditions. Virginia Agr. Exp. Sta. Tech. Bui. 6. 

 1915; Murray, T. J. The effect of straw on the biological soil processes. Soil 

 Sci., 12: 233-260. 1921; Scott, H. The influence of wheat straw on the accumu- 

 lation of nitrate in the soil. Jour. Amer. Soc. Agron., 13: 233-258. 1921; 

 Martin, T. L. Effect of straw on accumulation of nitrates and crop growth. 

 Soil Sci., 20: 159-164. 1925. 



19 Kellerman, K. F., and Wright, R. C. Relation of bacterial transformations 



