NUTRITION 73 



ing the whole crop under. * Chemical analysis of soils origi- 

 nally alike show that that of a clover field gains in nitrogen 

 as well as in carbon, while that of a grass field gains only 

 in carbon. In other words, plowing a grass-crop under re- 

 turns to the soil only what the plants took from it, plus 

 ,the amount of carbon- absorbed from the air. On the other 

 hand, plowing a clover-crop under returns to the soil what 

 the plants took from it, plus the amount of carbon ab- 

 sorbed from the air, and nitrogen. Where did the nitrogen 

 come from? Obviously it could come only from the air. 



If clover, pea, alfalfa, or other leguminous seed be sown 

 in sterilized soil of known composition, no increase in ni- 

 trogen will be discovered on analyzing the soil after the 

 crop has been turned under. These seeds sown in soil of 

 exactly the same composition, unsterilized, but otherwise 

 treated in the same way before and during cultivation, 

 will yield a larger crop, which will be found to have added 

 nitrogen to the soil. These seeds, sown in soil of the 

 same composition, sterilized and then inoculated, either by 

 the addition of a small quantity of unsterilized soil or of 

 clear water first sterilized and then shaken with unsterilized 

 soil, will produce a crop as large, and yield as large a gain 

 of nitrogen in the soil, as those sown in unsterilized soil. 



From these experiments t the only possible inference is 

 that the micro-organisms of the soil, and not the legumi- 

 nous plants alone, are effectively concerned in the increase 



* See Year Book of U. S. Dep't. Agriculture for 1897 and other publica- 

 tions of the national and state agricultural departments for statistics of 

 relative values of different crops as "green manures" in adding to the 

 available nitrogen content of soil. 



t For details see Frank, A. B. Die Assimilation des freien Stickstoffs 

 durch die Pflanzenwelt. Bot. Zeitung, Bd. 51, 1893, and elsewhere, to 

 which references are given in this paper. 

 Also various papers in the Deutsche Landwirthschaftliche Presse, 



" " " Landwirthschaftliches Jahrbuch, 

 " " " " " Annales Agronomiques, 



" " " " " Year Book U. S. Department of Agriculture, 



" " " " " Reports of " " " " 



" " " " " " Agricultural Experiment Stations 



of different states, etc. 



