Relations of Higher Plants to the Formation of Nitrates 107 



large part of its nitrogen in some form other than as nitrates; the com- 

 bination of these conditions may accomit for the very high nitrate 

 content of the soil under maize. 



Under both maize and oats the nitrate content was higher during the 

 period when the crop was making its greatest draft on the soil nitrogen 

 than in the later stages of growth, in spite of the fact that the nitrates 

 in the uncropped soil were increasing while those in the cropped soil were 

 disappearing. Nitrates under these crops and under millet failed to 

 increase late in the season, when nitrogen absorption had practically 

 ceased, although micropped soil showed a very large increase in nitrates 

 at that time. 



This, in conjunction with facts before mentioned, indicates a further 

 influence of the crop on the process of nitrate formation, and may be 

 accounted for on the supposition that during their later period of growth 

 the plants exert in some manner a depressing influence on nitrate formation. 



Aside from the influence of cultivation, the source of the great differ- 

 ences in the nitrates under the crops mentioned may be sought in the 

 inherent differences between plants of different species in their stimu- 

 lating or inhibiting influence on the production of nitrates, as well as 

 in their relative rates, amounts, and forms of nitrogen absorption. 



Changes in the moisture content or in the temperature of the soil after 

 early summer had no important effect on the nitrate content of the soil 

 under plants. On the uncropped soil an increase in moisture content 

 was sometimes accompanied by an increase in nitrates and sometimes by 

 a decrease. 



Determinations of the rate of nitrification of soil from plats planted 

 to alfalfa and timothy, respectively, showed the alfalfa soil to nitrify 

 more rapidly than the timothy soil, both in the soil on which the crops 

 had been grown continuously and in that from which they had been 

 removed and on which the soil had been kept bare for two seasons. The 

 formation of nitrates was in the same order when the soils were incubated 

 with dried blood. It seems probable that the character of the organic 

 matter left in the soil by the plants determines to some extent the rate 

 of nitrate formation in this soil. 



Plats of land planted to certain crops in 1910 were kept bare of vegeta- 

 tion during the early part of the growing season of 1911. Determinations 

 of nitrates in the soil of these plats showed a distinct and characteristic 



