Relations of Higher Plants to the Formation of Nitrates 55 



Both the yield of plant substance and the nitrogen in the crop are least 

 on the scraped plats, which are the ones that exhibit a tendency for the 

 nitrates to increase during the growing period. This would indicate that 

 the utilization of nitrates is greater on the plats producing larger yields; 

 but that the high nitrate content of the maize plats cannot be due entirely 

 to this is evidenced by the fact that the nitrates under the plants are higher 

 in August than are the nitrates in the unplanted soil. 



During the later stages of growth the maize plats harvested at maturity 

 all decrease in nitrates at about the same rate. In this respect they 

 differ from the plats that were harvested August 1, which maintain a 

 higher nitrate content during the latter part of the season as has already 

 been mentioned. 



The marked falling-off in the nitrates after the middle of August on the 

 plats of maize harvested at maturity as compared with the high nitrate 

 content earlier in the season is not in keeping with the nitrogen absorption 

 by the crop during the early and the later part of the season. Yields and 

 analyses of the maize on these plats showed tha.t by August 1 the maize 

 plants had absorbed 44 per cent of all their nitrogen. During the early 

 period the nitrate content of the planted soil was increasing nearly as rap- 

 idly as, and in some cases more rapidly than, that of the unplanted soil. 

 After the middle of August, by which time fully 50 per cent of the nitrogen 

 must have been absorbed, the nitrate content of the planted soil decreased 

 very markedly while the nitrate content of the unplanted soil increased. 

 If the decreased nitrate content in the planted soil were caused by nitrogen 

 absorption exclusively, there would be a tendency for the nitrates in the 

 planted soil to increase as do the nitrates in the unplanted soil; but in 

 general there is no such tendency even late in the season, when nitrogen 

 absorption must have ceased. This again indicates a depressing influence 

 of the plant on nitrification during the later stages of growth. 



Comparing the oat plats planted to maize after the oats were harvested 

 with those kept unplanted, it ^vill be seen that there is a tendency for the 

 nitrates to run higher in the latter. This indicates that the growth of 

 the young maize plants has stimulated the formation of nitrates and has 

 overcome the depressing influence of the mature oat plants. 



In diagrams VIII to XV these relations of nitrates under the plants 

 to those in the unplanted soil of the corresponding plats are shown 

 graphically. 



