18 SEASONAL NITRIFICATION AS INFLUENCED BY CROPS, ETC. 



progressive downward feeding of the corn roots seems clearly shown. 

 The characteristic manner of surface feeding of corn roots would early 

 draw upon the plant food in the surface soil, as indicated in figure 4 

 by the curves of the 6 and 12 inch layers. The soil samples, however, 

 were taken not between the corn rows, where the lateral roots only 

 were abundant, but as near the corn plant as they could be taken, 

 not more than 3 or 4 inches away. Such sampling would encounter 

 the primary roots of the corn, as well as penetrate the soil area in 

 which the lateral roots were feeding most actively, and it seems 

 probable that the reduction of the amounts of nitrates in the 18 and 

 24 inch layers was due to the penetration of these primary roots to 

 that depth. This would account for the tardy removal at the lower 

 depth, as the primary roots would be longer in reaching these lower 

 layers. The moisture determinations showed that the moisture was 

 being removed quite rapidly from the 18 and 24 inch soil layers at 

 the same time. 



The most striking feature about figure 3 is the general parallelism 

 of the nitrate curve of the wheat plat with the nitrate curve of the 

 fallow plat. The seven-day lagging of the maxima in the wheat plat 

 is not, however, as clearly shown here as in figures 1 and 2, but is 

 evident in the early part of the season. The only other point of 

 difference in the two curves is in the period from May 29 to June 12, 

 during which time the summer-fallow plat steadily accumulated 

 nitrates, while there was a bare increase in these salts in the wheat 

 plat. This is the period during which the fallow plat laid in its 

 increased stock of water-soluble nitrates, and from June 12 on there 

 is a constant difference of about twenty to thirty-five parts per 

 million in favor of fallowing. It is probable that this difference 

 represents in general the continuous amount of soluble nitrates used 

 up by the plants. It is, however, also likely that had more nitrates 

 been formed the plants would have used them and that the curves 

 represent in a general way the minimum amount to which the corn 

 and wheat roots can reduce the nitrates in this heavy soil. Traps ° 

 found that the nitrogen content of crops was increased with an 

 increased nitrifying activity in the soil. 



In figure 3 no close relation is evident between the weekly crop- 

 increase curve and the decrease in the nitrates, using the fallow curve 

 as a check. 



RELATION OF NITRIFICATION TO FIELD FACTORS. 



The nitrification in the soil depending, as it does, on so many 

 factors — temperature, moisture, aeration, organic matter, kind of 

 crop, cultivation, etc. — it is difficult in the field to run down the 



« Traps, G. S. Bulletin 106, Texaa Agricultural Experiment Station, p. 4. 

 173 



