Relations of Higher Plants to the Formation of Nitrates 85 



depressing effect of the oat crop on nitrate formation was sho"^Ti, by other 

 experiments, to be carried over to the succeeding year, when land pre- 

 viously in maize contained uniformly a higher nitrate content than did 

 land previously in oats. 



The depressing influence of the plant is thus showTi to be an indirect 

 one, and to remain with the soil for a considerable length of time. Neither 

 influence is completely nullified by aeration, at least by such aeration as 

 is involved in the process of sampling the soil and transferring it to bottles 

 for incubation, although this operation necessitates a very thorough 

 stirring of the soil. It is altogether probable, however, that the depressing 

 influence is decreased by this process. It is much more marked in field 

 tests than in incubation tests; but the former always involve the removal 

 of nitrates by the crop, which introduces an unknown factor. Data will 

 be presented later, showing that freezing dispels in part the depressing 

 effect of the plant on nitrate formation. 



Formation of nitrates in soil after freezing and thawing 



It has been suggested by the writers that plants which mature on the 

 soil have a depressing effect on nitrate formation in that soil for some time 

 afterwards. If this is the case, it seems probable that the inhibiting effect 

 of the crop may be removed in greater or less degree by subjecting the soil 

 to certain treatments. The effect of aeration on formation of nitrates is 

 well Imown. To what extent freezing may affect the nitrifying property of 

 a soil is not so well miderstood, as few experiments on that subject have 

 been published. 



Recently Brown and Smith (1912) reported that in ammonification tests 

 in peptone solutions and in soil to which infusions of field soil were added 

 at intervals from October 17 to March 1, the ammonifying power was 

 greater after the soil had become frozen than before. Similar tests of the 

 nitrifying power of the field soil did not indicate an increase in the nitri- 

 fying power of the frozen soil, but these investigators did not consider 

 their results in this experiment conclusive. Four pots of soil that had 

 been in the greenhouse for a year and had raised a crop of wheat and a crop 

 of millet during that time were used in the experiment. Pots 127 and 128 

 contained the same kind of soil and had received the same treatment, and 

 pots 343 and 344 corresponded with each other in these respects. The 



