77 



been reported, notably by Stevens and "Withers, 1 and is not infrequently 

 met with in studies on nitrification. Where it occurred in the present study, 

 it was a characteristic of extremely poor soils and it is probable that only in 

 suclTsoils is the condition ever to be found. 



SUMMAEIZING DISCUSSION. 



The studies on nitrification of ninety-three Virginia soils revealed some 

 interesting facts regarding that process. Cultivated soils showed decidedly 

 higher nitrifying qualities than virgin soils. This fact was exemplified in 

 that twenty-seven of forty-four pairs of virgin and cultivated samples, the 

 latter exhibited superior nitrifying powers,, the average excess of nitrates 

 was higher in cultivated soils than in the virgin samples, and the total 

 nitric nitrogen was considerably higher from the cultivated soils. 



There appeared to be considerable difference in the nitrifying power of 

 virgin soils of various texture. This quality in the light open sandy soils 

 was strikingly low, in the loams and clay loam it reached its maximum 

 height, and in the heavy clays there was again depression, yet not so low as 

 in the extremely open soils. The high nitrifying power of certain cultivated 

 soils irrespective of texture, suggests however that this power may be largely 

 increased by proper cultivation. The deficiency of light soils is probably 

 due to a lack of organic matter ; the depression in the clay soils could doubt- 

 less be ameliorated by aeration from tillage. 



From the results of this investigation it is unwarranted to conclude that 

 soils with high nitrifying power would exhibit also unusual qualities for 

 free nitrogen fixation or vice versa. In 50 percent of the soils which were 

 notably superior in either process such a relationship could not be traced, 

 but in the remaining 50 percent there was close agreement in the vigor of 

 the two processes. 



The soils were examined also for their accumulation of nitrates over a 

 period of six months. There was not exhibited in these tests the distinctive 

 nitrifying superiority of cultivated soils over virgin soils as was evinced in 

 the tests with ammonium sulphate, due perhaps to the higher organic nitro- 

 gen content of the virgin soils and the longer duration of the experiment. 

 The clay loam failed in these tests to show nitrifying powers commensurate 

 with those they exhibited with ammonium sulphate, though the relationship 

 as regarded other soil types was quite consistent in the two series of tests. 



A number of soils evinced no power whatever to nitrify. Some soils 

 which failed to nitrify ammonium sulphate in six weeks accumulated quite 

 significant amounts of nitrate from their own organic nitrogen in six months. 

 On the other hand there were some which nitrified the inorganic medium 



'Stevens and Withers. Science. Vol. XXIX. No. 743, p. 506. 



