232 NITRIFICATION 



repeated cultivation of fallow soil increased the number of organ- 

 isms in the soil, while Hiltner maintains that no nitrification occurs 

 in soils where legumes are growing vigorously and fixing large 

 quantities of nitrogen. This latter view, however, is the extreme, 

 as is shown by much of the literature on the subject. 



Welbel and Winkler found that fallow not only increased the 

 assimilable nitrogen, but also the available phosphoric acid of 

 the soil, and that the increased yield of wheat after fallow is due 

 to these factors. But Bychikhin and Skalski point out that fall 

 fallow is even more wasteful of soil nitrates than is summer fallow, 

 for here the excessive rains wash the soluble nitrates from the soil 

 as fast as formed. The cultivating of fallow further increases the 

 nitrate content, as was shown by Richardson. Nitrification is 

 related to fallow and crop, as may be seen from the following results 

 obtained by the author: 



Milligrams Milligrams 



nitric nitrogen nitrogen 



formed. fixed. 



Cultivated 4.16 14.28 



Virgin soil 2.09 6.99 



Wheat soil 4.00 11.83 



Alfalfa soil .......... 2.25 12.24 



Fallow soil, potato fallow, etc. ... 6.22 22.88 



The results reported under milligrams of nitrogen fixed indicate 

 that in an arid soil the increased nitrogen fixation in a fallow soil 

 more than offsets the loss of nitrates, even though rapidly formed, 

 for little, if any, would be lost in the drainage waters. These 

 results have recently been confirmed by Reed and Williams. More- 

 over, the number of organisms in the soil and the rapidity of the 

 bacterial activity within the same is going to vary greatly with 

 the thoroughness and time of cultivation, as shown by Deherain, 

 Neish, King and Whitson, Chester and Quiroga, while the number 

 and activity of the organisms in the soil may in a degree determine 

 the speed with which the water evaporates from a soil. 



The work at the Rothamsted station early demonstrated that 

 the nitrates in the drainage water from the various plats varied 

 greatly, depending upon the crop growing upon the soil, thus indi- 

 cating a relationship between the available nitrogen in a soil and 

 crop growing upon the soil. Since that time many experiments 

 have confirmed this conclusion. Furthermore, King and Whitson 

 found 22 per cent, more nitrogen developed from soil after clover 

 than from soil after corn, and 13 per cent, more than after oats. 

 Later work by them showed that there are greater quantities of 

 nitrates throughout the entire season in soil under corn or potatoes 

 than in soil under clover and oats. Stewart and Greaves found 

 that different plants show a marked difference in their demands 



