CROP AND FALLOW 231 



cent yielded 95.7, that containing 11 per cent, yielded 132.5 mg., 

 whereas that containing 16 per cent, of oxygen yielded 24(>.(> mg. 

 of nitric nitrogen. 



Plummer found there to be an optimum mixture of carbon 

 dioxid and oxygen for the best production of nitrates. This he 

 found to be one containing from 35 to (>() per cent, of oxygen. But 

 Hutchinson found complete nitrification of ammonium sulphate took 

 place under semi-anaerobic conditions in which no nitrification of 

 oil cake occurred. 



Stirring and pulverizing the soil is, therefore, of great importance, 

 as further shown by the experiments of Deherain. A number of 

 pots were filled with soil. Part of them were allowed to stand 

 undisturbed, while the others were poured out upon the floor and 

 frequently stirred. Those stirred invariably contained from ten 

 to forty times as much nitrates as did the unstirred. 



The work of King also shows that the stirring of the soil affects 

 nitrification. He further found land plowed in the fall contained 

 a different amount of nitrates than did the unplowed land, the 

 difference being apparent throughout the following summer. 



Crop and Fallow. Even as early as 1855 the work at Rothamsted 

 had demonstrated that the beneficial effects of fallowing lies in 

 the increase brought about in the available nitrogen compounds 

 of the soil. Deherain and Demoussy's work indicated that there 

 is a larger production of nitrates in fallow than in cropped soils, 

 and Pt'eiffer considers fallowing an extreme form of soil robbery, 

 for he found that it promotes the activity of the soil organisms, and 

 hence hastens the exhaustion of the nitrogen supply. But, as it 

 is so clearly pointed out by Warington, these results may not hold 

 in a dry climate or during dry seasons; for here bare fallow may 

 not necessitate this loss and much is to be gained by its practice. 

 But it must always be borne in mind that if there be sufficient 

 moisture the loss may be great. For instance, Schneidewind, 

 Meyer and Miinter record a loss in fallow plats of 85.5 pounds 

 per acre, which even exceeded the nitrogen removed by the growing 

 plant on the cropped soil. 



On the other hand, McBeth and Smith claim that plats con- 

 tinuously cropped to alfalfa, potatoes, oats, and corn all show a 

 higher nitrifying power than do corresponding fallow plats and 

 that the stimulating effect of crop production on the nitrifying 

 power of a soil is most marked in alfalfa soil. This is in keeping 

 with the recent findings of Welbel, but is contrary to the findings 

 of many other investigators, for Heinze found fallow to increase 

 the pectin, cellulose and humus fermenters and also the ammoni- 

 fiers, nitrifiers and Azotobacter. Russel finds that late summer 

 fallow land is richer in nitrates than is cropped, even after allowing 

 for the nitrogen taken up by the crop; and Heinze shows that 



