EFFECT OF THE GROWING PLANT 



137 



bacterial numbers with the nature of the crop, but the data hitherto 

 obtained are inadequate for satisfactory discussion. There is, however, 

 considerable evidence to show that nitrate accumulates more readily 

 on uncropped than on cropped soils even after allowance is made for 

 the quantity absorbed by the plant. Table 57 gives some of the 

 results obtained at Rothamsted : 



TABLE LVII. 



NITRATES IN CROPPED AND UNCROPPED SOILS AT ROTHAMSTED EX- 



PRESSED AS N. Ib. PER ACRE. RUSSELL 



expressed as N. parts per million. 



N. as nitrate o to 9 in. depth 

 9 to 18 in. depth 



12 



9 



To some extent climatic factors come into play, the cropped land fre- 

 quently being somewhat cooler and drier than the fallow. But this 

 does not hold universally, and the phenomenon has been observed 

 under such widely different conditions that climatic factors seem to be 

 ruled out. In 1905 Warington showed that the amount of nitrate in 

 the drainage waters from Broadbalk field was considerably less than 

 was expected from the manure supplied and the crop reaped. He 

 thought that denitrification might account for some of the discrepancy 

 but not for all, as it could hardly be supposed to act in dry summer 

 weather ; he further suggested that the nitrate might be taken up by 

 the plant and then somehow lost before harvest. More recently Lyon 

 and Bizzell (190) found more nitrate on land cropped with maize 

 (after allowing for the nitrogen present in the crop) than on fallow 

 land of similar history, and concluded that the growing maize plant 

 in some way stimulated nitrification. During the latter part of the 

 life of the plant less nitrate was found in the cropped than in the fal- 

 low land, and the further conclusion is drawn that nitrification is 

 inhibited by the conditions accompanying the decreasing activities of 

 the roots. On the other hand where oats and potatoes were grown 

 the nitrates were never so high in the cropped as in the uncropped 

 land, again, apparently, after allowing for what has been absorbed by 



10 



