300 SOIL CONDITIONS AND PLANT GROWTH 



of the crop on the temperature or moisture content of the 

 soil. 



Two hypotheses are possible : there may be a diminished 

 production of nitrate in cropped land, indicating an adverse 

 effect of the growing crop on bacterial activity, or there may 

 be some process destructive of nitrates (possibly by conversion 

 into protein) ^ at work in the cropped soil that does not go on 

 in fallow soil. 



Destruction of nitrates is at least as probable as a dimin- 

 ished bacterial activity. It may quite well happen that 

 portions of the roots and rootlets are decomposed during the 

 life of the plant, and if their nitrogen content is insufficient, 

 the organisms might assimilate some of the soil nitrates 

 (p. 210). This view is consistent with certain observations 

 which are otherwise difficult to explain, and which seem to 

 indicate an increased rather than diminished bacterial activity 

 in the neighbourhood of the growing plant. Working with 

 the irrigated soils of Utah, McBeth, and Smith (i85<a:) found 

 that the " nitrifying power " {i.e. the power to produce nitrate 

 when incubated under optimum conditions with dilute am- 

 monium sulphate solution) was higher on cropped than on 

 fallow land. Again, Le Clerc found higher bacterial numbers 

 under cow-peas than in the fallow plots. ^ A similar result 

 was obtained in Joshi's pot experiments with other crops.^ 



On the other hand, the alternative view that plant growth 

 depresses bacterial activity seems to be supported by Harri- 

 son and Aiyer's investigations in paddy soils already quoted 

 (p. 213). The production of methane and hydrogen was 

 markedly less on the cropped than on the fallow land. There 

 is no question here of deficient water supply or temperature, 

 and the conclusion of the authors is that the crop reduced 

 bacterial action either by giving out some bacteriotoxin or by 

 absorbing some of the products of the early stages of decom- 

 position. 



1 Burd (62a, p. 304) suggests that this change occurs in Californian soils. 



^yourn. Ag. Res., 1916, 5, 439-447. 



^Memoirs Dept. of India, Bact. Set., 1920, i, 247-276. 



