272 



SOIL CONDITIONS AND PLANT GRO WTH 



Inspection of their curves shows that reliable conclusions 

 cannot be drawn when counts are taken at long intervals. 



Effect of Temperature. — Bacteria being living organisms it 

 is natural to expect that their activity increases with the 

 temperature up to a certain point. The amount of nitrate 

 does show this expected increase but the bacterial numbers do 

 not, there being no steady rise as the temperature of storage 

 increases (Table LXXI.). Field observations lead to the same 

 conclusion (p. 275). 



Table LXXI. — Effect of Temperature of Storage on Bacterial 

 Numbers and Nitrate Production. Russell and Hutchinson (241a). 



Effect of Moisture. — Increasing moisture supply causes in- 

 creases in bacterial numbers, but they are not regular (24 1<:). 

 Field observations give the same result (p. 275). The rate 

 of nitrate production increases up to a certain point, beyond 

 which it decreases, presumably because of the lack of air. 

 Traaen's results are plotted in Fig. 25.^ On the practical 

 side it is found that application of irrigation water in arid 

 regions has a distinctly beneficial effect on the ammonifying 

 and nitrifying powers, both of cropped and fallow soils, and 

 on the numbers of organisms on fallow soils. Excess of water, 

 however, washes out the resulting nitrate from the soil and so 

 deprives the plant of the advantage it would otherwise gain. 

 (J. E. Greaves, R. Stewart, and C. T. Hirst ,ii3(^).) 



Effect of Added Organic Matter. — As shown on p. 252 the 



1 Centr. Bakt. Par., 1916, 45, iig. Other results are plotted in Greaves and 

 Carter's paper (ii^d), where also a full bibliography is given. 



