SEASONAL VARIATIONS IN BACTERIAL NUMBERS 127 



Effect of Moisture, Increasing moisture supply causes increases 

 in bacterial numbers, but they are not regular. Nitrate production 

 does not appear to be much affected. 



Effect of Added Organic Matter. It is shown in Table XLIX. that 

 the addition of organic matter to the soil may increase the bacterial 

 numbers without, however, affecting the production of nitrates, indeed 

 sugar actually leads to nitrate decomposition. 



Effect of Previous Treatment of the Soil. Prolonged drought affects 

 the soil even after it has passed away and the soil has become moistened 

 The rate of production of nitrate and the bacterial numbers both in- 

 crease (240). 



Field Observations. The general phenomena observed in the 

 laboratory can be seen also in the field, but it is less easy to disen- 

 tangle the various factors, and particularly to separate the effects of 

 moisture and of temperature. But speaking generally the bacterial 

 numbers show no tendency to rise with increasing temperature ; they do, 

 however, increase more frequently with increased water supply, they 

 rise when organic matter is added in the form of dung and they tend 

 to be higher on land carrying grass and clover than on a bare fallow. 

 Numerous counts bringing out all these points were made by Hiltner 

 and Stormer (136) from plots of cropped ground, and of unmanured 

 and dunged fallows : 



TABLE LII. BACTERIA IN CROPPED AND FALLOW SOILS, MILLIONS PER GRAM. 



(HlLTNER AND STORMER (136)). 



The only marked effect is that of the dung ; the net result of the 

 clover and grass has only been small in spite of the organic residues 

 shed by the roots. On no plot has the warm summer weather 

 increased the bacterial numbers. 



Later on Engberding (94) made a more extensive series of counts 

 of the bacteria in plots of ground under known treatment and pub- 

 lished his results in very complete form, giving details of temperature, 

 moisture content, etc. Here again no connection could be traced 

 between temperature or moisture content and bacterial numbers. 



1 Dung applied in July at the rate of 130-140 Centner pro Morgen (10 to n tons per acre). 



