122 



SOIL CONDITIONS AND PLANT GROWTH 



respective rates of ammonia production by the different soils. Lohnis 

 has used this method a good deal (186) as also has J. G. Lipman who, 

 however, modifies it considerably, and among other things uses 

 sterilised soil as the medium and substitutes dried blood or cotton 

 seed meal for peptone (176). Percy Brown (63) used a similar 

 modification in his studies of Iowa soils, and found that the " ammoni- 

 fying power" ran along with the " nitrifying power" and, in four out 

 of the six plots, with the crop-producing power also (Table XLVIIL). 



TABLE XLVIIL BACTERIAL ACTIVITY IN SOILS OF KNOWN PRODUCTIVENESS. 



BROWN (63). 



But, as the figures indicate, the differences are very small, and 

 would be considered within the error of experiment were it not 

 for the fact that three other sets of determinations came out in 

 practically the same order. Stevens and Withers (271) bring other 

 evidence to show the limited nature of the connection between 

 ammonifying power and plant growth. 



"Nitrogen-fixing power" is measured by inoculating soil into 

 Beijerinck's or some similar solution (p. 93) and incubating for a 

 definite time. This reaction only proceeds slowly. 



Bacterial Counts. The method of counting the number of colonies 

 that develop on gelatin or agar plates is admittedly faulty, but it 

 has the advantage of showing whether the numbers are high or low 

 and whether they are increasing or decreasing. It has, unfortunately, 

 three serious defects. No medium is known that brings out all the 

 soil organisms, so that the results are invariably low, and their 

 quantitative appearance is wholly illusory. No medium even distantly 

 resembles the soil in composition or in structure, so that the flora 

 developing on the plates does not necessarily reflect the flora active in 



