INSTITUTE AND COLLEGE, PUSA, FOR 1911-12. 79 



The method of determining the biologic activity of a 

 soil by plate counts has been abandoned in favour of 

 methods depending upon measurement of the physiological 

 activity of the soil organisms. By the use of such methods 

 biological analyses of soils may be carried out which w^ill 

 provide indications as to their relative capabilities for 

 dealing successfully with organic manures. Far more 

 valuable information, however, is also obtained bv such 

 analyses. By the use of fairly large portions of soil it is 

 perfectly feasible to set up and maintain conditions of 

 moisture and aeration comparable with those in the field, 

 and to determine in the laboratory for any particular soil 

 under w^iat conditions of water supply, tillage, and addi- 

 tion of lime or manures, either natural or artificial, the 

 greatest amount of ammonification and nitrification will 

 result. 



It has been ascertained by numerous experiments that 

 the rate of formation of carbon dioxide in a soil is directly 

 proportional to the biologic activity of that soil, and that 

 conditions unfavourable to the latter will also adversely 

 influence the former. A simple method of measuring the 

 rate of formation of carbon dioxide in soils under various 

 conditions has been devised and by this means it is possible 

 to determine Avhat moisture content of any paHicular soil 

 is the optimum for bacterial activity; this has been found 

 to vary from as high as 37 per cent, in one case to 

 as low as 16 per cent, in another. By the same method the 

 effect of the addition of various manures, organic or mineral, 

 may be observed, and it has been found possible in this 

 way to prescribe the addition of certain substances, such 

 as lime or superphosphate, to a soil in order to promote the 

 decomposition of green manures, oil-cake, or cattle manure 

 buried in it. Similarly by determining the o])timum mois- 

 ture content, recommendations can be made as to variations 

 in field practice which would tend to dry out the soil in 

 one case or to conserve moisture in another; a very large 

 number of observations have been made as to the conditions 

 under which ammonification and nitrification take place in 



