442 Mineral Nutrition of Plants 



Effect of variations in soil moisture 

 tension upon microbiological activity 



The important role of microorganisms in the mineral nutrition of 

 plants is discussed in an accompanying paper by Dr. Norman. It fol- 

 lows, therefore, that any effect that varying degrees of soil moisture 

 tension would have on microbial activity may result in an indirect effect 

 on mineral nutrition. There have been numerous studies (//, 29, 5/, 132) 

 on the relation of soil moisture to soil microorganisms, but as pointed 

 out by Bhaumik and Clark (9), in most of this work the soil moisture 

 levels were expressed as a percentage of the maximum water-holding 

 capacity. This technic often fails to ensure even moisture distribution 

 throughout the soil sample, or to maintain constant moisture content 

 over the experimental period. 



Bhaumik and Clark (9) adjusted the moisture tension of samples of 

 five different soils at o, 0.001, 0.01, 0.05, 0.5, and 3.2 atmospheres, and 

 collected the carbon dioxide evolved during the course of incubation. 

 In two soils the peak rate of carbon dioxide production was at 0.5 atmos- 

 phere of moisture tension, and in the other three soils at 0.05 atmos- 

 phere of tension. For all soils, the peak rate of carbon dioxide produc- 

 tion was observed at or very near to the moisture tension at the aeration 

 porosity limit, taken by convention as 0.05 atmosphere. Total carbon 

 dioxide production was actually at a maximum in Thurman sand at 

 the highest moisture tension used, and it was relatively very low at the 

 lowest two levels of moisture tension. On the other hand, total carbon 

 dioxide production for the Wabash silty clay was only slightly less than 

 maximum on the saturated soil, but was relatively low at the highest 

 moisture tension. The diverse effect of moisture tension on microbio- 

 logical activity on these two soil types differing widely in texture is in- 

 deed intriguing. This difference may be partially explained by the 

 enormous increase in population of fungi at 3.2 atmospheres tension, 

 as compared with zero tension in the sample of Thurman sand. Novo- 

 grudsky (9/) studied the rate of nitrification in a chestnut soil as a 

 function of moisture content. No nitrification occurred when only 

 hygroscopic water was present, but it was evident when the moisture 

 content was equal to about \ x / z times maximum hygroscopicity (pre- 

 sumably slightly above the wilting percentage). Nitrification reached 



