33° MICROBIOLOGY OF SOIL 



organic matter; (g), presence of toxic substances; {h) physiological 

 efficiency of the nitrifying bacteria. 



The rapid disappearance of organic matter from sandy soils is due in 

 large measure to, their better aeration. On the other hand, the decom- 

 position of vegetable and animal substances in heavy, ill-ventilated soils 

 is materially retarded by the limited supply and very gradual renewal of 

 oxygen. An intimate relation exists here between air and water in that 

 the latter replaces the former to a more marked extent in heavy than in 

 light soils. The influence of both aeration and the range of moisture is 

 illustrated by an experiment of Lipman's in which equal quantities of 

 soil were kept in large boxes under different moisture conditions. At 

 the end of a year the following quantities of nitrate nitrogen were 

 found:, 



6.S.2 per cent 14.75 P^r cent 18.62 per cent 22.05 per cent 22.12 per cent 



Moisture 



content 

 Nitrate 



nitrogen \ 697 mg. 823 mg. 720 mg. Trace Trace 



found 



In exp,mining the figures recorded above, we find that moisture was the 

 controlling factor in the development of the nitrifying bacteria, when 

 the proportion of water in the soil was 6.52 per cent. As the amount of 

 water increased. to 14.75 P^'^ cent there was a marked increase in the 

 amount, of nitrate produced. Beyond that, however, the further in- 

 crease in the amount of water began to limit the supply of oxygen, and 

 the production of nitrate nitrogen with 18.62 per cent of water in the 

 soil was somewhat decreased. A still further addition of water up to 

 22.05 P^"^ csiit led, practically^ to saturation, and the encouragement of 

 reduction rather than oxidation processes. Hence, no nitrate was al- 

 lowed to accumulate in the soil. The data in question thus help to 

 explain why care was taken, on salt-peter plantations, to keep the 

 compost heaps moist, yet not too wet. 



The influence of temperature on nitrate formation has been observed 

 by mslny investigators. Already Schloesing and Miintz recorded that 

 at 5° nitrification is quite feeble, at 12° marked and at 37° at its best. 

 Other investigators have obtained substantially the same results, except 

 that the optimum has been found to be considerably lower, often be- 

 tween 25° and 30°. Under field conditions nitrification seems to take 

 place at relatively low temperatures, as is indicated by the rapid 



