260 MICROBIOLOGY OF SOIL. 



it is evident that the number of species concerned in nitrate production is 

 relatively small. 



Relation to Environment. The conditions that affect nitrate formation 

 in soils may be classified under the following heads: (a) supply of 

 oxygen; (b) range of prevailing temperatures; (c) amount and distribution 

 of moisture; (d) quantity of lime and of other basic materials; (e) quantity 

 of soluble mineral salts; (f) character and amount of organic matter; 

 (g) presence of toxic substances; (h) physiological efficiency of the nitrify- 

 ing 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: 



Moisture \ . 



> 6. 52 per cent 14.75 P er cent 18.62 per cent 22.05 percent 22. 12 per cent 



Nitrate 



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



found 



In examining 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 er 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 er cen t l e d> 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 saltpeter plantations, to keep the 

 compost heaps moist, yet not too wet. 



