39 2 MICROBIOLOGY OF SOIL 



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

 found: 



Moisture 

 content 



Nitrate 

 nitrogen 

 found 



6.52 per cent 14.75 P er cent 18.62 per cent 22.05 P er cent 22.12 per cent 



697 mg. 823 mg. 720 mg. Trace Trace 



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 cent 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 saltpeter plantations, to keep the 

 compost heaps moist, yet not too wet. 



The influence of temperature on nitrate formation has been observed 

 by many investigators. Schloesing and Mlintz 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 

 oxidation of ammonium salts in the Rothamsted experiments in Eng- 

 land; and the rapid decay and nitrification of clover and of other 

 legume residues in the experiments at the New Jersey Experiment 

 Station. These facts have, therefore, an important bearing on the 

 nitrogen feeding of crops in tropical, subtropical and temperate zones. 



The influence of lime and of other basic substances including the 

 carbonates of magnesium, potassium and sodium, and of the oxides of 

 iron is of far-reaching importance in all nitrification processes. It is 

 well known that applications of magnesian and non-magnesian lime, 

 marl or wood ashes promote nitrification in the soil and in compost 

 heaps, a fact that was well recognized by the ancient niter refiners. The 



