176 Bacteria in Relation to Country Life 



teria to form nitrates of lime, magnesia and, potash 

 substances not injurious to the bacteria even when 

 present in the soil in considerable quantities. In the 

 absence of basic substances, the nitric acid produced 

 by the nitrifying ferments accumulates and reacts 

 injuriously on the latter. 



Small amounts of nitric acid are sufficient to retard 

 seriously the nitrification processes in the soil. The 

 supply of air also exerts a direct influence on these 

 processes. Nitrates are formed most rapidly in sandy 

 loam soils well supplied with humus. On the other hand, 

 heavy clay soils are too compact and fine-grained to 

 allow satisfactory aeration; hence they allow a gradual 

 nitrification of their humus-nitrogen. In fairly open 

 calcareous soils, the nitrification processes may proceed 

 with great intensity, whereas, in coarse sandy soils 

 they may be quite irregular on account of the rapid 

 depletion of the soil-moisture. 



The process of nitrification in cultivated lands is in- 

 fluenced by the chemical and mechanical composition of 

 the soil as well as by the prevailing climatic conditions. 

 We are indebted for much of our knowledge on the 

 subject to the careful researches at Rothamsted, Eng- 

 land. Many interesting facts contributed by these in- 

 vestigators show that, among other things, the pro- 

 duction of nitrates takes place almost exclusively in the 

 surface soil. It was found at Rothamsted that drain 

 gages situated at a depth of 40 inches and 60 inches 

 respectively, yielded no more nitrates than did the 

 drain gages situated at a depth of 20 inches. It is not 

 difficult to account for this fact, since the nitrifying 



