148 



SOILS. 



gions. It also follows that thorough and frequent tillage and 

 fallowing greatly favor nitrification ; thus explaining one of 

 the beneficial results of these operations. At the same time, 

 it is true that we may thus in a short time seriously diminish 

 the reserve stock of nitrogen contained in the soil in the form 

 of humus-amids; and since nitrates are exceedingly liable to be 

 lost from the soil in several ways, such excessive nitrification 

 is to be avoided. 



Unhuniificd Organic Matter docs not Nitrify. There can be 

 little doubt that the formation of ammonia from the amido- 

 compounds in humus is also the work of bacteria; but this, 

 really the initial phase of the nitrogen-nutrition of plants, has 

 not yet been fully elucidated. That, however, it is essentially 

 only the reacly-formed humus and not the unhumified debris 

 of the soil which participate in nitrification was shown by the 

 experiments of the writer, see chapter 19. 



Denitrifying Bacteria. Among the sources of loss of ni- 

 trates in the soil is the action of denitrifying bacteria ; some of 

 which cause merely the reduction of nitrates to nitrites and 

 progressively to ammonia, while others cause gaseous nitrogen 

 to be given off from nitrites and nitrates, resulting in their 

 complete loss to the soil. \Yhile there are probably several 

 kinds of the latter class, the most rapidly effective is an organ- 

 ism contained abundantly in fresh horse dung, and also on the 

 surface of old straw. This can readily be shown by subjecting 

 a very dilute solution (1-3 per cent.) of Chile saltpeter to the 

 action of fresh horse dung in a close flask, when nitrogen 

 and carbonic dioxid gases are evolved, 

 and in a few days the nitrate has totally 

 disappeared. In the course of time this 

 power of horse-manure disappears ; so that 

 " rotted manure " is practically free from 

 it and under proper conditions serves nitri- 

 fication so effectively, that in the past it 

 FIG. 17 Bacillus denitri- has served extensively for the production 

 of saltpeter in the " niter- plantations " for 

 the industrial purposes; the material of which was loose 



