Conditions of Activity.—Dumont and Crochetelle found that up to .25 per cent, potassic carbonate acted favorably on the process; which was, however, completely stopped by as much as .8 per ct. Warington has shown that ammonic carbonate similarly prevents nitrification when exceeding about .37 per ct. Ammonia salts in general appear to be antagonistic to the transformation of nitrites into nitrates.
Aside from the carbonates, some neutral salts favor nitrification very markedly; while others tend to depress it. Deherain found that .5 per cent of common salt suffices to prevent nitrification altogether, while smaller amounts retard it proportionally. According to Dumont and Crochetelle, potassium chlorid acts favorably up to .3 per cent, but at .8 per cent suppresses nitrification. Earthy and alkaline sulfates, on the contrary, seem to act favorably throughout, at least up to .5 per cent. This is especially true of gypsum, which, according to Pichard, accelerates the process more than any other substance known. Taking the effect of gypsum as the maximum, he found that, other things being equal, the amounts of nitrates formed were as shown in the table below, the effect of gypsum being taken as 100:
| Gypsum | 100 |
| Sodic Sulfate | 47.9 |
| Potassic Sulfate | 35.8 |
| Calcic Carbonate | 13.3 |
| Magnesic Carbonate | 12.5 |
The above estimates are markedly confirmed by the observations of the writer in the alkali soils of California. In these, nitrates exist most abundantly when the salts contained in the soil are mainly sulfates; while wherever common salt or sodic carbonate are present in considerable amounts, the amounts of nitrate found are notably less. In saline seashore lands nitrates are usually present in traces only. Wollny has moreover shown that the nitrates themselves exert a repressive influence on nitrification.
Effects of Aeration and Reduction.—While the fostering effect of sulfates upon nitrification is very energetic in well aerated soils, they become injurious whenever by a reductive process in ill-drained lands, the sulfates are reduced to sulfids. Under such conditions the process will in any case be much impaired. On the other hand, the favoring effect of abundant aeration was strikingly shown in the experiment made by Deherain, in which a cubic meter of soil was left unmoved for several months, while a similar mass was thoroughly agitated once a week during the same time. The proportion of nitrates formed in the latter case was as 70 to 1 formed in the quiescent soil mass. It follows that the intensity of nitrification is essentially dependent upon the porosity of the soil; and that it is thus greatly favored in the pervious soil-strata of the arid regions. 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.
Unhumified Organic Matter does 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 ready-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].
Fig. 17.—Bacillus
denitrificans I.
(Burri.)
Denitrifying Bacteria.—Among the sources of loss of nitrates 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. While there are probably several kinds of the latter class, the most rapidly effective is an organism 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 nitrification so effectively, that in the past it has served extensively for the production of saltpeter in the “niter-plantations” for the industrial purposes; the material of which was loose earth, marl and manure, kept moist and frequently forked over for better aeration. Saltpeter is similarly produced in stables, corroding the mortar of brick foundations. Nevertheless, it is necessary to avoid the use, either together or at short intervals apart, of Chile saltpeter and fresh manure; the manure if used first should be allowed to remain at least two months in the soil before saltpeter is applied.
The reduction of nitrates to nitrites and ammonia is brought about by quite a number of bacteria, mostly anaerobic, and such as consume combined oxygen in their development. Thus the butyric ferment, which in the absence of readily reducible compounds evolves free hydrogen, will in presence of nitrates reduce the latter to nitrites, or form ammonia by addition of hydrogen to nitrogen just set free by reduction. Such reductive processes of course occur chiefly in soils rich in organic matter, or ill-aerated. The ammonia so formed, while at first simply combining with any humus acids present, may in the course of time be itself reduced to the amidic condition, being thereby rendered relatively inert, until again brought into action by ammonia-forming bacteria.