Nitrification

“ It is as nitrates that most plants absorb the nitrogen they require to build up their tissue. In nature the nitrogen, present in the soil as ammonia and different organic forms, is constantly being converted into nitric acid. This conversion of nitrogen into nitrates, known as nitrification, is a process of very great importance, and, as has been already pointed out in the Introductory Chapter, is effected through the agency of micro-organisms (ferments) . ”

“ I need hardly add that such results as the nitrification of sewage by passing it through sand, or the nitrification of dilute solutions of blood prepared without special precaution, are no evidence whatever against the ferment theory of nitrification. If it is to be shown that nitrification will occur in the absence of any ferment, it is clear that all ferments must be rigidly excluded during the experiments; the solutions must be sterilized by heat, the apparatus purified in a similar manner, and all subsequent access of organisms carefully guarded against. ”

“ Certain Conditions having Great Influence in the Process of Nitrification.—If we suppose that a solution containing a nitrifiable substance is supplied with the nitrifying organism, and with the various food constituents necessary for its growth and activity, the rapidity of nitrification will depend on a variety of circumstances:
1. The degree of concentration of the solution is important. Nitrification always commences first in the weakest solution, and there is probably in the case of every solution a limit of concentration beyond which nitrification is impossible. ”

“ The processes of nitrification and the isolation and determination of the nitrifying organisms will be considered in a special chapter of this manual. By reason of the great solubility of the nitrates, and the inability of the soil to retain them, there can never be a great accumulation of nitric acid in the soil save in localities deficient in rain-fall or in specially protected spots, such as caves. The nitric acid, therefore, produced in the soil passes at once into growing vegetation, or is found eventually in the drainage waters. ”

“ The ammonia produced in the soil under normal field conditions is rapidly oxidised successively to nitrite and to nitrate, a process known as nitrification. The process of nitrification is more rapid than that of ammonia production, with the consequence that no more than traces of ammonia are able to accumulate. ”

“ Raising sewage to the boiling-point entirely prevents its undergoing nitrification. The heating of soil to the same temperature effectually destroys its nitrifying power. Finally, nitrification can be started in boiled sewage, or in other sterilized liquid of suitable composition, by the addition of a few particles of fresh surface soil or a few drops of a solution which has already nitrified ”

“ Nitrification, however, according to Warington, cannot be started in a solution if the initial temperature is 40°.
Desiccation has the same retarding influence on nitrification 455that a high temperature has. Even thoroughly air-drying a soil may destroy its nitrifying qualities.
Darkness is also necessary to the proper progress of nitrification. In a strong light, the activity of the organism is very much diminished or destroyed altogether. A bright light like sunshine may even stop nitrification which has set in. ”

“ Conditions Necessary for Nitrification.—In order to properly understand the reasons for many of the steps in investigating a soil for nitrifying organisms, it will be useful to state the general conditions on which nitrification depends.
The nitrifying organism, like every other one, first of all feels the necessity for food. In general, food which is given to microbes of all kinds consists of some organic matter together with the addition of mineral substances necessary to growth. These substances in general are phosphoric acid, potash, and 454lime. ”

“ This is the general process of nitrification in which the nitrifying bacteria transform or transfer the nitrogen from insoluble organic compounds into soluble compounds in which it may serve as available plant food. The nitrate which is thus formed may be calcium nitrate or magnesium nitrate or potassium nitrate or even sodium nitrate, depending upon which of these alkaline elements is present in the must suitable form. If no alkaline element is present in available form then no nitrates can be made in the soil. ”

“ In the present state of our knowledge, moreover, we can hardly regard the presence of nitrites as an obstacle to complete nitrification. On the other hand, it seems to be well established that the production of nitrites or ammonia is a necessary step between organic nitrogen and nitric acid.
421. Occurrence of Nitrifying Organisms.—According to the observations of Schloesing and Müntz the nitrifying organisms are widely distributed. [279] Arable soil containing considerable humus seems to be the medium in which they grow most freely and in which they accomplish their most important functions. ”

“ THE production of nitrates during the decay of nitrogenous organic matter under suitable conditions of moisture, aëration, and temperature, is a reaction of no little importance both technically and agriculturally: technically, as the sole natural source of saltpeter; agriculturally, on account of the fact that the nitrates formed in the soil constitute the chief if not the only supply of nitrogen to the plant. But, while the conditions of nitrification have long been well known, it is only within the past eight or nine years that its true cause has been recognized. ”

“ Nitrification of organic material takes place very rapidly the year round in this tropical region. These nitrated soils are everywhere abundant around caves, as described by Humboldt, caves which serve as the refuge of birds and bats. The nitrogenous matters, which come from the decay of the remains of these animals, form true deposits of guano which is gradually spread around, and which, in contact with the limestone and with access of air, suffers complete nitrification with the fixation of the nitric acid by the lime. ”

“ You understand, Girls, that nitrogen-fixation is simply getting nitrogen from the inexhaustible supply in the air by means of little microscopic organisms called bacteria, which live in little balls called tubercles attached to the roots of certain plants called legumes, like cowpeas and clover. Corn and wheat and such crops can't get this nitrogen. Now, Mr. Johnston is telling about nitrification, a process which is entirely different from nitrogen-fixation. ”

“ A variety of nitrogenous substances have proved susceptible to nitrification in solution. The weight of evidence, however, appears to show that in all cases the nitrogen first assumes the form of ammonia, and that the latter is, strictly speaking, the only substance capable of being nitrified. ”

“ Some moisture is necessary to nitrification, inasmuch as the nitrifying ferment does not act in perfectly dry soil, and in many localities 16in Chile where the nitrates are found it is too dry to suppose that any active nitrification could now take place. ”

“ The Theory of Nitrification.—The production of nitrates in soils, and in waters contaminated with sewage, are facts thoroughly familiar to chemists. It is also well known that ammonia, and various nitrogenous organic matters, are the materials from which the nitric acid is produced. Till the commencement of 1877 it was generally supposed that this formation of nitrates from ammonia or nitrogenous organic matter was the result of simple oxidation by the atmosphere. ”

“ The organic matters become oxidised into ammonia, nitrites, and nitrates, and these are eagerly assimilated by vegetation.
Nitrification is effected by micro-organisms in the soil. Ordinary garden mould and agricultural humus contain large numbers of micro-organisms. Their number diminishes with the depth of the soil, and below 12 to 15 feet there are few. Apart from the occasional presence of pathogenic (disease-producing) micro-organisms, the most important are those producing oxidation of organic matter, especially nitrification. ”

“ The rapid nitrification that takes place during the purification of sewage is an example of this. The conditions in culture, with regard to aeration and the removal of metabolic products from the neighbourhood of the organisms, are very different from those in the soil, and perhaps account for the discrepancies found.
The oxidation of ammonium carbonate by nitrosomonas results in the formation of nitrous acid. The organisms are very sensitive to acidity, and can only operate if the nitrous acid produced is neutralised by an available base. ”

“ The nitrous acid is then determined by potassium permanganate and the nitric acid by difference.
A great difference is to be noted between freshly taken earth and that which has been kept for a long while, especially when sealed. With fresh earth taken near the surface a mere trace is sufficient to produce nitrification. With samples of earth which have been kept for a long while and thoroughly dried, several grams must be added in order to secure perfect nitrification. ”

“ Moreover, there are certain species which can fix the free nitrogen of the atmosphere, and start the cycle of up-building of this inert element into the complex higher compounds we term organic. It is impossible to over-estimate the importance of these processes of nitrification and denitrification going on in the soil about the root-hairs of the higher plants. ”

“ Nitric acid is formed very rapidly, but only during periods of rest. Nitrifying bacteria are always present in ordinary town’s sewage, but it would appear that other micro-organisms besides Winogradsky’s bacteria assist in the process of nitrification. Nitric acid is formed very rapidly, but only during periods of rest, and besides aeration other less powerful influences are at work. ”

“ The supply of oxygen for the formation of nitrates in the soil comes from the air, which consists of about twenty percent oxygen, seventy-eight percent nitrogen, and two percent of other elements and compounds, as argon, carbon dioxid, CO2, water vapor, H2O, etc. One of the important effects of cultivation, or tillage, is that it permits the air more freely to enter the soil, and thus promotes nitrification. ”

“ The organic nitrogen may be in a form easily nitrified and rendered available for plant food, or it may be inert and resistant to nitrification, as in humus, or exist in an amid state. The ammoniacal nitrogen may exist in small quantities as gaseous ammonia, or be combined with mineral or organic acids. As nitric or nitrous acid the nitrogen will be found combined with bases, or perhaps in minute quantities as 421free acid, in passing under the influence of the nitrifying ferment from the organic to the inorganic state. ”

“ R. Warrington reported to the British Association that the results of his later experiments at Rothampstead showed a far deeper diffusion of the nitrifying organism in the soil than had been concluded from the earlier experiments. The power of producing nitrification was now found to exist generally down to three feet from the surface. Below this point the occurrence of the organism became less frequent, though at five and six feet about half the trials resulted in nitrification; with soil from seven and eight feet no nitrification was obtained. ”

“ The nitrifying action goes on best in well-ventilated soils. In poorly drained soils just the opposite process, denitrification, is apt to occur, reducing plant food to unavailable simple nitrogen. The subject of nitrifying bacteria is very complex, as they possibly also exert a fermenting influence on the minerals of the soil. Experiments have shown that an acre of cowpeas at the Louisiana Experiment Station produced 65 pounds of nitrogen, and an acre of crimson clover at Cornell University produced 156 pounds, 30 of which were in the roots. ”

“ The process for the formation of nitre, is called nitrification.
Although animal substances, by putrefaction, furnish azote, and nascent azote unites with facility with the oxygen of the atmosphere, by which nitric acid is generated— (hence the spontaneous decomposition of nitre composts) —yet Vauquelin is of opinion, that the presence of calcareous or alkaline substances is indispensable, and that the production of carbonate of ammonia from the animal matter, is another compound, which results from the same decomposition. ”

“ Again, in arranging a filter bed for the oxidation of sewage, it is obvious that, with a heavy soil lying in its natural state of consolidation, very little will be gained by making the filter bed of considerable depth; while, if an artificial bed is to be constructed, it is clearly the top soil, rich in oxidizing organisms, which should be exclusively employed.
The Substances Susceptible of Nitrification.—The analyses of soils and drainage waters have taught us that the nitrogenous humic matter resulting from the decay of plants is nitrifiable ”

“ We had in 1898 a very hot summer, followed by an unusually warm autumn, affording a long and favourable time for the natural nitrification of the organic nitrogen of the soil, so that the land must have been unusually well supplied with natural nitrates by the time that winter arrived, if, indeed, the term winter can properly be applied to such mild weather as was experienced during the calendar winter months of 1898-99. ”