Popular Science Monthly/Volume 29/October 1886/Nitrification
By Professor H. P. ARMSBY.
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. Pasteur, in 1862, appears to have first pointed out the similarity of nitrification to the various oxidations of organic matter known to be effected by the agency of mycoderms, and of which the acetic fermentation is the typical example.
In 1873, A. Müller advanced the opinion that nitrification was due to the action of a ferment. He based his opinion the fact upon that solutions of pure ammonium salts and of urea are very stable, while the same bodies in sewage are rapidly nitrified, holding that the difference was due to the presence of a ferment in the latter case. In 1877 Schloesing and Müntz published the results of experiments which indicated that Pasteur's suggestion and Müller's opinion were correct, and that nitrification might really be classed as a fermentation. These experimenters were engaged in investigating the oxidizing effect of the soil upon sewage. They filled a glass tube one metre long with a mixture of quartz, sand, and a small quantity of powdered limestone, and caused sewage to filter slowly through this artificial soil, so that it occupied eight days in passing through the tube. For twenty days the sewage passed through unaltered. Then nitrates began to appear in it, and rapidly increased in amount until all the nitrogen of the filtrate was in this combination. If nitrification is due to simple oxidation, it is difficult to see why it was so slow in commencing; but, if it is due to an organism which required time to develop in the artificial soil, the delay is at once explained.
Sewage was passed through the soil in this way for four months, with complete oxidation of its nitrogen. As soon, however, as vapor of chloroform, which is known to be inimical to the action of organized ferments, was caused to penetrate the soil, nitrification ceased, and did not recommence after the chloroform was withdrawn. After the sewage had passed unchanged for seven weeks, a small amount of turbid washings of a soil known to nitrify with ease was poured upon the top of the soil. After eight days (i. e., exactly the time required for the liquid to traverse the column of soil), nitrates reappeared in the strata, and continued to be formed as long as the experiment was continued. All these facts point plainly to an organism as the cause of nitrification. It developed in the soil during the first twenty days of the experiment from germs introduced by air or sewage; it was killed by the chloroform-vapor, and reintroduced in the soil-washing.
In 1878 appeared the results of experiments made by Warrington in the Rothamsted Laboratory, which fully confirmed those of Schloesing and Müntz. He first showed that a very considerable nitrification took place in a good garden-soil when a current of air was aspirated through the moist soil, but that hardly any formation of nitrates took place when this air contained vapors of chloroform or carbon disulphide, while vapor of carbolic acid seemed to produce the same effect so far as it was brought in contact with the soil. Thus far the results were simply confirmatory of those of Schloesing and Müntz. Further experiments, however, developed the important fact that nitrification could be brought about in dilute solutions of ammonium salts, by seeding them with a small amount either of a nitrifying soil or of a similar solution which had undergone nitrification. The first experiments were made with the dilute solutions employed in the determination of ammonia by Messler's method, with the addition of small quantities of tartrate and phosphate of potassium, and precipitated carbonate of calcium. The solutions used in later experiments had the following composition per litre:
Precipitated calcium carbonate was added to supply the necessary base. By this discovery the way was opened for the easy and fruitful study of the process and of the conditions affecting it.
Since the publication of Warrington's paper, a large amount of work has been done in this direction both by this investigator and by others. As a result, the ferment theory of nitrification has been very thoroughly established, the organism producing it has been isolated, and considerable progress made in the study of the conditions affecting nitrification, particularly in fluid media.
That nitrification is due to the action of a living organism is shown in various ways. Sterilized solutions, otherwise suitable for nitrification, have been preserved for as long as three years unchanged. But, if to such a solution a small amount of a solution or a soil in which nitrification has recently taken place be added, the solution nitrifies within a short time.
Nitrification is strictly confined to the range of temperature within which the action of low organisms is possible. It does not take place unless all the nutritive materials necessary for such organisms are present, absence of phosphoric acid, for example,-completely preventing it. Antiseptics, as already illustrated, inhibit nitrification. The action of heat likewise confirms the ferment theory. The temperature of boiling water at once stops nitrification, and it is not resumed until the medium is seeded again from some external source.
Some of the more important conditions affecting nitrification in liquids (and presumably also in porous solids, such as soil) are: 1. Alkalinity of the solution; 2. Concentration of the solution; 3. Character and amount of the ferment; 4. Temperature.
1. While nitrification does not take place in the absence of a salifiable base, any considerable degree of alkalinity greatly retards it, and, if it exceeds the equivalent of about three hundred and fifty parts of nitrogen per million, stops it.
2. Under like circumstances, nitrification begins more promptly the more dilute the solution. No definite limit of concentration can be stated, beyond which nitrification can not take place on account of the great differences caused by differences in the—
3. Character and amount of the ferment. The character of the ferment is determined by its previous history. A strong ferment, producing prompt and rapid nitrification, is obtained by repeated cultivations in moderately strong solutions well supplied with nutritive matter, while the opposite course produces a weak ferment. The stronger the ferment, and the greater the amount of it used for seeding, the sooner the nitrification begins, and the greater is the admissible concentration of the solution.
4. Nitrification has been observed to take place at a mean temperature of 3·2° C. The superior limit seems to be 40° to 50° C, the optimum 35° to 37° C.
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. In the case of urea this has been observed to lead to some interesting results. Thus, if nitrification is induced in a solution of urea containing no salifiable base, the process stops when one half the nitrogen has been oxidized, ammonium nitrate being produced. If the concentration exceeds a certain limit no nitrification occurs, the alkalinity produced when the urea is converted into ammonium carbonate being sufficient to prevent the action of the ferment. If, however, gypsum be present, the well-known double decomposition into calcium carbonate and ammonium sulphate takes place, and, the latter having a neutral reaction, nitrification proceeds unhindered.
An interesting and hitherto unexplained fact which was noticed in Warrington's experiments is, that sometimes nitrous and sometimes nitric acid was produced, and at times both in the same solution. The experiments thus far published suggest the possibility of the existence of two ferments, a nitric and a nitrous, but on this branch of the subject we may expect more light when investigations now in progress at Rothamsted are made public.
Some investigations into the distribution of the nitric ferment in natural soil are summarized by Warrington as follows: "I am disposed to conclude that in our clay soils the nitrifying organism is not uniformly distributed much below nine inches from the surface. On much slighter grounds it may perhaps be assumed that the organism is sparsely distributed down to eighteen inches, or, possibly, somewhat farther. At depths of from two feet to eight feet there is no trustworthy evidence to show that the clay contains the nitrifying organism. It is, however, probable that the organism may occur in the natural channels which penetrate the subsoil at a greater depth than in the solid clay. In the case of sandy soils we may probably assume that the organism will be found at a lower depth than in clays."