nitrite-producing organisms. Nitrobacter are among the smallest of all known organisms being only one-tenth of the size of an average nitrosococcus. They exist as slender, pear-shaped cells, possessed of slow powers of increase in comparison with their capacity for nitrate-production. If cultivated in a liquid medium they grow in the form of a thin scum which adheres persistently to the sides of the vessel. CHEMISTRY OF NITRIFICATION. We are at present without knowledge as to the complete cycle of chemical changes which are brought about in the process of the nitrification of ammonia, and can only deal with the end-products of the vital activity of the nitrifying bacteria. We have already noted that the first stage of nitrification is the production of a nitrite by the agency of the "nitroso" organisins. So far as the mere process of oxidation is concerned, the following equation expresses the first stage of nitrification; Stage I. 2 NH, +302 +Ca CO3=Ca (NO2)2+ CO2 Ammonia+oxygen + chalk calcium = nitrite 2 +3 H2 O carbonic + water. acid gas This change goes on only in the dark and is independent of all organic matter. The bacteria themselves, however, contain protoplasm and the usual organic constituents of living organisms, and in order to obtain the carbon necessary for their growth and increase they avail themselves of carbonic acid gas. The separation of carbon from its union with oxygen in this gas requires the expenditure of a considerable amount of energy. Green plants obtain their supply of this requisite energy from the rays of the sun, but nitrifying bacteria live and work in the absence of such light and are unable to make use of this source of power. Winogradsky, as also Godlewski, have shown that the nitroso' bacteria obtain their energy for the elimination of carbon from carbonic acid gas by the supply they derive from the oxidation of ammonia to nitrite. The former experimenter concluded from his investigations that the nitroso' bacteria were able to assimilate one part of carbon for every 42 parts of ammonia oxidised to nitrite. It would therefore appear that the main vital object of nitrifying organisms is that of the assimilation of carbon from carbonic acid gas and that the nitrification of ammonia is simply a means to that end. The effective character of these agents of nitrification is appare t from the enormous amount of ammonia they are forced to oxidise in order to gain a small supply of vital carbon. The chemistry of the conversion of nitrite into nitrate by the nitrobacter organisms is of the simplest. Stage II. Ca (NO). +02 =Ca (NO3)2 It is necessary to draw attention at this point to a fallacy that is very constantly repr duced in current agricultural literature. It is often stated that ammonia is oxidised to nitric acid by nitrifying organisms and unless enough chalk to neutralise the acid be present, the organisms are destroyed or prevented from free activity. It is clear from the work of Winogradsky that free nitrio acid is not produced at any stage of the process, but that two stages of oxidation take place, (1) oxidation of ammonia to an alkaline nitrite, (2) complete oxidation of the nitrite to nitrate. Unless oxygen (air), chalk, carbonic acid gas and a trace of such mineral foods as phosphates and potash be present, the necessary raw materials for nitrification are lacking and its progress impossible. It is certainly true that a salifiable base such as chalk is necessary for nitrification but it is a gross misrepresentation to state that this necessity lies in a need for the neutralisation of free nitric acid, which is perhaps the most corrosive substance that could be brought into contact with living organisms. CONDITIONS AFFECTING THE NITRIFICATION OF AMMONIA. A.-Presence of organisms. Fortunately for the agriculturist, the bacteria responsible for nitrification are universally distributed and no practical cultivator runs risk of loss through the actual absence of nitrifying organisms. Cultivated soils from all sources, desert sand and rocky fragments from lofty mountain tops have all yielded proof of the presence of nitrifying bacteria Warington found that all the samples taken from the cultivated surface of the soil which he tested contained nitrifying organisms. At a depth of two feet, powers of nitrification were occasionally lacking, while at a depth of six feet and over the soil had lost all such powers. Nitrifying organisms, therefore, are mainly present in the upper tilled surface of the soil and do not exist in the lower depths of unstirred soil. Such a distribution is obviously due to the fact that conditions favourable for nitrification are alone possible in the upper surface of the soil. B.-Air. The atmosphere contains one fifth of its volume of oxygen gas and as this latter material is requisite for the purpose of oxidising ammonia, a full supply of air is necessary for the free progress of the change. Drainage, cultivation with plough and harrow, spade, fork and hoe are time-honoured tributes to this fundemental requirement of cultivated ground A water-logged soil in which the pores are saturated with water is an impossible medium for nitrification owing to the absence of air. The wonderful improvements that have been brought about in the case of stiff, impervious soils by drainage and good cultivation are closely associated with the improved aëration of the soil and the consequent promotion of nitrification. C.-Presence of salifiable base. Chalk. It is desirable that special emphasis be laid on the absolute necessity of such an alkaline carbonate as chalk for the general requirements of the process of nitrification Those traditions of good cultivation which have been evolved through centuries of experience and observation by generations of practical men, have received marked confirmation and a rational explanation through the latest discoveries as to the causes and conditions of nitrification. The recommendations of science, based upon a comprehension of the causes at work, are singularly in harmony with the general maxims of good cultivation, based upon a shrewd appreciation of obvious effects. Of all the conditions favouring nitrification, lime or chalk is the one that is most frequently lacking in practice and that merits the careful consideration of every agriculturist who seeks to obtain the best returns from the cultivation and manuring of his land. Chalk is rapidly washed out of cultivated soil through the action of water and carbonic acid, and moreover it is an essential for the working of sulphate of ammonia at two stages of its history in the soil. First, chalk is required to convert the sulphate into carbonate of ammonia, and again when this ammonia is undergoing oxidation through the agency of the nitroso' bacteria, chalk (or under certain conditions magnesium or potassium carbonate in its place) is essential for the production of the nitrite. It is difficult to draw a hard and fast line as to the minimum amount of chalk in a soil for the adequate nitrification of ammonia. From the theoretical point of view, an acre of loamy soil to a depth of 6 inches would lose about 95 lbs of lime (previously existing as carbonate) for each hundredweight of sulphate of ammonia applied to the soil. This spells a theoretical minimum of one part of lime in 20,000 parts of soil for the nitrification of 1 cwt. of sulphate of ammonia per acre. A soil containing such a small proportion of lime as this however, would be practically sterile, and it would appear that soils which prove capable of free nitrification and adequately respond to the application of a liberal manuring contain, as a rule, at least a hundred times this amount of lime (0.5 per cent.) At the same time, there are hundreds of soils containing as little as 1 part in a thousand of lime (0.1 per cent.) in which ammonia or dung prove capable of normal nitrification. The results of soil analyses are frequently misleading on this point, as the analyst often gives his results for the total lime in every form of combination in the soil and neglects to state the proportion of lime as carbonate which is alone of service in assisting the efficacy of sulphate of ammonia Many of the cases on record in which chemical fertilizers have failed to produce adequate results ha e been due to a deficiency of chalk in the soil. Such a failure would as inevitably follow the use of dung, guano, or dried blood and is the outcome of a fundamental deficiency in soil fertility. In emphasizing the great necessity of chalk in the soil for the successful use of ammoniacal manure, we are but urging a matter of crucial importance as regards the general fertility of the soil. Chalk is a necessary foundation of the fertility of the soil. In its absence, sulphate of ammonia cannot be nitrified, dung and organic manures are incapable of normal results and acid phosphates or potash salts become ineffective or even injurious manurial applications. The surprising results which have been obtained on many soils by the use of basic slag are largely due to its alkaline nature and the assistance it re ders to the progress of nitrification in the soil. Liming.-If lime be a necessity for the progress of nitrification, and its constant wasting from the soil a necessary result of cultural conditions, its addition to soils in need of it is clearly a matter of some moment. The practical solution of the lime problem is not as simple as it appears at first sight. Lime is itself a powerful and caustic sub stance and when dissolved in water imparts to it the pronounced alkaline properties of 'lime-water'. Warington and Winogradsky have each shown that concentrated lime water destroys the nitrifying bacteria, and that these organisms only flourish when their surroundings are feebly alkaline in character. Με Many a farmer has found that the application of 3 or 4 tons of slaked lime per acre has had a depressing effect on the crop immediadiately following the application. So large a dressing as this applied in the spring of the year is practically certain to have a markedly injurious effect on the soil-bacteria and to prevent their free growth and activity until the alkalinity of the lime has been destroyed by the carbonic acid and vegetable acids derived from the humus of the soil. The following and subsequent seasons, however, will yield gratifying proof of the benefits of lining, provided the soil was one really in need of such treatment It is rarely wise to apply more than a ton per acre at a time, and this is best applied in temperate climates in the winter when nitrification is practically suspended. Excellent results have followed the use of such moderat dressings as 6 or 8 cwt. per acre; the German Agricultural Society, for instance, found in their field experiments with sulphate of ammonia which were conducted at many different centres under the direction of Professor Maercker, that the application of half a ton of lime per acre to the soil before applying a dressing of sulphate of ammonia yielḍe i an increased crop averaging over 2 cwt. of corn in the case of barley and oats.* Chalk or marl is often obtainable at small cost and although at least four times as much is required to equal the effect produced by lime, there is no risk of injury to the soil or crop. D.-Moisture. When soil is dust-dry, nitrification ceases Schloesing found, for instance, that provided the soil was not water logged and free aëration was secured, the rate of nitrification increased with the proportion of moisture in the soil. In one series of experiments he obtained the following results : 4 Generally speaking, nitrification is appreciable in a soil containing per cent. of moisture, and is at its maximum when the soil contains about half the total quantity of moisture which it is capable of retaining From the point of view of practice, nitrification is thus seen to have a close connection with the accidental variations of season, other conditions being the same. A season of intermittent and fairly liberal rainfall is most favourable for nitrification. Excessive wetness, however, depresses this activity by reducing the temperature of the soil and overloading it with moisture. It is important to note that cultivation is of great service in promoting and maintaining the progress * Jahrbuch der Deutschen Landw. Gesellschaft, 1889, p. 450. of nitrification during a trying time of drought. The addition of dung and bulky organic manures greatly promote the water-holding properties of a soil, and the constant preservation of a loose surface tilth by the use of cultural implements prevents the free escap of soil-moisture into the air. It is clear that, after all, the water supply is a most crucial factor in the development of our crops, for not only does this limit the direct feeding capacity of the plant but also the rate of production of nutritive nitrates from the humus of the soil and such manures sulphate of ammonia. E.-Temperature as The most favourable temperature for nitrification is about 100°F., at which temperature Schloesing found it to be ten times as active as at 57°F. In hard frost the action entirely ceases, but, as Warington suggests, in an average English winter the change is generally going on to a small extent. In tropical climates which combine abundance of moisture with a brisk heat, great intensity of nitrification is assured and this is one of the explanations of the remarkable luxuriance of tropical vegetation. Nitrification goes on both night and day in the soil, and the great advantage of warm nights in promoting this fermentation suggests an adequate explanation of the favourable effect of such conditions on vegetation. The temperature of the soil is obviously a question of climate and season, although cultivati n can modify it to a certain extent. Efficient drainage and good surface cultivation, each conduce to an economy of the heat of the soil, and thereby induce improved nitrification on this account. F-Cultivation. Although nitrifying organisms are apparently ubiquitous, their rate of reproduction is relatively slow. A reclaimed soil which has been hitherto unproductive generally requires one or two seasons to develop normal intensity of nitrifying power. From the peculiar natural properties of the nitrobacter organisms, it would appear that constant stirring and turning of the soil should promote their uniform distribution and rapid increase in the soil, and experiments by Schloesing as to the rate of nitrification in soil under various conditions of cnltivation favour this idea. G.-Potash and Phosphates. Both types of nitrifying bacteria require a general mixed diet of mineral food, b sides the carbonic acid gas and ammonia which form their staple nourishment. Of these minerals, potash and phosphates are of the chief importance. It is thus evident that a well nourished soil not only feeds a crop directly, but by promoting nitrification of ammonia exercises a very impo: tant secondary action. CONCLUSIONS. (1) Sulphate of ammonia when applied to fertile soil gives up its acid to the chalk with which it comes in contact. (2) The ammonia is then absorbed by the soil and prevented from loss by drainage owing to the guardianship of humus and clay. (3) Ammonia is converted first into nitrite and finally into nitrate of lime by two distinct types of bacteria. |