NI'TRIFICA'TION. The term applied to the formation of nitrates in soils and manures through the agency of micro-organisms. It in cludes three different stages. viz., (I.) (Immuniza tion, the transformation of organic nitrogen com pounds into ammoeia ; (2) nitrozation, or the conversion of ammonia into nitrites; and (3) nitration, or the formation of nitrates from the nitrites. Modern investigation has shown that all three of these changes are due to the activity of micro-organisms, the first being brought about by a variety of organisms of the putrefactive class, the most prominent being Bacillus my coides and Proteus vulgaris, while the second and third are the work of specific organisms I nitruso Inas, nitrous organisms, and nitrobacteria, nitrie organisms), which have been isolated and studied with some minuteness. The retrograde action known es denitrification, by which nitrates are reduced to the less highly oxidized forms and even to free nitrogen, has also been shown to be the work of micro-organisms. mainly Bacillus deuitrificans 1. and II_ although a large number of other organisms bring about denitrification. The first of the denitrifying organisms named works best in absence of oxygen; the second is aerobic and works most energetically in connec tion with Baeilhis coli-communis. The discovery of the true nature of nitrification was made by Sehloesing and Miiutz irr 1s77. Since that date the nature of the organisms causing nitrification and the conditions best suited to their activity have been carefully studied by many other in vestigators. These studies have shown that the activity of the nitrifying organisms is limited by a certain range of temperature, viz, from slightly above freezing to about 50° C., the organisms be ing most active, according to Sehloesing and Miintz, at a temperature of about 37° C. Other essential conditions are an adequate supply of air (oxygen). for which reason activity is generally confined to the surface layer of soil and moisture, and the presence of certain mineral plant food constituents, especially phosphoric acid and a satiable base (lime as carbonate).
Nitrification will go on only in a slightly alkaline medium, but excessive alkalinity is as fatal to the process as acidity. Winogradsky, Warington, and Frankland have demonstrated the interesting fact that the nitrifying organisms do not require organic matter. but can grow• in a purely mineral medium, deriving their carbon from carbon dioxide. The organisms are widely distributed and very abundant in all soils except such as are deficient in lime compounds and are acid. Mintz found them in abundance on the hare surfaces and in the creeks and fissures of rocks at the smninit of mountains in the Pyre nees. Alps, and Vosges. They are also found in well water, river water, and sewage. _Ns a rule they do not occur in rain or in the air. They decompose carbon dioxide most readily, and hence cause nitrification most rapidly in the dark.
While nitrification probably goes on to some extent during the winter. the conditions are usually most favorable to rapid nitrifieation dur ing the summer months, especially in fields lying in bare fallow-. The rate is of course very vari able. Warine-ton found it in an unfertilized Rot hamsted soil to be 1.3 pounds of nitric nitro gen per day per acre to a depth of nine inches. Similar soil fertilized with sulphate of ammonia showed nearly twice this rate. Much higher rates than these have been reported. Ammonium salts, since they have already passed the first stage of nitrification, are considered more readily nitritiable than organic substances which must first be converted into ammonium compounds. Miintz and Oirard place them first in order of nitritiability; then follow guano, green manures, dried blood and meat, powdered horn, poudrette, wool, and leather. Frequently, however, the rate of nitrification of ammonium salts is found to be much slower than that of organic manures. This may be due to a deficiency of lime or to other un favorable conditions, but Withers and Fraps re port experiments in which the order of nitrifica tion of different fertilizing materials in presence of an abundance of calcium carbonate was: dried blood, cottonseed meal, dried fish, hat guano. tankage, ammonium sulphate, bone. A portion of the nitrogen of soil humus is readily !ari while a part strongly resists the action of nitrifying organisms. The rate of nitrification in barnyard manure in the soil is very variable, but is probably greater than that of soil nitrogen. The old-time nitre beds or plantations were simply examples of rapid nitrification under peculiarly favorable conditions.
The constant production of nitrates in the soil and the readiness with which they are washed out in the drainage furnish a strong argument in favor of the practice of a system of cropping which keeps the soil covered with vegetation as constantly as possible. The conditions favorable to denitrifieation are exactly the opposite of those which favor nitritieation, viz. excess (and de ficiency) of water and a limited supply of air (oxygen). Under certain conditions denitrifica tion is especially rapid in barnyard manure and results in considerable losses of nitrogen from this material.
BIBLIOGRAPHY. AikMall, Man fires and ManurBibliography. AikMall, Man fires and Manur- ing (London, 1894) ; Roberts. Fertility of the Land. (New• York, 1897) ; Conn, Agricultural Bacteriology (Philadelphia. 1901); United States Department of Agriculture, Office of Experiment ta lions, Bulletin 8 (Washington, 1892) Comptes ?Twins de r.lead emir! de Science, vol. lxxxiv. (Paris, 1877) ; Bulletin (lc l'Arachltnie Royale de Belgique. series 3, No. 25 (Brussels, 1893) ; wiles agronamiques, vol. xix. ( l'aris, A nnales de linstitut Pasteur, iv., v. (Paris, 1890-91) ; Worth Carolina Experiment Station Bulletin. 176 (Raleigh, 1900).