Page:The New International Encyclopædia 1st ed. v. 14.djvu/681

NITRIFICATION. 581 NITRO-BENZENE. NITRIFICA'TION. The term applied to the foiniatiun of nitrates iu soils and manures through the agency of micro-organisms. It iu- chides three ditferent stages, viz., ( 1) (ii)imunizu- tioii, the transformation of organic nitrogen com- pijtuids into anunonia; (2) iiilioziiliun, or the conversion of ammonia into nitrites; and {'.i) nittution, 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 ny- coides and Proteus vulgaris, while the second and third are the work of specific organisms {nitrosu- mtis, nitrous organisms, and nilrobuctcria, nitric organisms), which have been isolated and studied with some minuteness. The retrograde action known as 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 denitrificans I. and II.. although a large immber of other organisms bring about denitrification. The first of the denitrifying organisms named works best in absence of o.xygen : the second is aerobic and works most energetically in connec- tion with Bacillus coli-connnunis. The discovery of the true nature of nitrification was made by Schlocsing and Jliintz in 1877. Since that date the nature of the organisms causing nitrification and the conditions best suited to their activity have been carefuHj' 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 ."jO" C, the organisms be- ing most active, according to Schloesing 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 salifiable 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, Waringtnn, and Frankland have demonstrated the interesting fact that the nitrifying organisms do not require organic matter, hut can grow in a purely mineral medium, deriving their carl)on from carbon dioxide. The organisms are widely distributed and very abundant in all soils except stich as are deficient in lime compnunds and are acid. Miintz found them in abundance on the hare surfaces and in the cracks and fissures of rocks at the summit of movmtains in the Pyre- nees. Alps, and Vosges. The_y are also found in well water, river water, and sewage. As a rule they do not occur in rain or in the air. They decompose carbon dioxiile 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 nitrification dur- ') ing the summer months, especially in fielils lying I in bare fallow. The rate is of course very vari- I able. Warington found it in an inifertilized Rothamsied 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 anmionia showed nearly twice this rate. Much higher rates than these have been reported. Ammonium salts, since they have already i)assed the first stage of nitrification, are considered more readily nitritiable tliaii organic substances which nuist first be converted into ammonium compounds. Miintz and Girard place them first in order of nitrifiability ; then follow guano, green manures, dried blood and meat, powdered horn, poudrette, wool, and leather. Frequently, however, the rate of nitrification of anmionium salts is fouml to l)e much slower than that of organic nuinures. This may be due to a deficiency of lime or to other un- favorable conditions, but Witliers and Kraps re- port experiments in which the ord<'r of nitrifica- tion of diU'erent fertilizing materials in presence of an abundance of calcium carbonate wa,s: dried blood, cottonseed meal, dried fish, bat guano, tankage, ammonium sulphate, bone. A portion of the nitrogen of soil humus is readily nitri- fiable, while a part strongly resists the action of nitrifying organisms. The rate of nitrification in barnj'ard manure in the soil is very variable, but is probably greater than that of soil nitrogen. The old-time nitre be<ls 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 denitrification are exactly the opposite of those which favor nitrification, viz. excess (and de- ficiency) of water and a limited supply of air (oxj-gen). Under certain conditions denitrifica- tion is especially rapid in barnyard manuie and results in considerable losses of nitrogen from this material. BiiiLiOGR.PiiY. Aikman, Manures and Manur- ing (London, 1894) ; Roberts, FertiUtij of the Land (New York, 1807); Conn, Afiricultural Bacteriology (Philadelphia, 15)01) ; United Htntvs Department of Agrieiilturr, 0/}ice of Experiment titulions. Bulletins (Washington, 1892) ; Comples rendiis de VAeademie de Seience, vol. Ixxxiv. (Paris, 1877) : Bulletin de I'AcadHiie Uogale de Belgigue, series 3, No. 25 (Brussels, 1893) ; An- nates agronomiques, vol. xix. (Paris, 1893); Annates' de I'lnstifut Pasteur, iv., v. (Paris, 1890-91); North Carolina Experiment Station Bulletin nCf (Raleigh, 1900). NITRITE OF AMYL. See Amyl Nitrtte. NITRITES. The salts of nitrous acid (q.v.). NITRO-BENZENE, or Nitro-Benzol, C„Hr. NO.. An aromatic nitro-compound manufac- tured from the benzene derived from coal-tar. and in its turn transformed into the aniline that is used in making dye-stuff's. Nitro-benzene is an oilv yellow liquid of specific gravity 1.2 at 0° C. (32° F.) and boiling at 20.5° C. (401° F.) : it may be distilled withont decomposition. It has a "sweet taste, is insohi!)le in water, but dis- solves freely in alcohol and ether. Its odor is very similar to that of oil of bitter almoiuls. which has led to its use in perfumery, under the name of essenec of mirbane. Most of the nitro- benzene of commerce is. however, transformed into aniline. On a large scale nitrobenzene is made by gradually adding a quantity of benzene to a mixture of concentrated nitric and sulphuric acid jilaced in a large iron vessel, the tempera-