80 PHYSIOLOGY OF NUTRITION 



Under natural conditions these plants may assimilate minute quantities of nitrogen 

 from the ammonia of the air, but this source of nitrogen is generally quite negligible. 



2. The Nitrogen of the Soil. — The soil contains free nitrogen (which cannot be 

 assimilated by ordinary plants), ammonia and ammonium compounds, nitrates, 

 nitrites, and organic nitrogenous substances. Nitrates in the soil are the main source 

 of nitrogen for ordinary plants, though some forms are apparently able to assimilate 

 some nitrogen in the form of nitrites or in the form of ammonia or ammonium salts. 

 Organic nitrogen compounds are first decomposed (by soil microorganisms), giving 

 nitrates, and then the resulting nitrates are assimilated by higher plants. Ammonium 

 salts are similarly converted to nitrates in the soil, by microorganisms, as also are 

 nitrites. Ammonium compounds and organic nitrogenous substances (arising from 

 the decay of animal and plant tissues, etc.) are held in the soil in considerable amounts, 

 but nitrates are readily washed out by percolating rain water, and carried away in the 

 soil drainage. More nitrates are gradually formed, so that there is always a supply of 

 these salts that may be absorbed through the roots of ordinary plants and assimilated. 



3. Nitrification in Soils.— The production of nitrates in the soil, from other nitro- 

 genous compounds (and from free nitrogen), occurs through the action of soil bacteria. 

 For the activity of these nitrifying organisms a continuous supply of oxygen is neces- 

 sary in the soil. According to Vinogradskii's work, ammonia and ammonium salts are 

 assimilated by nitrite bacteria in the soil, which give off nitrites, and nitrites are assimi- 

 lated by nitrate bacteria in the soil, which give off nitrates. These two groups of soil 

 bacteria derive their carbon compounds by synthesis, from carbon dioxide or carbo- 

 nates as source of carbon. The energy for this synthesis is derived from the oxidation of 

 ammonia or of nitrites; the nitrites and nitrates that are produced may be considered 

 as by-products. In the presence of organic compounds that may be readily oxidized 

 (like sugars), these bacteria secure their carbon compounds directly, without synthesis 

 from carbon dioxide, and they do not then alter the organic nitrogenous compounds that 

 may be present, nor does nitrification occur. Nitrogenous organic compounds are 

 not assimilated by the nitrite and nitrate bacteria, but they are used by another group 

 of soil bacteria, the ammonifying forms, which give off ammonia as a by-product. 

 When bacteria of all three groups are present, the nitrogen of other nitrogenous 

 compounds is ultimately converted, by three steps, into nitrate nitrogen. (1) The 

 ammonifiers produce ammonium compounds (NH 4 ) from nitrogenous organic sub- 

 stances. (2) The nitrite bacteria produce nitrites (N0 2 ) from ammonium compounds. 

 (3) The nitrate bacteria produce nitrates (N0 8 ) from nitrites. 



Ammonium salts are generally not largely assimilated by ordinary plants, and 

 ammonium nitrogen usually becomes readily assimilable only after nitrification, with 

 formation of nitrates. Wagner found that ammonium salts were beneficial, as ferti- 

 lizer, in lime soils, but not in other soils. In lime soils the lime prevents the develop- 

 ment of any considerable acidity. Nitrites are generally not largely assimilated by 



ordinary plants. 



4. [6] Assimilation of Atmospheric Nitrogen by Soil Bacteria.— Still another group 

 of soil bacteria assimilate free nitrogen, as was shown by Vinogradskii and Beijennck, 

 and these bacteria form organic nitrogen compounds or nitrates. The energy for this 

 nitrogen fixation is derived from the oxidation or fermentation of organic compounds, 

 such as sugars. Some of these nitrogen-fixing bacteria thrive in the presence of 

 oxygen, others are inhibited by oxygen. There are also soil bacteria, thriving 

 under special conditions, that convert nitrate nitrogen into nitrite or ammonium 

 nitrogen, or even into free nitrogen, these being denitrifying processes. 



