The amount of nitrogen fixed per unit of energy material decomposed varies greatly, according to the organism and the conditions. Winogradsky found that his _Clostridium_ assimilated 2-3 mgs. of nitrogen per gram of sugar consumed. Lipman found that _Azotobacter_ fixed 15-20 mgs. of nitrogen per gram of mannite consumed. [Illustration: FIG. 5. Caption: Azotobacter. Decrease in efficiency in N fixation with age of culture. (Koch & Seydel.) X-axis: Days. Y-axis: Milligrams of Nitrogen fixed per gram of dextrose consumed.] It is found, however, that in liquid culture, the ratio of nitrogen fixed to carbohydrates oxidised varies according to the age of the culture, falling off rapidly as the age increases[42] (Fig. 5). This decreasing efficiency in cultures may be due to the accumulation of metabolic products such as would not occur under soil conditions. Indeed, the efficiency of _Azotobacter_ in a sand culture has been found by Krainskii[39] to be considerably greater than in solution. It is thus probable that in soil the nitrogen-fixing organisms are less wasteful of energy material than under the usual laboratory conditions. It is to be hoped that future research will indicate what are the conditions that produce the greatest economy of energy material in nitrogen fixation. The fixation of nitrogen in soil is depressed by the presence of considerable amounts of nitrates. This is, in all probability, due to the fact that nitrogen-fixing organisms are able to utilise compounds of nitrogen where these are available. The energy needed to build up amino-acids and proteins from nitrate or ammonia is, of course, far less than that required to build up these substances from elemental nitrogen. It is, therefore, not surprising that where nitrate is available, _Azotobacter_ will use it in preference to fixing atmospheric nitrogen.[5] TABLE III.--ASSIMILATION OF NITRATES. BY AZOTOBACTER IN PURE CULTURE--(_Bonazzi_). +---------------------------+--------+-----------+------+ | | Nitrate| Organic |Total | | | and | Nitrogen |Fixed | | | Nitrite|and Ammonia| or | | |Present.| Present. |Lost. | +---------------------------+--------+-----------+------+ | | mgs. | mgs. | mgs. | |_Culture with nitrate_-- | | | | | At beginning | 8·55 | 0·76 | -- | | After growth | 0·2 | 8·71 | -0·4 | |_Culture without nitrate_--| | | | | At beginning | -- | 0·76 | -- | | After growth | -- | 4·50 | +3.74| +---------------------------+--------+-----------+------+ (Growth period--24 days at 25° C.)