It will be seen that, in the acquirement of energy from such a compound, ammonia is released as a by-product. It is not certainly known what is the exact course of the reactions brought about by bacteria in soil during the breaking-down of organic nitrogen compounds, but they result in the splitting off of most of the nitrogen as ammonia. Herein lies the great importance of the process, for the production of ammonia is an essential stage in the formation of nitrate in the soil, and on the supply of nitrate the growth of most crops largely depends. [Illustration: FIG. 4.--Quantities of ammonia produced by pure cultures from 5 grams of casein in the presence of varying quantities of dextrose. (After DORYLAND.) X-axis: Percentage of dextrose added. Y-axis: Milligrams of NH₃ produced.] It is very important to note that the production of this ammonia is only a by-product in the economy of the bacteria, the benefit that they derive from the reactions being due to the release of energy involved in the decomposition. The common ammonia-producing bacteria in the soil have been found equally capable of deriving their energy by the oxidation of sugars and similar non-nitrogenous compounds. Fig. 4 shows an experiment by Doryland,[17] in which cultures of common soil bacteria were grown in peptone solution, to which increasing quantities of sugar were added. One can see that, as the amount of sugar is increased, the production of ammonia is lowered, since the bacteria are obtaining energy from the sugar instead of from the nitrogen compound, peptone. Consequently, if soil contains a quantity of easily decomposible carbohydrate material, bacteria will derive their energy from this source, and the production of ammonia and nitrate will be lowered. Thus the addition of sugar or unrotted straw to the soil often lowers the nitrate production, and consequently reduces the crop yield. If the soil is sufficiently rich in carbohydrate material, the bacteria may multiply until the supply of organic nitrogen is used up, and then will actually assimilate some of the ammonia and nitrate already existing. There is thus a balance of conditions in the soil due to varying proportions of nitrogenous and non-nitrogenous energy material. When nitrogen compounds are the predominant energy source, the bacteria utilise them, and ammonia is released. When a non-nitrogenous energy source predominates, this is utilised and little or no ammonia is released, and in extreme cases ammonia may be assimilated.