Although a large number of the common organisms in the soil produce ammonia in culture media containing peptone, the relative importance of these in the soil has yet to be decided. It was supposed that the spore-forming organisms related to _Bacillus mycoides_ were of chief importance. This supposition dates from the work of Marchal,[49] who studied the production of ammonia by an organism of this group in culture solution, and found it to be a very active ammonifier. As already mentioned, however, there is some doubt as to whether the large spore-forming organisms are very active under soil conditions.[12],[13] The existence of rapid fluctuations in nitrate content, found to exist in soil, may in the future indicate which are the most active of the common bacteria in the soil itself by enabling us to observe which types increase during periods of rapid ammonia and nitrate formation. (3) _Nitrate Production._ The ammonia produced in the soil under normal field conditions is rapidly oxidised successively to nitrite and to nitrate, a process known as nitrification. The process of nitrification is more rapid than that of ammonia production, with the consequence that no more than traces of ammonia are able to accumulate. The rate at which nitrate is formed in the soil is consequently set by the slower process of ammonia production. The work of Schloesing and of Warington showed that the oxidation of ammonia was the work of living organisms. It is, however, to Winogradsky’s isolation and study of the causative organisms that we owe our present knowledge of the biology of the process. By a new and ingenious technique, he isolated from soil two remarkable groups of bacteria that bring about nitrification. The first group oxidises ammonium carbonate to nitrite, and was divided by Winogradsky into the two genera, _Nitrosomonas_, a very short rod-like organism bearing a single flagellum, and _Nitrosococcus_, a non-motile form found in South America. The second group oxidises nitrites to nitrates. They are minute pear-shaped rods to which he gave the name _Nitrobacter_. Winogradsky found that the first, or nitrite-producing group, would live in a culture solution containing:-- 2·25 grams ammonium sulphate, 2·0 „ sodium chloride, 1·0 „ magnesium carbonate, to the litre of well water.