16 THE FERTILITY OF THE SOIL [CH. 



nitrifying bacteria, and it was not till Winogradsky in 

 1891 hit on the brilliant idea of using a jelly of silica, 

 that they were grown and studied. Both organisms 

 are extremely small they are, in fact, the smallest 

 known in the soil. Unlike the others they do not 

 require organic matter as food, they make their own 

 supply from carbonates or, like plants, from carbon 

 dioxide. But unlike plants they do not want sun- 

 light for this purpose, indeed sunlight kills them. 

 Where, then, do they get their energy from ? Wino- 

 gradsky adduced very strong evidence, which has 

 never been disproved, that the energy comes from 

 oxidation of ammonia : he found a definite relation- 

 ship between the amount of ammonia oxidised and 

 the amount of carbon assimilated. It appears tha 

 ammonia is the only compound they can utilise. 

 Many other substances have been tried, but without 

 results, and we can take it as proved, as well as any 

 negative proposition can be proved, that ammonia is 

 the only substance from which nitrates are made, 

 and that all the nitrate we find in the soil has pre- 

 viously been ammonia. This conclusion is very im- 

 portant and leads us to look for ammonia in the soil. 

 But in no arable soil yet examined has more than 

 a trace been discovered at any time of the year. We 

 must, therefore, conclude that the rate at which 

 ammonia is oxidised to nitrite is greater than the 

 rate at which it is formed. But nitrites are never 



