790 BACTERIA : THEIR WORK 



cessively minute and rank as the smallest of all living organ- 

 isms. None of them are capable of attacking ammonia or its 

 salts. 



The rate of production of nitrites and the conversion of the 

 latter into nitrates in the soil appear to be equal, for free 

 nitrites cannot be detected in soils in which nitrification is going 

 on. 



Both types of nitrifying bacteria are present in manure heaps, 

 in sewage, river-water, and in all soils, especially near the 

 surface. In order that their work may proceed rapidly, an 

 adequate supply of oxygen and water are necessary as well as a 

 suitable temperature and the presence of alkaline salts, preferably 

 the carbonates of calcium, magnesium or potassium : darkness is 

 also essential to nitrification. 



In excessively dry soils or those which are insufficiently 

 drained, and therefore imperfectly supplied with air, the process 

 is stopped : in winter, and whenever the temperature falls below 

 5° or 6° C., the organisms cease to work. 



The presence of very small amounts of easily oxidisable 

 organic compounds is detrimental to the growth and activity of 

 both types of nitrifying organisms, the nitrite-forming bacteria 

 being more sensitive in this respect than the nitrate-forming 

 species. On this account the nitrification process does not 

 begin until all the organic material has been fermented by other 

 species of bacteria. 



The nitrate-forming bacteria are excessively sensitive to 

 ammonia, five parts of the latter in one million of the nutrient 

 medium being sufficient to check their growth and work. 



With the exception of nitrate of soda, practically all nitro- 

 genous manures, such as sulphate of ammonia, dung, rape-dust, 

 wool and fur-waste, bone-meal, fish-meal, and guano, must first 

 be nitrified before they can be of service to crops. It is the 

 business of the farmer to promote the change by judicious 

 addition of lime where this substance is deficient in the soil, and 



