222 NITRIFICATION 



the minimum retarding amount of ammonia to be about 2 parts 

 per million. It is seldom sufficient ammonia accumulates in soils 

 under natural conditions to interfere with the multiplication of 

 nitrifying bacteria. 



Just as all organic nitrogen must be ammonified before it can 

 be changed by the nitrosomonas to nitrous acid, so all ammonia 

 compounds must be oxidized to nitrous acid before the nitrobacter 

 can convert them into nitric acid. The nitrite organism readily 

 oxidizes nearly all nitrites in solutions containing 0.5 to 1 gram 

 per liter, but larger quantities of the nitrites are toxic even to the 

 nitromonas. 



Organic Matter. Winogradsky early learned that the nitrifying 

 organisms will not grow in a medium containing soluble organic 

 matter, and since that time numerous experiments have been 

 made to account for this apparent discrepancy. It was well known 

 that nitrification takes place in the soil and compost which contains 

 organic material. Hence, the theory was soon advanced that 

 organic matter in the form of humus is not injurious and may 

 actually be beneficial, as is illustrated by the work of Smirnov: 



Humus NITRIC NITROGEN IN 100 GRAMS SOIL 



Per cent. At beginning After 19 Days After 36 Days After 73 Days 



0.42 . . . . 1.5 ing. 14.0mg. 25.5mg. 28.0mg. 



3.55 .... 0.5 21.0 38.0 53.0 



Miintz later concluded that humus even in larger quantities 

 does not interfere with nitrification, but on the other hand it is 

 favorable to it. Nor is an abundance of humus a necessary condi- 

 tion to nitrification, since soils poor in this constituent gradually 

 develop intensive nitrification. He considers that the humus 

 favors the multiplication of the nitrifying organisms and a soil 

 which contains a large amount of humus is more abundantly sup- 

 plied with these organisms and more apt to enter into rapid nitri- 

 fication. 



Coleman found dextrose, cane sugar, glycerin, and lactose, in 

 small amounts, to favor nitrification, and in some cases even as 

 much as 1 per cent, of dextrose has proved beneficial. This con- 

 clusion has been confirmed by numerous other workers. Where 

 larger quantities of sugars are used there is usually a disappearance 

 of nitrates. This is probably due to its favoring other organisms 

 which produce protein from the nitrates rather than interfering 

 with nitrification or accelerating to a great extent denitrification. 

 The optimum amount of organic matter for most rapid nitrification 

 varies with the moisture and nature of the 'soil. Fischer found 

 even peat extract to favor nitrification, while Niklewski claims 

 that nitrification occurs in solid stable manure when there is not 

 much liquid manure mixed with it, and that on the first day nitrite 

 bacteria are found in the manure coming originally not from the 



