No. 4.] NITROGEN AND FERTILITY^ 173 



tran^forniations that are going on in soils and manures ; and 



to add somewhat to the complexity of the problem, many of 



these chan<»es involve more than one kind of bacteria, and 



many different kinds of transformations may be in progress 



at the same time in the soil, the manure pile or compost 



heap. 



Changes due to Bacteria. 



The decomposition bacteria are those which produce decay 

 tmd putrefaction. Decay takes place in- the presence of 

 plenty of oxygen ; putrefaction occurs in its absence. The 

 decomposition bacteria occur practically everywhere in 

 nature. They are so widely distril^uted and are so abun- 

 dant that they are sure to iind any dead nitrogenous organic 

 matter which can serve them as food. With such food sup- 

 ph^ they multiply with exceeding rapidity until they have 

 used up the food material, thereby breaking it up into very 

 simple compounds, such as water, carbonic acid, ammonia, 

 etc. Their growth is then checked, and those remaining lie 

 in the soil ready to grow again when more organic matter 

 comes within their reach. 



Another class of bacteria takes the ammonia that the de- . 

 composition bacteria have formed from the proteids of dead 

 animals or plants, and under suitable conditions change it 

 over into nitric acid (nitrate nitrogen), which is in the 

 form in which plants can use nitrogen as food. Still other 

 bacteria, under proper conditions, luay change this nitrate 

 nitrogen over to nitrite nitrogen, which is a form that puts 

 it out of the reach of plants ; and still another kind of bac- 

 teria may, if the conditions are right, take this nitrite nitro- 

 gen and convert it into free nitrogen, and it will then escape 

 into the air and be lost to aG:riculture. 



There is still another class of l)acteria, that will under 

 suitable conditions take the free nitrogen of the air, and out 

 of it build, either directly or in company with other kinds 

 of bacteria and plants, compounds of nitrogen that plants 

 can use for food. The most important, apparently, and 

 certainly the ones we know most about, Avork in or upon 

 the roots of such plants as peas, clover and other legumes, 

 and enable them to acquire from the air free nitrogen that 

 otherwise is out of their reach. 



