NITROGEN FIXATION BY NODULE BACTERIA 175 



Nitrification Leads to Loss of Organic Matter.— The bacteria 

 engaged in nitrification do not add one atom of nitrogen to the soil 

 supply; they simply change insoluble forms of nitrogen into soluble 

 forms which are either used by plants or are leached from the 

 soil. This explains in a large measure how the soil organic matter 

 is used up; and it also emphasizes the necessity of maintaining a 

 good supply of this material in soils as a source of easily available 

 mineral elements as well as of nitrogen. 



ORGANIC MATTER 



(confoming complex , Insoluble 

 n if rag en compounds ) 



Nitrates- soluble salts confammg nitrogen 

 ^\ (plants absorb these salts) 



\Nifrif ICQ f ion = the conversion of 

 \ ammonia into nitrates by 

 ) nitrifying bacteria. 

 \mmonia ' 



(NHs) 



FiQ. 115 — Diagram illustrating nitrification. 



Some Soils Lack Nitrifying Organisms.— Some peat and muck 

 soils, on being reclaimed through drainage, fail to produce satis- 

 factorily even though mineral fertilizers be supplied. Such results 

 are sometimes due to a lack of ''available'' nitrogen; in spite of 

 the fact that these soils may be well supplied with, or are composed 

 aknost entirely of, organic matter. DecompoMng and nitrifying 

 organisms may be lacking. Since good manure, especially horse 

 manure, contains myriads of these kinds of organisms, they may 

 be supplied to such soils through manuring. This explains in 

 part the beneficial effect of man-ore, as a first treatment, to many 

 peat and muck soils. 



NITROGEN FIXATION BY SOIL BACTEKIA 



Nitrogen Fixation by Nodule Bacteria.— For many years 

 scientists were puzzled to know three things: (a) Why a clover 

 plant could grow perfectly well when no available nitrogen was 



