BACTERIA OF THE SOIL 247 



able period for the content in nitrates. Determination of 

 the ammonifying and nitrifying power of soils is among 

 the more valuable determinations made in the laboratory in 

 an effort to estimate soil fertility. 



Assimilation of Nitrogen by Plants. All plants which 

 live in the soil, the bacteria, the yeasts, the molds, the 

 higher fungi, many alg, the mosses, the ferns, and the 

 seed plants all require nitrogen for their growth. The 

 process of taking up nitrogen and utilizing it in metabolism, 

 particularly in the building up of protoplasm, is termed 

 nitrogen assimilation. 



It was previously noted that bacteria of different kinds 

 utilize many different sources for their nitrogen supply. A 

 few species are capable of taking it directly from the air. 

 Many more can utilize amino acids, ammonia or nitrates. 

 Nitrogen taken up by bacteria and built into protoplasm is 

 locked up, of course, in a form unavailable to higher plants. 

 Molds and yeasts resemble bacteria in their ability to use 

 various compounds of nitrogen in their metabolism. It is 

 not strange, therefore, that in fertile soils an appreciable 

 amount of the total nitrogen contained in the soil should be 

 found in the cells of the yeasts, molds, bacteria and other 

 fungi present. 



Some higher plants have the ability to assimilate amino 

 acids, but these latter are present usually in very small 

 quantities and only at intervals in most soils. Many species 

 likewise can utilize ammonia, and most species can utilize 

 nitrates. Nitrites in general are poisonous to plant roots. 

 The nitrogen upon entering the plant is rapidly built up 

 into more complex nitrogenous compounds; first, probably 

 into amino acids, then into peptids, then finally into the 

 complex plant proteins. Inasmuch as animal proteins are 

 in all cases derived directly or indirectly from plant pro- 

 teins this may be said to complete the nitrogen cycle. There 

 are, however, several transformations of nitrogen not in- 



