BIOLOGICAL FACTORS 197 



These bacteria are associated with the plant tissues in all stages of 

 development from seed to maturity, but nodules form only on 

 leaves. Although these bacteria have been credited with nitrogen- 

 fixing ability, it is certain that the plants are not dependent upon 

 them for their nitrates. Certain products of their presence are 

 necessary, however, for without the bacteria, seedlings do not ma- 

 ture. 127 The relationship is, therefore, truly symbiotic (Fig. 92). 



Other Soil Flora.— In addition to the symbiotic fungi and bac- 

 teria, great numbers of bacteria, fungi, and algae occur free in the 

 soil. Their importance to natural plant communities cannot be 

 evaluated accurately, but their significance is indicated by their 

 general functions of making nitrogen available by fixing it, or 

 releasing it with other nutrients through their activities in decom- 

 posing organic matter. 



The fixation of nitrogen as nitrates by free soil organisms is 

 known to be accomplished by a number of bacteria under both 

 aerobic and anaerobic conditions and even in practically sterile 

 soils. Some are inhibited by acidity or chemical constituents of the 

 soil, and temperature ranges may affect their activity, but, in gen- 

 eral, some are present almost everywhere. Certain algae are also 

 thought to be capable of nitrogen fixation. 



All nitrates appearing in the soil from sources other than fixa- 

 tion are the products of organic decomposition, particularly of 

 proteins. The breakdown involves a series of chemical changes 

 accomplished by a succession of bacteria and fungi. The first of 

 these causes the proteins to break down into the less complex pro- 

 teoses, peptones, and amino acids. This digestive process allows 

 the bacteria and fungi to use a part of the nitrogen for themselves, 

 and, in so doing, they release ammonia as a waste. Few plants can 

 use ammonia directly, and many are injured by its accumulation 

 in the soil. Ammonification is followed by nitrification, in which a 

 group of nitrite bacteria convert the ammonia to nitrites by partial 

 oxidation. Subsequently, the activities of nitrate bacteria cause 

 further oxidation and the formation of nitrates. Now, finally, the 

 nitrogen is usable by higher plants. Digestion of proteins, am- 

 monification, and nitrification must all take place before organic 

 nitrogen can be used by plants, and the succession of bacteria must 

 be present if the processes are to occur. 



