SOIL SOLUTION 



157 



The nitrogen-fixing Bacteria take nitrogen from the air and 

 oxidize it to nitrates. They add nitrogen to the soil while other 

 kinds of Bacteria simply make available to higher plants the 

 nitrogen already present in the organic matter of soils. Some 

 kinds of the nitrogen-fixing Bacteria live independently of higher 

 plants, getting their food and energy 

 from the humus of the soil, while other 

 kinds live on the roots of higher plants, 

 such as Clover, Alfalfa, Beans, and other 

 Legumes. ' Those living on higher plants 

 enter the roots through the root hairs 

 and become established in the cortex. 

 As a result of their presence, the roots 

 develop nodules in which the Bacteria 

 live and multiply. (Fig. 138.) They 

 use some of the carbohydrates in the 

 roots, but the nitrates they form by fix- 

 ing the nitrogen of the air more than 

 compensates for the damage they do. 



The denitrifying Bacteria decompose 

 nitrates, thus freeing the nitrogen which 

 then escapes from the soil as a gas. 

 Thus they are actually harmful, tending 

 to reduce the amount of nitrogen in soils. 



FIG. 138. Nodules on 

 the roots of a Pea. After 

 C. M. King. 



They are most active 



in soils containing an excess of organic matter and poorly aerated. 

 In soils well cared for the effects of these Bacteria are not great. 



The animals of the soil are also of many kinds. The smallest 

 are the Protozoa, the one celled animals of which the Amoeba is 

 a representative. The Protozoa are abundant in moist, rich 

 soils. They do no doubt exert an influence on soil fertility, but 

 their influence is not definitely known. They feed upon the 

 Bacteria of the soil and some think that they prevent the Bacteria 

 of the soil from becoming too numerous. There is also evidence 

 that at times they destroy too many of the useful kinds of soil 

 Bacteria and in this way are detrimental, to soil fertility. In 

 some laboratory experiments with soils, nitrates accumulated 

 more rapidly in soils in which the Protozoa had been killed. 



The numerous worms of the soil have some influence on soil 

 fertility. The holes they make in the soil permit better aeration 

 and drainage. They also digest the organic matter of the soil 



