THE BIOLOGY OF THE SOIL 



95 



2. Soil-Bacteria. Countless swarms of bacteria a few 

 malignant, but the most so beneficent as to be indispens- 

 able to plant-life inhabit the upper layers of the soil. 

 It seems likely that most of the chemical changes taking 

 place in the soil, especially in its organic constituents, 

 are due to their activities. The most important are the 

 following : 



(a) Nitrate - Bacteria. The nitrogenous compounds 

 present in decaying organic matter are decomposed in 

 three stages. Each stage is effected by a different kind 

 of bacterium. In the first, compounds of ammonia are 

 produced, but these can 



only be effectively util- 

 ized as food by fungi. 

 If air is plentiful, these 

 ammonium - compounds 

 are converted into ni- 

 trites, and finally into 

 nitrates. In the form of 

 nitrates, they constitute 

 the sole source from 

 which the green plant 

 obtains its nitrogen. 



(b) Nitrogen - Bacteria. 

 A few bacteria, how- 

 ever, are able to assimi- 

 late the free nitrogen 

 that exists in the atmo- 



sphere, just as green FIQ 27 . _ BmD>s . FooT TEEFOIL 



plants assimilate car- (Lotus comiculatus), SHOWING ROOT- 



bonic acid gas. These NODULES (a). 



bacteria are present in 



all well-aerated soils. By their agency the soil is always 



being replenished with nitrogen, for when they die their 



bodies are decomposed and nitrates produced. 



(c) Root-Nodules. Other bacteria which utilize atmo- 

 spheric nitrogen inhabit the bodies of green plants. They 

 swarm in the nodules, or swellings, which occur so abun- 

 dantly on the roots of leguminous plants e.g., peas, beans, 

 clover (Fig. 27). For this reason, leguminous crops may 

 be raised in a soil from which nitrates are excluded ; they 

 make no demands upon the soil for this salt, and if they 

 are dug into the ground at the end of the season, they 



