THE BIOLOGY OF THE SOIL 



95 



three stages. Each stage is effected by a different kind 

 of bacterium. In the first, compoundB 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. 



(6) Nitrogen - Bacteria. 

 — A few bacteria, how- 

 ever, are able to assimi- 

 late the free nitrogen 

 that exists in the atmo- 

 sphere, just as green 

 plants assimilate car- 

 bonic acid gas. These 

 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 

 actually increase the amount of available nitrogen present 

 in it (see Symbiosis, p. 132). 



2. Soil Protozoa. — Minute unicellular animals occur in 

 enormous numbers in the soil and play a prominent part 

 in determining its fertility. These protozoa feed on the 

 soil bacteria, and if present in too great abundance 

 hinder, or even prevent, the formation of nitrogenous 

 salts. In such cases, by partially steriHzing the soil by 

 means of antiseptics, and particularly by heating it, its 



FiQ. 27. — Bird's - Foot Tbefoii, 

 (Lotas comiculatus), showetq Root- 

 NODTJLES (o). 



