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, espectally 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. 
(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 
em Fic. 27. — Bmop’s - Foor Trerom 
plants assimilate car- (Lotus corniculatus), sHow1NG 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 
