AA NATURE’S FOOD SUPPLY. THE CARBON CYCLE 
in unfrozen soil, but that then their growth is kept down by the 
rivalry of other microorganisms that grow only at warmer tem- 
peratures. This theory is a possible explanation, but it is also 
possible that the increase is only an apparent one. Determina- 
tions of the number of bacteria in soil are made by means of the 
plate method (see Experiments 2 and 24 in Chapter XXT). In 
employing this method, itis assumed that each colony on the gela- 
tin or agar plate develops from a single organism It is well- 
known, however, that a colony often arises from a clump of bac- 
teria instead of from a single organism, and for this reason the 
plate count is always an inaccurate one. It will be readily seen 
that if anything happens to break up the clumps of bacteria in 
soil, the plate count will be increased, although the actual number 
of bacteria present may remain the same. Hence, it is still 
uncertain if there are actually more bacteria in frozen than in 
unfrozen soil. 
The microédrganisms found in the soil include true bacteria, 
thread bacteria, and smaller numbers of the higher fungi and 
yeasts. Microscopic animals (Protozoa) have also been observed. 
The most common soil bacteria are very small non-spore-forming 
and generally non-motile rods. A few larger motile spore-forming 
rods (Bacilli) are also found. Cocci are found, but not in great 
abundance. Thread bacteria (Actinomyces) are very common, 
and give to soil its characteristic odor. The Protozoa in soil may 
possibly be of great importance because of their ability to destroy 
and feed upon the bacteria. It is claimed by some that the 
Protozoa are so active in destroying bacteria that they are the 
most harmful factor present in the soil. 
Each of the classes of microdrganisms just mentioned is repre- 
sented by many varieties, and each has an important share in the 
complex activities going on in the soil. These functions and the 
relation of the soil microorganisms to them may best be under- 
stood by noticing in succession their relation to the various soil 
ingredients that constitute plant foods. 
