Soil, Changes Produced By Micro-organisms. 7 
grow for generation after generation on the same virgin soil with¬ 
out any apparent loss of vigor, the soil remaining as fertile as ever. 
Clearly, the problem for the scientific farmer is to ascertain the 
factors which have kept up the fertility of the virgin soil and apply 
them intelligently to cultivated land. By this means alone, can we 
hope to maintain undiminished the productiveness of our agricul¬ 
tural lands. 
Number and Distribution of Bacteria in the Soil. 
The prevailing idea of a soil is that it is a mixture of finely 
ground rock particles and organic matter capable of supporting 
plant life. Now, there was a time when this definition would fulfill 
the requirements, but, in the light of our present knowledge, it is 
altogether inadequate. It ignores absolutely the presence of mil¬ 
lions upon millions of microscopic plants and animals whose pres¬ 
ence is indispensable to the continued fertility of the soil. Let us 
remember, then, in passing, that a soil is not a dead mass of inert 
material, but rather that it is a teaming complex of living beings. 
Of the three groups of plants which constitute the microbial 
flora of a soil, bacteria are considerably more abundant than either 
molds (fungi) or yeasts. Just as with the larger plants, with which 
we are more familiar, there are certain factors that affect the 
number of bacteria in a soil. Among the most important of these, 
may be mentioned moisture, air, organic matter, mineral foods, 
acidity, or alkalinity, toxic bodies and temperature. As a rule 
sandy soils contain the fewest microorganisms, relatively speaking, 
because of the deficiency of organic matter and the poor water 
holding power. However, if such soils are plowed deeply and a 
liberal amount of green material is thoroughly incorporated, the 
germ content may be increased considerably. An ordinary sandy 
soil will often contain less than 100,000 bacteria per gram (one 
twenty-eighth of an ounce), but after some attention has been given 
to its cultivation, the count may increase to several millions per 
gram. 
In the rich loam soils, the numbers range from 1,000,000 to 
several millions and even into the billions, due to the high organic 
content which favors bacterial growth. 
In the heavy, compact clay soils, such as our typical adobe, 
the germ count is considerably less, owing both to the poor aera¬ 
tion and absence of organic matter. 
Under artificial conditions, such as are offered in a green¬ 
house, where the moisture, temperature, and food supply are 
especially favorable, it is not uncommon to find numbers ranging 
