8 
The Colorado Experiment Station. 
around 100,000,000 per gram. The same is true of soil from sew¬ 
age farms and of soil which has been partially sterilized either by 
disinfectants or heat in order to destroy certain animal forms that 
have been shown to feed upon bacteria. On the whole, it may be 
said that microorganisms are most abundant in the soils which are 
in the highest state of cultivation, and as a rule those soil condi¬ 
tions which are favorable to the growth of the higher plants are 
also well suited to the development of bacteria. 
Distribution of Bacteria at Different Depths. 
From what has been said already concerning the factors which 
favor the growth of soil bacteria, it would be natural to suppose 
that the organisms would be most abundant in the surface layers 
that receive manure and more thorough aeration through cultiva¬ 
tion.. This is exactly the case. Bacteria are most numerous in the 
first seven to eight inches of an arable soil; the germicidal action 
of the sunshine and the drying at the very surface reduce the num¬ 
bers there somewhat. Below sixteen inches, the count falls off very 
rapidly, until at three feet, in the rainfall sections, it is said the soil 
is nearly sterile. The texture of the soil, however, makes consid¬ 
erable difference in the lower limit, since with a more open, loose 
structure, organisms are very apt to find their way to the lower 
levels. In irrigated regions, we know that bacteria frequently 
occur at depths greater than three feet, for it is not uncommon to 
find tubercles on alfalfa roots, produced by bacteria, eleven to 
twelve feet below the surface. Other bacteria have been found as 
deep as eight or ten feet in open soils where the drainage is good 
and where alfalfa roots have penetrated the earth to considerable 
depths. They have undoubtedly been carried down from the sur¬ 
face with the irrigating water and have followed the paths of 
alfalfa roots to the lower regions. In the more compact soils, we 
find the lower limit of microbial activity much nearer the surface 
since the conditions for aeration are poorer, and the filtering action 
of the finely divided soil tends to prevent a very extensive penetra¬ 
tion even under a head of irrigating water. 
THE DECOMPOSITION OF ORGANIC MATTER. 
The Availability and Source of Plant Food. 
The superior quality of well rotted manure, which has not 
been allowed to become worthless by leaching, as compared with 
the fresh product, is so well known to all experienced farmers that 
any comment on this point would be presumptuous. Again, when 
a heavy stand of alfalfa, red clover, or vetch is plowed under in a 
