Fixation of nitrogen from the air by bacteria. The air is made 
up largely, nearly 80 percent, of nitrogen. If green plants could 
use this nitrogen directly we would not need to use sodium nitrate 
from Chile nor to employ other expensive means of nitrogen 
fertilization. 
There are, however, two general groups of soil bacteria that 
have the power of taking the free nitrogen out of the air and 
“fixing” it, by changing it to nitrates and possibly other nitrogen 
compounds. 
One group constitutes the “ legume bacteria" (Pseudomonas 
radicicola) of which there are said to to be six varieties (the 
alfalfa— sweet clover, the clover, the vetch — garden pea, the cow- 
pea, the soybean, and the garden bean bacteria), which grow on 
the roots of plants of the legume family, in tiny swellings called 
“root-tubercles,” or “nodules.” These will not grow well except 
in soils which are sweetened with lime and well aerated. Further, 
the application of gypsum (calcium sulphate) to soils seems to 
have a remarkable stimulating effect on these legume bacteria, 
causing an increase, sometimes, equal to 100 percent, and caus- 
ing the number of root nodules to be greatly increased. 
The second group of nitrogen-fixers live free in the soil, not 
associated with the roots of plants. These bacteria are often 
called Azo-bacteria , and the process of nitrogen-fixation Azofica- 
tion. There are two kinds of these Azofiers: one which lives under 
conditions of poor aeration (the anaerobic species), and one 
which requires plenty of air (the aerobic species) and called 
Azotobacler. 
Harmful soil organisms —The harmful organisms of the so-called 
"sick" soils in greenhouses are thought by Professor Russell, of 
the famous English Rothamsted Experiment Station, to be 
protozoa, such as the amoeba and other forms of minute animal 
life, which he says apparently destroy the useful bacteria. 
Also may be classed as harmful, the so-called “ denitrifying ” 
bacteria, which flourish in water-logged soils from which the air 
is largely excluded, and which destroy the nitrates which plants 
require and thus allow the escape of valuable plant food in the 
form of ammonia and nitrogen gas. 
Some of the soil-decay organisms are the main cause of the 
acidity of low-lying fields, of swamp and muck lands. In fact, 
cultivated soils in general tend to become more and more sour, 
due to the accumulation of the more inert humic and other organic 
acids and the more rapid removal by drainage of the lime and 
and other bases. Or, the use of acid phosphate and similiar 
fertilizers also is said to cause an increase in soil acidity. Lime 
(calcium carbonate — ground limestone; or calcium hydroxide — 
