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EXTENSION COURSE IN SOILS. 41 
LESSON VI. THE NITROGEN SUPPLY OF THE SOIL. 
(Ref. No. 2, pp. 110-119.) 
As stated in Lesson II, nitrogen is one of the most important 
elements of plant growth. Nearly four-fifths of the atmosphere, or 
over 70,000,000 pounds over each acre of land, is nitrogen. While 
this is sufficient to support plant growth for thousands of years, yet 
atmospheric nitrogen can not be utilized directly in plant growth but 
must first be combined in the soil with other elements before plants 
can absorb it. It will be the purpose of this lesson to explain how the 
nitrogen of the air becomes transformed so as to be used by plants 
and to discuss briefly the practical means of maintaining the soil- 
nitrogen supply. 
Combined nitrogen in the atmosphere (Ref. No. 1, p. 22). — From the 
decay of vegetable and animal materials, burning, electrical dis- 
charges, and other causes the atmosphere derives certain substances, 
among which are ammonia and nitric acid, both compounds of 
nitrogen. These gases are readily absorbed by the moisture of the 
atmosphere, and when this moisture condenses and falls as rain or 
snow it carries with it into the soil the nitrogen compounds which 
it contains. While the available nitrogen thus added to the soil is 
not large, yet it is an appreciable quantity and contributes in a 
small way to the soil's fertility. 
The -fixation of atmospheric nitrogen in the soil (Ref. No. 7, pp. 213- 
223) . — The nitrogen of the soil which plants require comes ultimately 
from the atmosphere. A large supply of this nitrogen is collected 
from the atmosphere in the soil through the action of microorganisms 
called bacteria. The nitrogen-fixing bacteria of the soil may be 
divided into two classes. One class fives independently in the soil 
and secures nitrogen direct from the air for its growth. After these 
bacteria perform their life's work their bodies decompose and the 
combined nitrogen which they contain becomes available for the 
growth of plants. The amount of nitrogen fixed by this class of 
bacteria in ordinary cultivated soils has been estimated by different 
investigators at from 15 to 40 pounds per acre. Probably the latter 
figure is much above the general average, even under favorable con- 
ditions. The other class of nitrogen-fixing bacteria lives in connec- 
tion with the roots of certain plants, viz, of the family of legumes, 
including clovers, alfalfa, beans, peas, and others. These bacteria 
form nodules or tubercles in which the chemical combination of nitro- 
gen with other elements takes place and from which the host plant 
obtains much of its nitrogen for growth. 
Inoculation (Ref. No. 7, pp. 223-228). — The bacteria which form 
tubercles on the roots of leguminous plants are generally different for 
different species of plants. Those which live on alfalfa, however, are 
