EXTENSION COURSE IN SOILS. 43 
corn, cotton, and other nonlegumes secure all of the nitrogen from the 
soil for their growth, clovers, alfalfa, and other legumes secure a 
substantial part of their nitrogen by fixation from the air. Since 
there is always under field conditions a larger or smaller amount of 
nitrogen compounds made available to legumes, it is extremely 
difficult to determine just how much is fixed from the air. Under 
conditions of average fertility it is probable that about one-third of 
the nitrogen used by clover or alfalfa is taken direct from the soil, 
while about two-thirds is secured from the nitrogen of the air in the 
soil. When these crops are cut for hay, about one-third of the total 
amount of the nitrogen contained in the entire plant is left in the 
roots and stubble and about two-thirds is removed in the hay. 
Figuring from the above estimates, when a crop of clover or alfalfa 
is removed from the land the soil is left with practically the same 
amount of nitrogen that it had before the crop was grown. This, 
however, does not take into account what is lost by leaching. Cow- 
peas, soy beans, and other legumes restore to the soil from roots and 
stems a somewhat smaller percentage of nitrogen than do the clovers 
and alfalfa. • When leguminous crops, therefore, are sold from the 
farm there results at least no gain of nitrogen to the soil. On the 
other hand, if these crops are fed to stock and the manure produced 
returned to the land, much of the nitrogen contained in the crops 
will go back to the soil and an actual increase of the nitrogen content 
of the farm will result. But when only corn and other grains or 
hay from timothy and other nonlegumes are grown, there results a 
positive gradual loss in the nitrogen content of the soil, no matter 
what may be the disposition of the crops. 
Nitrification (Rei. No. 4, pp. 135-140). — Nitrogen is used for 
growth by plants in the form of chemical compounds called ammonia 
and nitrates. It is now known that rice takes up ammonia directly, 
while, as far as is known, all other farm crops absorb nitrogen chiefly 
in the form of nitrates. Organic matter can not, therefore, be uti- 
lized for plant growth until it has first undergone a process of decompo- 
sition. This decomposition is caused by microorganisms, or bacteria, 
living in the soil, which use the organic matter, mostly vegetable, for 
their nourishment and produce as by-products ammonia and nitrates, 
which can then be absorbed by plants. The normal process of 
decomposition of organic matter and the formation of nitrates 
through bacterial action is called nitrification. These organisms 
perform their work only under favorable conditions of moisture, 
aeration, and temperature. Nitrification is twice as rapid at 70° as 
it is at 50° and twice as rapid at 90° as it is at 70°, but the maximum 
temperature is probably between 95° and 100°, and if a much higher 
temperature is reached the bacteria do not grow well. If the soil is 
poorly aerated and water-soaked from a lack of proper drainage, an 
