ed in the sap or juice of certain plants, and these
or others are formed when crop residues are decomposed in the soil.

In the ultimate decomposition of organic matter the carbon appears
in the form of carbon dioxid which when combined with water forms
carbonic acid. Though this is a very weak acid, its solvent action
is very important.

But, in addition to the various organic acids and carbonic acid, we
have also to consider the formation of nitric acid in connection
with the decomposition of organic manures. Nitric acid is one of the
strongest known, and in solvent power it is excelled by no single
acid. The nitrogen contained in crop residues and other organic
manures is chiefly in chemical combination with carbon, oxygen and
hydrogen, much of it in insoluble protein compounds. Normally this
organic nitrogen is transformed in the soil, first into ammonia
nitrogen, next into nitrite nitrogen, and lastly into nitrate
nitrogen, these three transformations being effected by biochemical
action produced by different kinds of living microscopic organisms
called bacteria. Though detectable amounts of free nitric acid do
not accumulate during this process of nitrification, the soluble
nitrate or final product is formed by the action of nitric acid upon
a mineral base, such as calcium, magnesium, or potassium, which may
have been in the soil in insoluble form, so that the nitrogen must
pass through the form of nitric acid in the transformation into
nitrates.

While the organic matter applied to the soil contains about twenty
times as much carbon as nitrogen, and while corresponding amounts of
carbonic acid and important amounts of intermediate organic acids
must be formed, it is of much interest to know that even the nitric
acid formed in the transformation of organic nitrogen to nitrate
nitrogen in sufficient quantity for a given crop is seven times as
much acid as would be required to convert raw rock phosphate into
soluble phosphate to furnish the phosphorus required for the same
crop. A knowledge of this definite quantitative relationship should
help us to appreciate the possibilities of decaying organic manures
in the important matter of making plant food available, including
potassium, calcium and magnesium as well as phosphorus and nitrogen.

The value of rye, rape, buckwheat and other non-legumes when used as
green manures is very largely due to the liberation of plant food by
their decomposition in contact with