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248

QUANTITATIVE AGRICULTURAL ANALYSIS

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1

Organic Matter.—In a natural soil there is a close relationship
between the proportion of organic matter and the fertility.
The cause of this is partly physical (improving the texture of a
soil increases its absorbing capacity) and partly biological in
that promoting the growth of bacteria, molds and protozoa helps
to release essential elements to further availability. Organic
matter also furnishes plant food directly. Many definite
chemical compounds have been isolated1 from the complex soil
organic materials.

Methods for Determining Total Organic Matter.—An ap-
proximate calculation of organic matter may be made from the
per cent of carbon, the average carbon of soil organic matter
being taken as 58 per cent.2

Loss on ignition, as already determined, is sometimes taken as
an approximate measure of organic matter. The results obtained
by this method usually differ considerably from those obtained
by calculating the organic matter from carbon, for reasons already
explained.

Of the various methods that have been used for the determina-
tion of carbon, direct combustion and oxidation by a mixture of
chromic and sulphuric acids have been most widely adopted. At
present, due chiefly to the efficiency of the modern electric furnace
and to failure to obtain complete oxidation by other methods, the
direct combustion method has found greatest favor. By any
of these methods, carbon dioxide of carbonates is measured
along with that produced by the oxidation of organic carbon
and this occasions an error in organic matter calculations, unless
carbonate carbon is determined and a correction applied.

The combustion method is similar to that used for the deter-
mination of carbon in iron and steel. It depends upon the direct
combustion of the soil in a current of oxygen, the carbon dioxide
produced being absorbed in standard barium hydroxide and the
excess of base titrated.

Warrington and Peak illustrate the discrepancies between
the results obtained by calculating organic matter from
loss on ignition and from carbon determinations by the
following table:

1 U. S. Dept. ofAgr., Soils, Bull. 74 (1910).

2 See also READ AND RIDDGELL, Soil Sci., 13, 1 (1922).

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