Introduction to Soil Chemistry

analysis 65
3.7.1. Analysis for Animals and Plants
Determination of the number of animals in a cubic meter of soil is usually
done by simple isolation and counting. Plants are usually counted as the
number per square meter, hectare, or acre. Determination of root numbers or
mass is extremely difficult and is commonly done by assuming that there is a
certain mass of roots associated with a plant’s top, that is, the ratio of roots to
top masses. Determination of microorganisms is usually done using standard
microbiological techniques such as dilution plate counts. Because of the
extreme diversity of soil microorganism, it is impossible to determine all the
organisms present at one time by any one technique. In some cases direct
microscopic observation, as was done to produce Figure 3.6, is used to estimate
the numbers of microorganisms present [2–4].
3.7.2. Determination of Soil Organic Matter
The analysis of soil for organic matter 4 is straightforward, involving oxidizing
it to carbon dioxide and water. Oxidation can be accomplished in a number
of different ways, such as by applying air at high temperature, pure oxygen, or
chemical oxidation using various common chemical oxidizing agents such as
hydrogen peroxide, permanganate, and dichromate followed by titration. Of
these, oxidation by hot dichromate followed by titration of unreduced dichromate
is most commonly used.
The real problem is the complete oxidation of soil organic matter. As noted
previously, this can be associated with mineral surfaces or in pores to which
there is difficult access. For these reasons, simple oxidation will not be sufficient,
and drastic oxidation procedures will be necessary.
3.7.2.1 Direct Oxidation Using High Temperature and Atmospheric Air
The simplest and least expensive method of determining the organic matter
in soil is high-temperature oxidation using air. In this case a weighed soil
sample in a crucible is placed in an oven and dried. Subsequently, after a prescribed
time at an elevated temperature, it is again removed and weighed, and
the difference is taken as the amount of organic matter. This procedure is
seemingly simple, straightforward, and easy to perform and does not involve
the expense of waste disposal. While oxidation of organic matter with a consequent
loss of weight does occur, so do other reactions, which lead to a loss
of weight. One such reaction is the loss of waters of hydration from soil minerals.
Another is the decomposition of soil minerals. For instance, carbonates,
which are very common soil minerals, decompose at high temperature to form
4 Specific directions for determination of soil organic matter are given in the bibliographic
references.