Environmental Engineering Reference
In-Depth Information
Figure 18.12 The three main types of surface organic matter
in soils.
Source: After Avery (1990).
In agricultural soils the addition of organic material to the soil surface is lower than
under natural conditions owing to losses by harvesting, stubble burning and increased
rates of erosion. Cultivation and disturbance of the soil surface also increase the rate of
decomposition and mineralization. Manuring and the rotation of grass leys are important
as possible means of maintaining SOM levels. Mineral soils commonly contain 1-10 per
cent SOM, with arable soils usually at the lower end of this range. A simple method of
estimating the SOM is to ignite a sample at a high temperature and determine the loss in
weight.
Humus has many beneficial effects on soil. First, as already noted, the cation exchange
capacity of humic colloids is very high, due partly to its high specific surface and partly
to the density of phenolic (OH) and carboxyl (COOH) groups. The size of the negative
charge is heavily pH-dependent, typically doubling from 120 me 100 g −1 at pH 5·0 to 240
me 100 g −1 at pH 8·0. This is a result of the ionization of the OH groups at higher pHs.
The second role of organic matter is that it is the chief source in the soil of the plant
nutrients nitrogen and sulphur, and an important source of phosphorus. Ninety-eight per
cent of the nitrogen absorbed by plants comes from the mineralization of soil humus. The
microbial protein in humus is mineralized to nitrate, in which chemical form it is
absorbed by plants, whence it becomes a constituent of plant proteins. Sulphur is an
essential constituent of some plant proteins. Organic sulphur compounds in humus are
mineralized to give sulphate, which is the form absorbed by plants. Phosphorus plays a
fundamental role in a very large number of enzymic reactions in plants, and is a
constituent of the cell nucleus and essential for all cell division. About 50 per cent of the
phosphorus absorbed by plants comes from SOM and about 50 per cent from phosphorus
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