Geoscience Reference
In-Depth Information
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 enzyme 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 in
rock minerals, e.g. apatite.
The third role of humus is to act as a source of most
of the micro-nutrients which plants need. Micro-nutrients
are needed only in very small quantities, but they
are absolutely essential. The main micro-nutrients are
iron (Fe), manganese Mn), copper (Cu), zinc (Zn),
molybdenum (Mo) and boron (B). Those nutrients which
are also metals, e.g. iron, manganese, zinc and copper, can
be held in organic molecules in the form of
chelates
,
where the metal ion is held in the form of a 'chelate ring'.
The chelates will be decomposed by micro-organisms to
release the nutrient ion into the soil solution, whence it
can be absorbed by plant roots. As well as providing
nutrients to plants, it must not be forgotten that humus
also supplies nutrients for bacteria and other living
organisms essential to a productive soil. This is in addition
to the carbon supplied to heterorphic micro-organisms.
The final role of humus is its influence on the soil's
physical properties. Thus it acts to improve the soil's
water-holding capacity through its effects on soil struc-
ture. It is especially efficaceous in improving aggregation
in sands and sandy loams. Similarly, in heavy clays the
humic colloids improve structure formation and aeration.
The role of humus in darkening the soil surface influences
the thermal absorption and radiation characteristics of a
soil. A darker soil will heat up more rapidly than a lighter
soil, owing to its lower albedo, though it will also cool
faster at night.
PLANT
LIGNIN
PLANT
PROTEIN
PLANT
CARBOHYDRATE
DECOMPOSITION AND RESYNTHESIS
HUMUS
(
DEGRADED
LIGNIN
MICROBIAL
PROTEIN
MICROBIAL
POLYSACCHARIDES
+
+
of plant remains, and by synthesis of microbial products.
excludes many fauna and soil micro-organisms. Decom-
position is very slow, and the remains of plants build up
into
peat
(p. 446).
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 oxidation and erosion. 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.
Beneficial effects of humus on soil
Humus has many beneficial effects on soil. First, as already
noted, the cation exchange capacity of humic colloids is
very high, due partly to their high specific surface and
partly to the density of phenolic (OH) and carboxyl
(COOH) groups. The size of the negative charge is pH-
dependent, typically doubling from 120 me 100 g
-1
(cmol
c
kg
-1
) at pH 5·0 to 240 me 100 g
-1
(cmol
c
kg
-1
) at pH 8·0.
This is a result of the ionization of the OH groups at
higher pH. 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 acidity and alkalinity
The reaction or pH of a soil greatly influences the growth
of higher plants and of micro-organisms in the soil. pH
is defined as the negative index of the logarithm of the
hydrogen ion (H
+
) concentration. For pure water, the
amount of dissociation into H
+
and OH
-
ions is very
soil humus. The
small. Thus:
