Geoscience Reference
In-Depth Information
the most important store is the exchangeable cation store
on the clay and humic colloids. When the concentration
of a particular cation in the soil solution is lowered by
plant absorption, cations immediately leave the exchange
sites and enter the soil solution to maintain equilibrium.
Exchangeable cations are thus readily available to the
plant. Another nutrient store is the organic matter, but in
this case the organic molecules need to be decomposed
or
mineralized
first to release their nutrients. Nutrients
may also be bound up in the crystalline structure of
minerals in soil, which in that case have to be weathered
in order to release the ion into the soil solution or on to
the colloidal exchange sites.
Carbon pool
Size (Gt)
Atmosphere
720
Ocean
38,400
Rock carbonates
>60,000,000
Kerogens (e.g.coal)
15,000,000
Living biomass
600
Soil
1,500
Peat
250
Coal
3,510
Oil
230
Natural gas
140
Nutrient availability
An important concept in soil fertility is that of
available
nutrients
, i.e. those nutrients in the soil solution and soil
stores which the crop can reasonably be expected to
absorb over the course of its growing season. Below are
listed the forms in which a nutrient or indeed any
chemical element may exist, in order of
Soil fertility
The fertility of a soil is its ability to support a desired crop
at an adequate level of yield and quality. It must be capable
of providing sufficient water, air and nutrients for
satisfactory crop growth. Large areas of the world's soils
still suffer from limitations on agricultural productivity
because of their inability to provide one or more of these
three in an optimal amount. In the early years of the
nineteenth century the foremost agricultural chemist, the
German Baron Justus von Liebig, expounded the idea that
the amount of the least favourable element would be the
one that limited plant growth. This is still known as
Liebig's
law of the minimum
. Although the amounts of the
essential nutrients required by different plant species vary,
and therefore soil fertility depends on the particular
species, in general any crop removes a large amount of
nutrients, and therefore losses need to be replaced by
manures and fertilizers. Nitrogen and phosphorus are
needed in large amounts by all plants, by both natural
vegetation and crops. These two nutrients are analysed in
Chapter 21,
as are biogeochemical nutrient cycles.
Plant roots absorb nutrients from the soil in the form
of cations (ions carrying a positive charge, e.g. potassium,
K
+
) and anions (ions carrying a negative charge, e.g.
nitrate, NO
3
-
). Absorption requires the expenditure of
a large amount of energy by plant roots, as nutrients
concentrations are 100 times greater in the plant sap
than in the soil solution. The energy comes from plant
carbohydrates which are oxidized and converted into
carbon dioxide (CO
2
) by respiration. The concentration
of nutrients in the soil solution is very dilute, so, when a
plant absorbs a nutrient, supplies in the soil solution need
to be replenished quickly from a nutrient store. For cations
increasing
availability:
1
Nutrient in the mineral crystal structure.
2
Nutrient as part of organic molecules.
3
Nutrient ions adsorbed on clay and organic colloids.
4
Nutrient ions in the soil solution.
The overall sum is the
total nutrients
, which can be meas-
ured by digesting the soil with a strong acid. The content
of available nutrients is more problematic to measure,
as there are many possible dilute extractants. The value
obtained is always a function of both the chemical ex-
tractant used and the type of soil. Exchangeable and
dissolved ions are available, and will be replenished by
amounts of primary mineral and organic nutrient released
by the weathering of minerals and the microbial decom-
position of humus.
A final important feature of nutrient availability in soils
is that of
ion antagonism
. By this mechanism one nutrient
element 'blocks' the absorption of another. For example,
calcium and iron are antagonistic, as are calcium and
potassium. There may be ample iron or potassium in a
soil for normal plant growth, but if the content of calcium
is high the iron and potassium will become
unavailable
for plant growth. This condition of iron or potassium
deficiency shows itself by a yellowing of the plant leaves
(
chlorosis
). Physiologically the plant root is unable to
absorb sufficient iron and potassium in the presence of
