Agriculture Reference
In-Depth Information
In animals, potassium similarly plays important roles in osmoregulation and in main-
taining the acid:base balance. It is also necessary for nervous conduction, muscle
excitability and carbohydrate metabolism. It is the major cation within cells in contrast
to sodium, which is the major cation at the physiological level (McDonald
et al.,
1988).
In soils, potassium enters the soil solution largely through the weathering of such
primary minerals as micas, feldspars, etc. It is slowly released into the soil solution where
it equilibrates with potassium on the exchange complex or is taken up by plants. It may
also be fixed on to 2:1 clays in non-exchangeable forms. Potassium is continuously
leached from the aerial parts of plants, mineralised biological materials and soil organic
matter. As with other nutrient elements, it is taken up and released by micro-organisms
under constraints imposed by environmental conditions.
Likens
et al.
(1994) consider that potassium reserves in natural soils are not large but
are maintained by weathering and conservative cycling mechanisms. Plant uptake may
account for considerable proportions of the potassium available in the soil, particularly
in high-producing agricultural crops. To maintain crop production in these situations,
it must be replaced by appropriate fertiliser inputs (Brady and Weil, 1996). Because potas-
sium is highly mobile in soils, particularly those of coarse texture, the potassium cycle
is sensitive to such disturbances as the removal of a forest cover. Such practices can lead
to large site losses that may continue over many years (Likens
et al.,
1994).
3.1.3
GENERAL NUTRIENT SOURCES
The nutrient elements absorbed by plants are derived from geological, biological and, to
a lesser extent, atmospheric and anthropogenic sources. Chemical weathering of saprolite
below the soil horizons, fragments of the parent rock, or of other parent materials
resident in the soil may all release nutrient and other elements into the soil solution
(Chapter II.3.1). Mineral nutrient elements continue to be released from this source until,
over geological periods of time, all the weatherable minerals in the rooting zone have
been broken down. Because of this, many highly-weathered tropical soils have little
capacity to supply plant nutrient elements from this source.
Within the soil, nutrient elements are available through displacement from exchange
sites on soil colloids. These may include both phyllosilicate and other clays and organic
matter. Nutrient elements may also become available as a consequence of organic mat-
ter decomposition, through either the consumer or decomposer food webs (Chapter IV),
or from turnover of the microbial biomass. In particular, organic matter decomposition
supplies much of the nitrogen, phosphorus and sulphur available in unfertilised soils.
It has become clear from work carried out over the past few years that the microbial
biomass is an important component of plant nutrition. The microbial biomass comprises
a small pool of nutrient elements with high turnover rates and it may act as either a sink
or source of these elements (Singh
et al.,
1989). The phosphorus in microbial biomass
comprises 3-24
per cent.
of total organic phosphorus in soils, although most values
lie in the lower part of this range; equivalent values for nitrogen are 1.7
-
6
per cent.
(Singh
et al.,
1991), and for sulphur are 1-3
per cent.
(Banerjee
et al.,
1993; Wu
et al.,
1994).
Microbial biomass and its contained nutrients vary with climate, the type of vegetation
present, land use and cultural treatments (Wardle, 1992) (see also Chapter III
.
2.2.2).
