Page 110 - Soil and water contamination, 2nd edition

Major dissolved phase constituents 97

In contrast to the K-containing minerals, potassium salts are readily soluble in water, so
are not subject to precipitation–dissolution reactions, except when water evaporates under
semi-arid and arid conditions and salts are left behind. The major mechanisms that control K
concentrations in soil and water are sorption to clay minerals and plant uptake . Adsorption
of K to organic matter plays only a minor role in K retention . Due to its strong affinity with
illitic clay minerals, K is partly irreversibly retained in sediments (K fixation ). In general,
there is equilibrium between the following pools of K:

Dissolved K + Exchangabl e K + Fixed K (5.2)

The equilibrium between dissolved and exchangeable (adsorbed) K is reached relatively fast,
whereas the equilibrium between K adsorbed to the clay mineral surfaces and the K fixed
between the clay lattices is reached much more slowly. The exchangeable K content in soil
generally increases with increasing clay content . In acid, sandy soils the exchangeable K
content only amounts to a few mg K per 100 g soil; in basic, clayey soils it is up to about
100 mg K per 100 g soil (= 0.1 percent). Because of the fixation of K in clay minerals , K
seldom occurs in very large concentrations in natural waters and its concentration is usually
much lower than the concentration of Na. Because of the relatively strong K sorption in soil,
K generally moves slowly through the soil.
The second important factor (after sorption and fixation of K to clay minerals ) that
controls the availability of K for solution in subsurface water is the biological cycling of
K. The potassium taken up by plants comes from the exchangeable and dissolved K pools.
The fixed K pools are generally not available for uptake by plants. When exchangeable and
dissolved K are available in sufficiently large amounts, luxury consumption by plants may
occur, which means that plants take up K in excess amounts. The average K content of
living plant material is about 0.3 percent, but concentrations in dry plant material and ash
are considerably greater (1.5–4 percent). Potassium assimilated by plants becomes available
immediately after the plants die or when leaves and other parts are shed at the end of the
growing season. The K present in the litter or detritus is readily released in dissolved form
during the dormant season. The released K leaches into the soil where it is adsorbed to cation
exchange sites of clay minerals and recycled during the subsequent growing season. A small
part of the K may be lost from the soil due to leaching to groundwater or runoff to surface
water. In agricultural soils, part of the K is exported by crop harvesting and removal of crop
residues.

5.4 CALCIUM

Calcium (Ca) is the most abundant of the alkaline-earth metals ; under natural conditions
it occurs only in the 2+ oxidation state . Calcium is an essential element for plant and
animal life forms: it is needed for the physical integrity and normal functioning of cell
membranes and it plays a vital role in cells that are dividing. In animals and humans, Ca is
also used to build bones and teeth and to control muscle activity and transmission of nerve
signals. In addition, it plays a role in the coagulation of the blood and the regulation heart
activity. Calcium is non-toxic, but large concentrations in drinking water are considered as
undesirable, because it causes the formation of boiler scale (i.e. calcareous deposits) in much
household equipment such as boilers, washing machines, and kettles. The sum of the ions
which can precipitate as ‘hard particles’ from water is called the hardness . The hardness is
usually reported as mg CaCO per litre or meq/l or in hardness degrees. One German degree
3
corresponds to 17.8 mg CaCO /l and 1 French degree corresponds to 10 mg CaCO /l. The
3 3
-
2+
2+
temporary hardness is the part of Ca and Mg ions which are balanced by HCO and can
3

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Soil and Water.indd 109 10/1/2013 6:44:24 PM