Soil Chemistry (continued)
The macrosolute composition of a soil solution will vary
depending on pH, pE (negative common logarithm of the
electron activity), organic matter content, input of chemical
elements from the biosphere, and effectiveness of leaching.
Under conditions of near-neutral pH, high pE, low organic mat-
ter content, no solute input from agriculture, and good (but not
excessive) drainage, the expected macrosolutes are Ca2+, K+,
Mg2+, Na+, Cl-, biocarbonate ion (HCO3-), silicic acid [Si(OH)4],
and sulfate ion (SO42-). If the pH is low, H+ and Al3+ should
be added to this list; if it is high, carbonate ion (CO32-) should
be added. If the soil has been fertilized, nitrate ion (NO3-) and
hydrogen phosphate ion (H2PO4-) become important.
The important microsolutes in soil include trace metals, such
as iron, copper, and zinc, and trace element oxyanions, such as
those formed by arsenic, boron, molybdenum, and selenium.
The tableau of microsolutes in a given soil solution is more de-
pendent on inputs from the lithosphere and the biosphere and
less on proton or electron activity and hydrologic factors than
is the composition of macrosolutes. The trace metals present,
for example, usually are derived from the chemical weather-
ing of specific parent rocks, from the application of fertilizers,
pesticides, and urban wastes, and from air pollution.
Fig. 3: Factors influencing the chemistry of a soil solution.
(Credit: Based on an original illustration created by J. F. Hodson)
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