Agriculture Reference
In-Depth Information
3
4
5
6
10 ppm Zn
20 ppm Zn
40 ppm Zn
70 ppm Zn
7
3
4
5
6
7
8
9
10
pH
Figure 3.13
Solubility of Zn
2
+
versus pH in soil at four Zn levels. The straight lines
indicate the solubilities of possible precipitates at atmospheric CO
2
pressure (McBride,
1994). Reproduced by permission of Oxford University Press
whereas precipitation is the process by which a substance accumulates to form
a new solid phase. Both imply a net removal of solute from solution, but one
is inherently two-dimensional and the other three-dimensional. However the dis-
tinction is blurred because similar types of chemical bond are often involved.
Figure 3.14 shows the formation of a precipitate on the surface of an oxide or
layer silicate in an analogous way to the formation of inner-sphere complexes
in Figure 3.11. In practice there is a continuum between the two ranging from
extremely insoluble inner-sphere complexes to precipitates that are much more
soluble. In general when the concentration of a sorbed species is small sur-
face complexation dominates and when it is large precipitation dominates. In
submerged soils very large concentrations of dissolved ions and gases develop
following anaerobic metabolism and reductive dissolution of solid phases, and
so precipitation reactions often dominate.
Precipitation is generally much slower than adsorption. Table 3.14 compares
rates of precipitation with rates of adsorption and other surface phenomena in
soil systems. Rates vary greatly between precipitating compounds. They are also
often subject to inhibition and catalysis by other solutes and solid phases present.
