Agriculture Reference
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in which the sorbed solute diffuses. It can be measured by assuming that the
solution pathway is the same as that for a nonabsorbed ion, and then deducing
the movement in the solid from the excess movement not accounted for by the
solution pathway.
While f S is negligible for strongly sorbed anions, such as phosphate, which
are covalently bound to the surface, it is often substantial for exchangeable
cations (Staunton and Nye, 1983, 1987; Staunton, 1990; Nye and Staunton, 1994).
Although the values of f S are much smaller than those of f L , the flux on the sur-
face may exceed that in solution because a much larger proportion of the cations
are held on the surface. Staunton (1990) measured f S for different exchangeable
cations (Na + , Ca 2 + ,Rb + and Cs + ) in soils with a range of clay contents and
mineralogies, and found that f S decreased as the degree of binding to the sur-
face, measured by the ratio S S /C L , increased. Thus the two tend to offset each
other and as a result the contribution of surface diffusion to D is variable. For
the cations and soils considered by Staunton the contribution of surface diffusion
to D ranged from 27 to 97%. There was no correlation between f S and soil
mineralogy or clay content. Staunton and Nye studied alkali and alkaline earth
cations sorbed in freely exchangeable forms, over reaction times of a few hours.
More strongly sorbed cations, such as those of transition and heavy metals at
trace concentrations, are likely to have less surface mobility.
A consequence of high surface mobility is that equilibrium between inter- and
intra-aggregate pores is maintained more easily. Thus Pinner and Nye (1982), for
Cl , and Staunton and Nye (1983, 1987), for exchangeable cations, found no evi-
dence of slow equilibration between inter- and intra-aggregate pores for diffusion
in naturally structured soils, and the soil pore network behaved homogeneously
to the diffusant. However for solutes with little surface mobility, such as strongly
sorbed anions, access to exchange sites through narrow access pores is likely to
limit equilibration between inter- and intra-aggregate pores. It is important to
consider this in defining continuity equations and in measuring rates or reaction.
Effects of Flooding and Redox Conditions on f S . I know of no published data on
this. But it is likely that the nature of particle surfaces in intermittently flooded
soils would restrict surface mobility. For ions to diffuse freely on the surface
there must be a continuous pathway of water molecules over the surface and
uniform cation adsorption sites. But in intermittently flooded soils the surface
typically contains discontinuous coatings of amorphous iron oxides on other clay
minerals, and on flooding reduced iron is to a large extent re-precipitated as
amorphous hydroxides and carbonates as discussed above, resulting in much
microheterogeneity with adsorption sites with disparate cation affinities.
The Derivative d C L /d C
This derivative is the reciprocal of the buffer power and describes the distribution
of the diffusing solute between the soil solid and solution. Its value varies by
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