E-VALUEExpected Value
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4, and also for experiments in which E-values are determined after only short exposure times of the isotope to the soil (Fig.
E-values determined (see below) from the fraction A solutions correspond to the `standard' E-value method adopted by many researchers, though the time of incubation with isotope can vary from 1 min to >3 days.
The theory underlying the concept of E-values using the standard methodology has been described by many researchers (McAuliffe et al.
1, it was therefore anticipated that problems might be expected in the determination of E-values for the soils KO, PG, QL, HAS, and KT.
Potential interferences in the determination of P E-values
There are several reasons advanced to explain the difficulty in determining realistic E-values for soils that have a high P sorption capacity and these are discussed below.
1994) have speculated that overestimation of E-values may be due to the presence of significant quantities of soluble silicate in the soil, which reacts with the molybdate reagent to form a blue complex, providing a positive interference in the measurement of MRP.
5 mg P/kg soil) either (a) had no effect on E-values calculated for the different soils, or, particularly for soils high in amorphous Fe, (b) led to a decrease in the calculated E-value.
However, their results could also be explained as E-values performed with [H.
p] was overestimated, and hence, E-values assessed in these soils via the standard method using 0.
It is also possible that inaccurate E-values may be obtained for soils where P is present as soluble aluminophosphate polymers or in other forms that are not molybdate-reactive, but nevertheless isotopically exchangeable (White and Taylor 1977).
Given the multiple advantages of this method, studies are also in progress to determine whether it can successfully be adapted for the determination of trace metal E-values using a cation exchange resin.