Uptake of Metals from Soil into Vegetables - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 336

this is the effect of pH on uptake of cationic metals. For example, different solution
culture studies have shown that metal uptake increases when H + activity is lowered
(pH increases). For example, Weng et al. ( 2003 ) showed that nickel uptake in oats
increased by about a factor of 3 per unit pH increase between pH 4-6, at constant
Ni 2+ activity. Concentrations of cadmium in soybean shoots was shown to increase
by a factor of 1.9 between pH 5-7 at constant Cd 2+ activity (Smolders and Helmke
unpublished), while shoot cadmium increased markedly larger, with factors of 4-13
in unbuffered nutrient solution between pH 5-7 for ryegrass, lettuce, cockfoot and
watercress (Hatch et al. 1988 ). Numerous other interaction effects in plants have
been identified for metal and metalloid uptake. Without attempting to be complete,
we cite those that are relevant for contaminated site Risk Assessment, i.e. Ca 2+ :Cd 2+
(Tyler and McBride 1982 ), Zn 2+ :Cd 2+ (McKenna et al. 1993 ), H + :Cu 2+ (Chen and
Allen 2001), H 2 PO 4 - :H 2 AsO 4 - (Khattak et al. 1991 ) and SO 4 2- :SeO 4 2 −

(Hopper

and Parker 1999 ).
The ion interaction effects are required to interpret effects of soil properties on
metal availability to plants. For example, increasing pH invariably decreases the free
metal ion activity in soil (see previous section), which is in contrast to the above
mentioned effects on the uptake of free metal ions from pore water into plants.
It is tempting to predict the net effect of pH on metal bioavailability by properly
'adding up' the interactions on both sides. Such calculations are the basis of the
Biotic Ligand Model (BLM - see next section) for predicting toxicity of metals and
this model is an extension of the FIAM by taking ion interactions into account. The
elegant study of Weng et al. ( 2003 ) is an example of this. That study showed that the
nickel concentrations in oats grown at different pH were reasonably well described
by combining solution culture data with data on free metal ion activities in pore
water. The difference between predictions and observation were largest (factor 2) at
lower nickel supply in soil, which may be due to lack of accounting for other ion
interaction effects or lack of modelling rhizosphere conditions. A similar attempt
was made to predict liming effects on cadmium uptake (Smolders and Helmke,
unpublished). However, that study showed that the ion interactions studied sepa-
rately did not add up, but predictions overestimated effects of liming (pH increase)
and only correctly predicted the trend. The general trend emerging from large sur-
veys are suggesting that the H + interaction for Cd 2+ uptake is important. Field data
on cadmium uptake by numerous plants (Table 8.4 ), for example, show that the net
effect of increasing soil pH on reducing cadmium bioavailability is, on average, only
a factor of 1.6 per unit pH increase. This impact is distinctly smaller than that effect
on reducing solubility, i.e. a factor of 3.6 per unit on average (Degryse et al. 2009 ).
Hough et al. ( 2005 ) similarly concluded from a large set of pot-trial data with rye-
grass that H + decreased the availability of pore water Cd 2+ and Zn 2+ to an extent
that the net effect of soil pH on decreasing crop cadmium is smaller than a factor of
1.5 per unit pH increase.
While the BLM concept improves our understanding of metal toxicity over the
FIAM, it is still incomplete and not ready for practical use. First of all, the BLM
is currently applied to pore water data while it is conceptually most correct to do
that for the solution in the rhizosphere. The ionic composition in the rhizosphere