Uptake of Metals from Soil into Vegetables - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 352

8.4.4.1 pH
Soil pH is perhaps the master soil variable governing the availability of metals
and metalloids to plants (Page and Chang 1985 ). The major effect that pH has
on both partitioning of metals/metalloids in soil (Section 8.3 ), and on response
of plant roots to uptake of ions (Section 8.4.1 ), means that this variable accounts
for most of the variation in metal-metalloid concentrations in vegetable tissues.
Acidic soils will tend to increase uptake and accumulation of the cationic metals
cadmium, copper, mercury, nickel, lead and zinc, while uptake and accumulation of
the anionic elements arsenic, molybdenum and selenium will be reduced. As noted
in Section 8.4.1 , the effect of pH on accumulation of cationic metals is not pre-
dicted well just by consideration of changes in partitioning, as H + competition for
root uptake mitigates the effects of pH on metal partitioning (Hough et al. 2005 ).
Hence, liming of acidic soils will generally reduce uptake by vegetables of cationic
metals in urban soils (Preer et al. 1980 , 1995 ).
8.4.4.2 Soil Texture and Depth of Contamination
Soil texture (content of sand, silt and clay) will affect metal availability to crops
as finer textured soils (clays) have a greater cation exchange capacity (CEC) and
hence a greater ability to retain cationic metals (higher K d ) compared to sandy soils.
Given the same total metal concentration in soil, clay-rich soils will produce crops
with lower (cationic) metal concentrations. Depth of soil is also important as shallow
contamination is likely to have a lesser effect on metal concentrations in vegetables
compared to deeper contamination (Tiller et al. 1997 ), as plants may extend their
root systems into less-contaminated subsoils.
8.4.4.3 Soil Organic Matter
Soil organic matter (OM) plays a similar role to clays in affecting metal concen-
trations in vegetables, as OM is a major contributor to the pH-dependent negative
charge in soils which gives rise to soils' ability to retain cationic metals (represented
by the CEC). Addition of OM to soil in the form of compost can therefore markedly
reduce cationic metal uptake by plants (Farfel et al. 2005 ; Jones et al. 1987 ;Narwal
and Singh 1998 ; Traulsen and Schonhard 1987 ; Verloo and Willaert 1988 ). Addition
of OM has also been found to reduce arsenic accumulation by vegetables from
contaminated soils (Cao and Ma 2004 ).
Many vegetable gardens use imported OM (e.g. composts, mulches or other soil
amendments) and these may be useful in minimising uptake of cationic metals by
vegetable crops. Also, high OM contents in many garden soils compared to agri-
cultural soils means that soil to plant transfer factors developed from agricultural
surveys (which may not identify OM as an important factor controlling plant uptake)
may overestimate metal uptake in garden scenarios.
However, it should also be noted that high concentrations of dissolved OM
in a soil may lead to increased potential for leaching of cationic metals, due to