Soil, lead concentration amounts

The uptake of lead via the root system depends on the amount of available lead in the soil moisture. Inhibition of plant growth may start at lead concentrations < 1 mgkg soil moisture, and becomes more completely inhibitory at levels between 3 and 10mgkg . Plant populations that are genetically adapted to high-lead soils may achieve 50% of their normal root growth at lead concentrations above 3 pgg Plants that absorb nutrients from deeper soil layers may receive less lead. 

Application of S, either as gypsum or from other S sources, generally decreases the uptake of Mo by crops. Studies of soybeans have shown that decreases in plant Mo are not limited only to Mo-S interactions in the soil, because foliar-applied Mo has also been shown to decrease Mo concentrations in soybean seeds and leaves in the presence of soil-added S. On low-Mo soils this can lead to induced Mo deficiency, which can decrease yields and crop quality. On soils with high amounts of Mo, application of S can prevent the accumulation of high Mo concentrations and decrease the potential for Mo toxicity to livestock. Competition between sulfate and molybdate anions, competition between bicarbonate and molybdate, and root-zone pH changes have been suggested as explanations for the action of S to reduce Mo uptake. 

Elevated lead contents were recorded in various species of plants from the vicinity of metal smelters, roadsides, soils heavily contaminated with lead, natural ore deposits, and lead recycling factories. Bioavailability of lead in soils to plants is limited, but is enhanced by reduced soil pH, reduced content of organic matter and inorganic colloids, reduced iron oxide and phosphorus content, and increased amounts of lead in soils. Lead, when available, becomes associated with plants by way of active transport through roots and by absorption of lead that adheres to foliage. Lead concentrations were always higher in the older parts of plants than in shoots or flowers. 

Many plants absorb considerable amounts of lead into the roots, but translocate only limited quantities into the above-ground portions. Table 4.4 shows the relationship between the lead content of lettuce and radish grown in contaminated soils and the concentration in the soil, in the absence of atmospheric contamination. In this instance lead concentrations in the soil are expressed as both total lead and available lead (AL-soluble Pb), the latter being defined as that soluble in an ammonium acetate-lactate solution [17]. In this study a significant correlation (P <0.05) between lead concentration in plants and AL-soluble lead in soil was found. 

The total amount of lead is an accumulation irom these combined sources. In addition, airborne lead may eventually settle on accessible soil and/or dust and thus become part of an additional exposure pathway. Even after airborne lead settles on streets, sidewalks, and nearby soil, it can become airborne again if it is disturbed by, for example, vehicular or pedestrian tralHc or excavation activities. The relative importance of any of these airborne lead sources depends on site-specific exposure iactors. Research has shown a relatively clear correlation between airborne lead concentrations and blood lead levels (ATSDR, 1988). 

---