Metal (Loid)s in Farmland Soils and Strategies to Reduce Bioavailability

High concentrations of heavy metal (loid)s (HMs) in farmland soils reduces crop yield and contaminates the food chain. Exposure to HMs in the diet results in several adverse health effects such as cancer, reproductive health problems and cardiovascular diseases. Understanding the origin and fractionation of these toxic substances will provide direction for reducing their bioavailability in contaminated farmland soils. HMs are added to farmland soils through activities such as irrigation, organic and inorganic fertilization, pest control, and mining. Weathering of parent material and atmospheric deposition can also increase the levels of HMs in the soil. Fractionation of HM contaminated soils provides information on availability of HMs such as Pb, Cd, As, Cr and Cu to soil biota and plants. Several studies have reported that Pb is mostly associated with Fe and manganese oxides (reducible fraction) while Cd is mostly associated with the most mobile fraction (exchangeable fraction). The application of organic and inorganic soil amendments such as vermiculite, zeolite, composts and crop residue to contaminated farmland soils converts HMs from the plant available fractions to the less mobile fractions. HM resistant microbes can change HMs to a less mobile fraction or less mobile oxidation state. The combination of HM resistant microbes, HM tolerant plants, and soil amendments can be used to reduce mobility of HMs in contaminated farmlands.</p

Organic acids in the rhizosphere and root characteristics of soybean (Glycine max) and cowpea (Vigna unguiculata) in relation to phosphorus uptake in poor savanna soils

Root characteristics associated with phosphorus (P) uptake under limiting soil-P conditions were examined in two sets of greenhouse experiments. Average diameter and length of soybean, cowpea,maize and sorghum roots were assessed after 7 weeks in three low-P soils amended with P fertilizer at 0, 3, 6, 11 and 23 mg P/kg. Organic acids in the rhizosphere of soybean, cowpea and pigeon pea wereseparately evaluated in one soil amended with or without rock phosphate, iron phosphate, aluminium phosphate, calcium phosphate, or triple super phosphate. Unplanted soil served as the control. Thegrowth of soybean, cowpea, maize, and sorghum was significantly improved with P application in all the soils and the amount of P applied played an important role. The shoot dry matter yield and P accumulation correlated significantly with the root length of cowpea and the average diameter of sorghum roots. Citric acid was the only organic acid detected in measurable quantities in the rhizosphere of all plants tested; on average, it varied from 4 (pigeon pea) to 17 (soybean) ìmol/g soil. For soybean, the secretion of citric acid appeared important for P acquisition in P-limiting environments whereas for cowpea, the size of the roots may be more important