Plant Availability and Uptake of Molybdenum as Influenced by Soil Type and Competing Ions

Irrigation has been proposed as a possible disposal method for large quantities of water having high concentrations of Mo (5–100 mg L⁻¹) resulting from mining and reclamation activities. To assess this possibility, laboratory and greenhouse experiments were conducted to evaluate the influence of competing ions in the soil solution on the relationship between the sorption of Mo by soils and Mo uptake by grass. Laboratory experiments assessed Mo sorption by Edroy clay (Vertic Haplaquoll), Olmos loamy sand (Petrocalcic Calciustoll), and Randado sandy loam (Ustollic Paleargid). Soil treatments included 1, 5, 10, 20, 50, and 100 mg Mo L⁻¹ added as ammonium molybdate in solutions of 0, 167, 333, and 500 mg L⁻¹ concentrations of Cl⁻ and 0, 226, 451, and 677 mg L⁻¹ concentrations of SO²⁻₄ salts. Sorption data fit the Freundlich isotherm. The Olmos soil had the highest CaCO₃ content and sorbed the most Mo. The Edroy soil had the lowest pH and sorbed the least Mo. The presence of Cl⁻ in solution increased Mo sorption, while SO²⁻₄ reduced Mo sorption in all three soils. In the greenhouse study, soils were treated with 0, 6.5, 13.0, and 26.0 mg Mo kg⁻¹ soil; 167, 334, and 501 mg Cl kg⁻¹ soil; and cropped with bermudagrass, Cynodon dactylon (L.) Pers. No toxic symptoms or yield decreases were noted as a result of Mo or salt additions. Plants grown in soil containing 26.0 mg Mo kg⁻¹ accumulated up to 600 mg Mo kg⁻¹ dry matter and would be toxic to ruminants. The Mo concentrations in grass grown on the three soils were ranked from highest to lowest as Edroy > Randado > Olmos. Chloride addition to the soil had no significant (P <0.05) effect on bermudagrass Mo content. Both the concentration of Mo in the vegetation and the amount of Mo taken up by the vegetative cover were well correlated (r² = 0.73 and 0.83, respectively) with the equilibrium solution Mo in the soil as calculated from the sorption isotherms. Thus, the results indicate that there is a strong relationship between the laboratory sorption data and the equilibrium solution Mo in the soil and that grass grown on soil irrigated with waters high in Mo may reach levels that are expected to be toxic to animals.