Sorption and Availability of Molybdenum in Soils of Western Oregon

Eight western Oregon soils were chemically characterized in an attempt to identify soil parameters affecting residual availability of fertilizer Mo to subterranean clover ( Trifolium subterraneum L.). Amounts of “exchangeable” Fe and Al, “amorphous” Fe and Al, and “crystalline + amorphous” Fe in the soil were determined. As a measure of surface colloid OH ‐ reactivity, the meq of H + required to maintain a mixture of H 2 O 2 ‐treated soil and 0.85 N NaF at pH 6.8 was measured over a 25‐min period. Anion‐exchange‐resin‐extractable Mo (AER Mo) was removed from soil with a strongly basic anion exchange resin. Molybdenum adsorption isotherms for each soil were determined by equilibrating soil samples at 25°C with Mo solutions ranging in concentration from 2 to 20 mg Mo/liter. The data for each soil conform to a Freundlich adsorption isotherm specific for that soil. Molybdenum adsorption was related to the (NH 4 ) 2 C 2 O 4 ‐extractable Fe in the soil ( R 2 = 0.85**), but not to other fractions of extractable Fe or to extractable Al forms. The OH ‐ released from each soil after 2 min of NaF treatment was correlated with NH 4 OAc‐extractable Al( R 2 = 0.65**). Molybdenum uptake by subterranean clover grown in field experiments was significantly correlated with AER Mo ( R 2 = 0.17*). The correlation was significantly improved when two soils which behaved differently from the rest were statistically analyzed separately; the resulting correlations for the groups of two and six soils were R 2 = 0.64** and R 2 = 0.72**, respectively.