MOLYBDENUM EXTRACTABILITY IN SOILS AND UPTAKE BY ALFALFA 20 YEARS AFTER SEWAGE SLUDGE APPLICATION

Abstract

Molybdenum (Mo) at elevated concentrations in nonacid soils is readily taken up into forages, particularly legumes, and can result in secondary copper deficiency or molybdenosis in ruminants. Because sewage sludge products are commonly higher in total and available Mo than soils, amendment of soils with sludges could cause health problems in livestock. To determine the long-term potential for sewage sludge amendments to raise forage Mo concentrations, alfalfa was grown on two controlled experimental sludge application sites at the University of Guelph, where three different sludge types (high Ca, high Al, high Fe) had last been applied 20 years earlier. Analysis of the alfalfa harvested from these two field sites, which had near-neutral soils of different texture, showed that more than about 5 kg ha -1 of cumulative Mo loading 20 years earlier could lower the forage Cu/Mo concentration ratio below 2/1, considered to be the lower limit to protect the health of grazing ruminants. The decreased Cu/Mo ratio was due to increases in alfalfa Mo concentrations, as alfalfa Cu was not significantly increased by the sludge amendments at either site. All six sludge treatments (three sludge types at two application rates) investigated at Elora and four of six treatments at Cambridge showed significant increases in alfalfa Mo relative to the controls. At Elora, the critical forage Cu/Mo ratio of 2 was reached with about 10 to 12 kg ha -1 total Mo applied historically from sludge, whereas this ratio was reached near 6 kg ha -1 total Mo applied at Cambridge. Soil analyses revealed that although CaCl 2 -extractable Mo was significantly increased in five of the six sewage sludge treatments at Elora and four of the six sludge treatments at Cambridge, most of the Mo applied in several sludge materials (particularly the high-Ca sludge) had been lost from the topsoil at both sites. Over all experimental plots at both sites, the alfalfa Mo concentration was correlated to readily extractable Mo (by 0.01 M CaCl 2 ) in the soil. Total soil Mo and past Mo loading to soil were less reliable predictors of Mo concentration in alfalfa than the soil test for readily extractable Mo. The results revealed that residual plant-available Mo in sludge-amended soils can persist for decades with some types of sludge materials, but leaching losses of Mo may reduce the impact of residual Mo in soils. The need for stronger regulation and monitoring of Mo in waste materials intended for forage, pasture, and range land application is indicated.