EFFECT OF PH OF AMMONIUM OXALATE EXTRACTING SOLUTIONS ON PREDICTION OF PLANT-AVAILABLE MOLYBDENUM IN SOIL

Molybdenum (Mo) is an essential element of plants and animals and is o f concern from human nutrition and environmental standpoints. Rational applications to soil of Mo in fertilizers, sewage sludges, or other s oil amendments requires information of the concentrations of Mo in soi ls and plants. Two greenhouse experiments were conducted at Lexington, Kentucky, using surface samples of 12 soils (11 soil types) derived f rom diverse parent materials in Kentucky with soil pH ranging from 5.1 8 to 7.46. Molybdenum (Na2MoO4 . 2H(2)O) was added at rates equivalent to 0, 0.3, and 0.6 mg Mo kg(-1) soil. Tobacco (Nicotiana tabacum L., cv. Ky14) and soybean [Glycine max: (L.) Merrill cv. McCall] were grow n to provide plant Mo data for Mo soil test correlations and compariso ns. The primary purpose of these investigations was to determine the e ffect of pH of NH4-oxalate extractant solution on the relationship of soil Mo and Mo uptake by tobacco and soybeans, and to evaluate the aut omated KI-H2O2 procedure for use in determining Mo in soil extracting solutions. The mean dry weight and Mo concentration of tobacco and soy bean were increased by applications of Mo fertilizer to soil in the gr eenhouse. Dry matter of tobacco was increased 11 to 25% and concentrat ion of Mo from 40 to 82% by each increment of added Mo fertilizer, The results of this study suggest that many soils in Kentucky are not mee ting the requirements for Mo sufficiency for tobacco and soybean plant s. The average amount of soil Mo extracted by NH4-oxalate decreased wi th increasing pH of extractant. Regression estimates for the relations hip of Mo uptake by tobacco or soybean and extractable soil Mo show th at the slope and the coefficients of determination increased with pH o f NH4-oxalate solution from pH 3.3 to 6.0 and then decreased again at pH 5.4. The greatest amounts of variation in Mo uptake by plants (67% and 20%, respectively, for tobacco and soybean) were accounted for by the soil Mo data at pH 6.0. Soil Mo values for the NH4-oxalate extract ant (pH 6.0) were related to values for anion exchange resin extractan t (r(2) = 0.61*), but not soil pH. However, values for anion exchange resin were more closely related to Mo uptake by tobacco (r(2) = 0.86 ) and soybean (r(2) = 0.60**) than were values for NH4-oxalate (r(2) = 0.65* and r(2) = 0.27**, respectively). Results of this study indic ate that the automated KI-H2O2 method used previously in analysis for plant Mo can be used to analyze Mo in soil extracts. Other instrumenta tion such as GFAAS and ICP may be effective in the analysis of extract s obtained by the NH4-oxalate (pH 6.0) or by anion exchange resin proc edures when the Mo concentration of extracts falls within the detectio n limits of the instrument.