Estimating available molybdenum in biosolids-amended soil using iron-oxide impregnated filter paper*1

Abstract

Various extraction procedures have been used to characterize molybdenum (Mo) in soils, and attempts have been made to correlate the results with plant response. However, few of these studies attempted to characterize available Mo in biosolids-amended soils, and none used Fe-oxide impregnated filter papers as an availability index. The purpose of this study was to determine the potential of Fe-oxide impregnated filter papers to assess the availability of Mo in biosolids-amended soils, and compare the assessments with those offered by a popular chemical extraction method (acid ammonium oxalate) and total soil Mo (EPA Method 3050A). Soil and plant samples were obtained from a field study with bahiagrass (Paspalum notatum Flugge) as the test crop and analyzed for Mo. The grass was grown in a sandy Spodosol, amended with two Class A biosolids at three rates. Though the effect of biosolids addition on total soil Mo was visible in Year 1, it gradually disappeared with time. However, tissue Mo contents varied significantly with biosolids application rates each year, suggesting that a part of the Mo in the biosolids was readily available for plant uptake. The changes in pH of amended soils did not significantly affect Mo extracted by any of the methods. The best correlation between tissue Mo and soil Mo was obtained using the Fe-oxide strips, followed by acid ammonium oxalate; total soil Mo was generally not well correlated with Mo uptake. Results suggest that the Fe-oxide strips can serve as an analytically satisfactory, theoretically reasonable, and practically acceptable procedure for assessing available Mo, even in soils amended with biosolids. *Contribution of the Agric. Exp. Stn. Journal Series No. R-07612.