Use of NH4 HCO3‐DTPA soil test to assess availability and toxicity of selenium to alfalfa plants

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Abstract
This study was carried out to determine if ammonium bicarbonate-DTPA soil test (AB-DTPA) of Soltanpour and Schwab for simultaneous extraction of P, K, Zn, Fe, Cu and Mn can be used to determine the availability index for Se. Five Mollisols from North Dakota [USA] were treated with sodium selenate and were subjected to several wetting and drying cycles. These soils were extracted with hot water and with AB-DTPA solution for Se analysis. Alfalfa plants were grown in these soils in a growth chamber to determine plant uptake of Se. Coal mine soil and overburden materials from Western Colorado were also extracted and analyzed. Hot water and AB-DTPA extracted approximately equal amounts of Se from Mollisols. A high degree of correlation (r2 = 0.96) was found between Se uptake by plants and AB-DTPA extractable Se. Extractable level of Se in treated soils was decreased with time due to change of selenate to less soluble Se forms and plant uptake of Se. An AB-DTPA extractable Se level of over 100 ppb produced alfalfa plants containing 5 ppm or higher levels of Se that can be considered toxic to animals. Soils with about 2000 ppb of extractable Se were highly toxic to alfalfa plants and resulted in plant concentrations of over 1000 ppm of Se. The high rate of selenate (4 ppm Se) was less toxic to alfalfa plants in soils of high organic matter content. This lower toxicity was accompanied with lower extractable levels of Se. The AB-DTPA solution extracted on the average about 31% more Se than hot water from the mine and overburden samples and was highly correlated with the latter (r2 = 0.92). The results indicated the presence of bicarbonate-exchangeable Se in these materials.