Selenium speciation in the soil solution and its relevance to plant uptake

The relationship between soil and plant selenium has been investigated using two ⁷⁵SeO₃ spiked silt loam soils from the Hoosfield series with long-term equilibration pH values of 4.5 and 7. Lolium perenne plants were grown in the soils over a 15-week period, and seven harvests were made. Plant samples and soil solutions were assayed for radioactivity, and selenium compounds found in the soil solutions separated by high voltage paper electrophoresis. Throughout the 15-week experimental period selenium speciation was observed to change in soil solution. Samples collected at day 2 from pH 4.5 soil revealed that selenate accounted for 71% and selenite 8% of the soluble selenium compounds present, whereas for the pH 7 soil the relative percentages were 51 and 23% respectively. After 105 days selenate accounted for 22% and selenite 20% at pH 4.5, and 12 and 22% respectively at pH 7. The occurrence of selenoglutathione was noted in the soil solution and its concentration increased significantly with time. Statistically significant correlations (r=0.82) were found between the concentration of selenium in Lolium perenne at harvest and selenate selenium removed from the soil solution.