Spatial and time-dependent patterns of selenium (Se) release from selected Se fertiliser granules

Abstract

The spatial and time-dependent patterns of selenium (Se) release from similar-sized granules of 4 Se fertilisers, 3 containing sodium selenate (Unitech Agsel, Ravensdown A, and Ravensdown B) and the fourth containing sodium selenate and barium selenate (Selcote Ultra), were evaluated using ryegrass grown on a Se-deficient soil under glasshouse conditions. Four granules (2–3 mm diameter) from each of the 4 fertiliser types having uniform shape and similar weight were placed on the soil surface of trays (0.51 m length, 0.42 m width, 0.15 m depth) at a distance 0.10 m from 2 adjacent sides of the trays, and the Se content of herbage in concentric rings around each granule was periodically measured over 1 year. The amount of Se applied to the trays, calculated by adding herbage Se to the residual fertiliser Se in soil at the end of the trial, was 12–31 g Se/ha. Soil Se concentration at the end of the trial for the Selcote Ultra and Ravensdown A treatments and herbage Se concentration at 10 harvests for all fertiliser treatments decreased exponentially with increased distance from the granules. Herbage Se concentration reached the level considered deficient to animals (0.05 mg/kg) at the centre of the tray (0.12 m from the granules) 4 months after Unitech Agsel application (7% of paddock area). By 9 months all plants in this treatment became deficient in Se. In contrast, 7% of the sward area in the Ravensdown A treatment, and 41% of the sward area in the Ravensdown B and Selcote Ultra treatments, were deficient in Se at this time, but the average herbage Se concentration for the entire tray was above the deficiency level for these 3 fertiliser treatments. The efficiency of the fertilisers in increasing herbage Se concentration per unit weight of Se applied for the first 3 months was: Unitech Agsel ≥ Ravensdown B ≥ Ravensdown A > Selcote Ultra. During the last 3 months, this order was: Selcote Ultra = Ravensdown A > Ravensdown B > Unitech Agsel.