Residual value of manganese fertiliser for lupin grain production

Abstract

The residual value of manganese fertiliser for seed (grain) production of lupin (Lupinus angustifolius L.) was measured in 2 long-term (16 years) field experiments on 2 different soils (sand, lateritic ironstone gravel–sand) when manganese sulfate was placed (drilled) with the seed at about 5 cm while sowing. Relative to the nil-manganese treatment, additions of manganese fertiliser increased yields by about 1.0 t seed/ha and decreased the amount of split seed from about 80% to less than 5%. Additions of manganese fertiliser did not affect plant density (measured 1 month after sowing) or the yield of dried shoots. The residual value of manganese fertiliser was measured in 1994 for grain production of lupin, for manganese applied once only to plots not treated with manganese before, either in 1994 (current manganese) or in a previous year (previous manganese), applied in one of the following years: 1978, 1982, 1988, 1991. The residual value of previous manganese for grain production decreased relative to the effectiveness of current manganese, the decrease increased with increasing time since application. The decrease in the relative value varied with soil type. For example, the sandy soil showed a larger decrease in the effectiveness of manganese compared with the gravelly sand. For the sandy soil, 16 years after application the manganese application was about one fifth as effective as current manganese, for lupin grain production. As measured using soil test manganese, critical soil manganese values for diethtlenetriamine penta-acetic acid, ammonium acetate, and ammonium acetate plus hydoxy quinone extraction varied with soil type. For example, the diethtlenetriamine penta-acetic acid critical concentration of manganese for grain yield for the sand was about 2 mg/kg compared with about 4 mg/kg for the gravelly sand. The critical concentration of manganese in the youngest vegetative bud for maximum seed yield was about 50 mg/kg while the critical concentration of manganese in the stem at mid-flowering of the main stem was about 20 mg/kg. Having a manganese concentration above the critical concentration of manganese in either plant tissues resulted in split seed levels of less than 5%.