Critical sulfur concentrations in oilseed rape (Brassica napus) in relation to nitrogen supply and to plant age

Abstract

Summary. Oilseed rape was grown in a sand culture experiment in a glasshouse to derive values for plant testing for the diagnosis of sulfur (S) deficiency and for the prediction of seed yield. Five rates of S, combined factorially with 4 rates of nitrogen (N), maintained constant throughout the experiment, were used to determine critical concentrations of S fractions and ratios (total S, S_t; sulfate-S, SO₄; total N/total S, N/S_t; SO₄/S_t). The most satisfactory indices of rapeseed S status for diagnosis or prediction were S_t and SO₄. Whole shoots and youngest fully expanded leaves exhibited similar critical values in plants at the rosette stage, and critical values (S_t = 0.20–0.25%; SO₄ = 230–460 mg/kg) changed little with time. Critical values for N/S_t changed with time, required 2 analyses, and gave no indication of the degree of deficiency when used to predict yield. Critical values of SO₄/S_t depended on N supply, so 3 analyses were needed. It is argued that high critical values reported previously for prediction of seed yield have been obtained when there was a decline in soil-available S and plants relied on S taken up during early growth.