CRITICAL SULFUR CONCENTRATIONS IN OILSEED RAPE (BRASSICA-NAPUS) IN RELATION TO NITROGEN SUPPLY AND TO PLANT-AGE

Oilseed rape was grown in a sand culture experiment in a glasshouse to derive values for plant testing for the diagnosis of sulfur (S) defic iency and for the prediction of seed yield. Five rates of S, combined factorially with 4 rates of nitrogen (N), maintained constant througho ut 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 fo r diagnosis or prediction were S-t and SO4. Whole shoots and youngest fully expanded leaves exhibited similar critical values in plants at t he rosette stage, and critical values (S-t = 0.20-0.25%; SO4 =230-460 mg/kg) changed little with time. Critical values fbr N/S-t changed wit h time, required 2 analyses, and gave no indication of the degree of d eficiency when used to predict yield. Critical values of SO4/S-t depen ded on N supply, so 3 analyses were needed. It is argued that high cri tical values reported previously for prediction of seed yield have bee n obtained when there was a decline in soil-available S and plants rel ied on S taken up during early growth.