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Use of plant analysis to assess the sulfur status of field grown subterranean clover

S. Duncan, G.M. Proudfoot and M.G. Browne

CSBP & Farmers Ltd, 40 The Esplanade, Perth WA 6000

Introduction

Widespread use of superphosphate has masked the incidence of sulfur deficiency in legume- based pastures. The increasing cost of sulfur has resulted in a trend toward the use of low sulfur fertilisers, increasing the likelihood of sulfur deficiency. Glasshouse experiments have indi- cated that, in addition to plant sulfur concentration, plant age and total nitrogen are useful in detectir - nutrient deficient subterranean clover (Trifolium subterraneum) plants (1).

This paper reports the results of long-term field trials to establish robust criteria for the prediction of sulfur response in pastures by plant analysis. These criteria are then used to evaluate the sulfur status of pastures from samples collected by Western Australian farmers in 1991.

Methods

Four permanent pasture trials were established in 1989 on a range of sandy and gravelly loam soil types in the high rainfall area (>900 mm/yr) of Western Australia. The trials included treatments with a range of sulfur application rates and basal supplies of phosphorus and potassium. The pasture was rotationally grazed. Pasture growth was measured at the beginning and end of each enclosure period with a capacitance meter calibrated against pasture cuts. Samples of clover were collected at each measurement for analysis of total nitrogen (modified Kjeldahl) (2) and turbimetric determination of total sulphur on a dry weight basis.

Results and discussion

The sulfur concentration in the tops of clover plants fluctuated through the growing season. A composite sulfur status index which combined sulfur concentration, plant age (estimated from sampling date), and nitrogen to sulfur ratio was able to differentiate between sulphur responsive and sulfur adequate situations (Fig. 1). However, there was a range of marginal sulfur status, which represented treatments responsive to sulfur application under some conditions. Analysis of the sulfur status of routine subterranean clover samples forwarded to CSBP's Soil Fertility Service Laboratory in 1991 shows that 10% were deficient, 49% were marginal and 41% were adequate.

Figure 1: Relative pasture growth rate related to sulfur index.

References

Spencer, K., Jones, M.B. and Freney, J.R. 1977. Aust. J. Agric. Res. 28, 401-412.

Searle, P.L. 1984. Analyst 109, 549-568.

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