Abstract

Sulfate sorption by the soil affects the rate of sulfate leaching, which impacts on the availability of soil sulfate for plant uptake. In Australia, plant-available sulfur is measured using 0.25 M KCl heated for 3 h at 40°C to extract soil sulfur (SKCl40). This paper describes a technique referred to as a sulfate buffering index (SBI), which provides a measurement of sulfate sorption. SBI when combined with the estimates of the q and b parameters of the Freundlich equation, can be used to define a sorption curve. The equation is S = acb – q; where S is the amount of sulfate adsorbed (mg S kg–1), c is the equilibrium concentration of sulfate measured in solution (mg S L–1) and a, b and q are coefficients that describe the soil sulfate sorption curve. Coefficients S and c were measured using six sulfate solution concentrations ranging from 0 to 250 mg S kg–1. The adsorption curve was fitted using the modified Freundlich equation including setting of b = 0.41 and q = SKCl40 using recently collected soil samples. The modified Freundlich a coefficient or SBI was calculated as SBI = (S + SKCl40)/c0.41; where S and c were determined using 50 mg S kg–1 of added sulfate. The SBI ranged within 1–40. The SKCl40 was related to SBI below a depth of 10 cm (r2 = 0.71) but not for the 0–10 cm soil layer where S sorption was minimal.

Highlights

  • Sulfur deficiency in wheat (Triticum aestivum L.) and canola (Brassica napus L.) occurs when sulfate fertilisers are not applied to soils low in natural sulfur reserves (Anderson et al 2006; Brennan and Bolland 2006)

  • Sulfate leaching during the growing season reduces the ability of soil test methods to accurately predict wheat and canola grain yield response to applied sulfate fertiliser (Anderson et al 2013)

  • The low sulfate sorption capacity combined with intense rainfall after sowing of crops can result in the rapid displacement of sulfate from the 0–20 cm soil layer, leading to a sulfur deficiency in wheat (Anderson et al 2006)

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Summary

Introduction

Sulfur deficiency in wheat (Triticum aestivum L.) and canola (Brassica napus L.) occurs when sulfate fertilisers are not applied to soils low in natural sulfur reserves (Anderson et al 2006; Brennan and Bolland 2006). Sulfate is readily soluble in soil solution and subject to loss by leaching (Till 2010) in soils with low sorption capacities. This is less of a problem in soils with high anion exchange capacities, which occurs in acidic subsoils. These soils are expected to have lower rates of nitrate and sulfate leaching (Wong and Wittwer 2009).

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