Use of elemental sulphur as an acidifying soil amendment for Brassica napus cultivation
Elemental sulphur is applied to alkaline soils as an acidifying material in order to improve nutrient availability and soil productivity. The oil industry extracts crude oils with ever higher concentrations of sulphur (S) that should not be released to the atmosphere and can be recovered by desulphurisation processes. The elemental S thus obtained has diverse applications, such as the manufacture of sulphuric acid, retreaded tyres, fungicides and acidifying agents. When used as a soil acidifying agent, S undergoes an oxidation process that depends on several soil-related factors: temperature, aeration, texture and organic matter content. The most important parameter appears to be the particle size of the S form applied to the soil. This study investigated the acidifying effects of two forms of elemental S, derived from an oil refinery, on an alkaline soil planted with oilseed rape. A trial was established in 300 L containers maintained outdoors in a site that received more than 1000 mm of precipitation during the study period, between October and the end of April. The treatments consisted of powdered (<0.5 mm) S at a dose of 1.5 t ha-1, and sieved (0.5-2 mm) S at doses of 3 and 5 t ha-1. Control soils were not treated with sulphur. The effects of the different treatments were studied by analysing leachates collected after every episode of rain during the crop cycle, foliar analysis of the crop produced, quantification of seed production and oil yield, and, finally, analysis of aqueous extracts of the soil obtained after harvesting the crop. The results obtained show that under the study conditions, sieved S (0.5-2 mm) was at least as effective as powdered S as a treatment for acidifying the alkaline calcareous soil used in the study. The sieved form offers two advantages relative to the powdered form: it requires less grinding and its storage and distribution in the field are simpler because the material does not disperse in the air (as the particles are larger) and is not explosive.