Biological oxidation of elemental sulphur added to three composts from different feedstocks to reduce their pH for horticultural purposes

The biological oxidation of elemental sulphur (So) added to three alkaline composts prepared with a range of organic wastes (CC, melon crop residues; MC, mixed manures; and BC, pine bark) to reduce their pH was studied. The titration curves showed that to achieve an equivalent pH drop, compost CC needed a larger dose of So than did composts MC and BC. The acidification efficiency was high in the three composts (⩾53%), but the pH reduction obtained from the titration curves in MC and BC composts was lower than expected. So oxidation in amended composts was found to be related to pH and CaCO3 content decreases, and to the rise in CaSO4 and electrical conductivity levels. A remarkable increase in the autotrophic bacteria population and a slight increase in heterotrophic bacteria along with So oxidation were recorded. Actinomycetes, fungi and yeasts were not affected by the addition of So to composts.