Sustainable growing media based on green waste compost and other organic recycled materials: Use of elemental sulphur to control pH

We tested the feasibility to partially or fully replace peat in growing media without reduction in quality. Experiments indicated that the lack of available raw materials with sufficiently low and/or stable pH values hinder peat replacement in growing media. Therefore an experiment on reducing growing media pH with elemental sulphur (S) was undertaken. Five mixtures with 0, 10, 15 and 25% (v:v) peat and 40 or 60% (v:v) green waste compost were tested. Sulphur was added at 3 rates: 0, 250 and 500 mg S L -1 substrate. A dose of 250 mg S L -1 substrate resulted in a pH-decrease of 1 unit after 5 weeks. After addition of 500 mg S L -1 substrate, however, EC increased to values far above the national legal threshold and thus reduced the quality of the substrate. No significant phytotoxicity was found in the acidified mixtures. Addition of elemental sulphur had also no significant effect on water-extractable N nor on plant-available P and K in the mixtures, which may explain the small effect of acidification on plant growth and flower characteristics, i.e., no significant effect on aboveground plant biomass nor on rooting characteristics or number of flowers. The feasibility and need for reducing pH of growing media mixtures containing green waste compost and alternative organic recycled materials is discussed.