The use of elemental sulphur to control pH in potting soil mixtures with green waste compost

We tested the feasibility to reduce or completely replace peat in potting soil without quality reduction. According to Belgian legislation, pH-range of potting soil for the consumer market is 4.5 to 7.0 and EC should be below 750 µS/cm. The first stage of the experiment indicated that – within the legislative framework – the high pH and EC values of green waste compost and other organic residues hinder peat replacement. Therefore, an experiment on reducing potting soil pH with elemental sulphur was started.
Five mixtures with 40 to 60% (v:v) green waste compost and 0, 10 or 25% peat were tested. Sulphur was added at 3 rates: 0, 250 and 500 mg S/l substrate. Organic fertilizer (9-3-3 NPK) was applied to all mixtures. Mixtures were brought to 30% moisture content and incubated at 20°C in complete darkness for 52 days, with regular sampling. Part of the mixture was removed after 14 days of incubation and used in a germination test and pot trail.
A dose of 250 mg S/l substrate resulted in a pH-decrease of 1 unit after 5 weeks. After 500 mg S/l substrate, however, EC increased to values far above the legal threshold. Acidification of the mixtures had no significant effect neither on germination of Lepidium sativum nor on water-extractable N or plant-available P and K in the mixtures, which may explain the small effect of acidification on plant growth and flower characteristics. The feasibility and need for reducing pH of potting soil mixtures with green waste compost will be discussed.