Coal and quarry waste turned into peat-free compost

More than 50,000 cubic metres of peat is used each year in the UK mostly extracted from peat bog sites – rich wildlife and species habitats. Producing a viable alternative from waste products would obviously reduce the need to extract peat from these sensitive locations.

The study was undertaken by Warwick HRI. Lead researcher, Ralph Noble, said: “Composted materials such as bark and green waste have already been introduced to partially or completely replace peat for some uses. This study looks at producing an alternative peat-like product that can be used to raise vegetable, bedding and pot plants in blocking media.”

The study used fine-clay like waste from mining and quarrying – the result of washing sand or mined ores and minerals and are usually disposed of in large pond lagoons with a significant amount of sand or gravel. In some quarries the coarser, saleable particles are removed from the washings and the remaining suspended fine particles flocculated with polymers so that the water can be recycled, leaving a clay-like product.

Two tests were conducted. The first one sowed pelleted seeds for iceberg lettuce into the media prepared from quarry clay, green waste compost and fine grades of composted spruce and pine bark.

It showed that the composted green waste was easier to form into blocks than the bark. However, no more than 20% of the overall volume of green waste could be used, otherwise the pH and conductivity of the growing medium became too high. Including at least 20% of quarry tailings clay bound the compost into a block. The inclusion of the quarry tailings also reduced the porosity and increased the water retention of the compost mix.

The best results were achieved with a mixture of 20% green waste compost, 55% composted bark, and 25% quarry tailings. The results were comparable to those using a commercial blocking peat.

The second test used begonia seeds. It found that using 12.5% volume of the quarry tailings clay was sufficient to bind a compost mixture without impeding the drainage of the cells. A mixture containing 75% bark, 12.5% green waste compost and 12.5% quarry tailings was found to perform best and was again comparable to commercial peat based substrates.

Commercial trials are now underway to determine the economics and large-scale feasibility of using a peat-free compost. The research was funded by a grant of £13, 960 from the Onyx Environmental Trust through the Landfill Tax Credit Scheme.

Margaret Cobbold, General Manager of the Trust said: “I am delighted to see such encouraging results from this Trust funded research project. Using peat-free product to cultivate foods and plants will mean we can preserve some important habitat sites throughout the UK, helping to protect many species of plant and wildlife for the future.”

By David Hopkins