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Conclusions and future prospects

This review on the use of organic waste compost in horticultural media in Germany outlined the research and development activities in this area over the past decade. This period saw a wide range of growth trials with compost based potting mixes, a major research project that evaluated potentially detrimental substances in compost and established thresholds in conjunction with recommendations for the use of compost in horticultural media, and finally the establishment of quality criteria for compost to be used for the production of commercial container media. Naturally, during that time the education of and co-operation with compost producers, the horticultural industry and manufacturers of growing media was an ongoing process.

During that time the following main lessons have been learned:

• Compost can be used successfully in horticultural growing media in all sectors of the industry.

• Results of compost amendment depend on compost quality, the amount of compost incorporated and the hardiness of plants cultivated (e.g. salt tolerance).

• Only top quality compost can be used in the production of horticultural growing media. Particularly a high salt content, adverse growth effects on indicator plants and a high potassium content preclude composts from being incorporated in container media.

• General compost quality standards are insufficient to determine composts fit or unfit for the use in container mixes. It is necessary to establish specific quality criteria for compost to be used in horticultural growing media.

• It is safe to incorporate 20 % or 40 % (v/v) of high quality compost in a growing mix, provided the product complies with the horticultural compost standard. A higher amendment might be possible but requires thorough evaluation.

• Compost operators need not only focus on the processing of waste materials but also on the marketing of the finished compost. It is necessary to cater for the need of the horticultural industry and its high quality requirements, develop compost-based products, widen the product range and produce composts for specific markets.

• The horticulturist needs assurance that compost based products are standardised, are in continuos supply, do not pose a risk, do not impose substantial additional costs and generate similar or better yields than alternative products.

This demonstrates that the use of compost in horticultural growing media has come a long way in Germany during the last decade. But have the initial goals of largely substituting peat moss with compost in container media and soil amendment products and utilising some 2 to 4 million m³ of compost in that way been realised?

Estimates presented last year suggest that 150,000 to 200,000 m³ compost have been used in the production of growing media (Deutsche Bundesstiftung Umwelt, 1997). This amount represents some 5 - 10 % of the envisaged potential compost use in the production of growing media. The moderate growth of this market sector in the past has certainly been linked to a high level of uncertainty among horticulturists with regard to the risks of using compost. However, with sufficient prove of the beneficial effects of compost in container media and the establishment of specific quality criteria for compost in growing media, the production and use of compost based growing media should increase. Much of the work done so far can be seen as a pre-requisite for the successful and commercial use of compost in horticultural growing media. Compost producers, manufacturers of growing media and horticulturists now have the chance to build on a sound basis and realise the full potential compost has in the horticultural industry.

The aim to substitute peat in growing media with compost has only been achieved partially. Due to the quality characteristics of organic waste compost, generally it is only possible to replace 20 % to 40 % (v/v) of peat with compost. Consequently, the reduction of peat mining will be limited and the destruction of wetlands ongoing. But there is no doubt about it: peat moss is a finite resource and will run out eventually.

The search has already begun to further reduce peat in container media and to substitute it with other, readily available recovered resources such as for example rice hulls, nut shells, coconut fibre, wood fibre or brick debris. There is also considerable research required into the adequate processing of these materials and into the development of high quality growing media containing these components and a minimum of peat moss.