Sustainable growing media based on green waste compost and other organic recycled materials: indicators for potential biodegradation and N immobilization

Composts and lignocellulosic materials allow for reducing or completely replacing peat in growing media. However, these materials may vary in their decomposition rate (as affected by biochemical composition) and interaction with nitrogen, thus affecting the quality of growing media. We identified, characterized and assessed different organic recycled materials that potentially can be combined with green waste compost, known as a qualitative sustainable recycled material for growing media. For better characterization of feedstock materials and growing media mixtures, we tested an indicator for biochemical composition based on the crude fibre analysis, and an indicator for N immobilization based on a 7-day incubation trial. Based in the biochemical composition, potential biodegradation was calculated as the holocellulose/lignin ratio. The materials were tested for immobilization of mineral N by adding 350 mg N L-1 substrate as KNO3 followed by one week of incubation at 37°C and comparing the actual mineral N concentration with the theorethical concentration. Selected raw materials were combined in different ratios. Most raw materials had high pH and/or EC values, limiting their share in the growing media. Based on feedstock selection, green waste compost with sufficiently low EC and pH could be produced. Mixtures complying with the Belgian Federal Legislation (pH-range from 4.5 to 7.0 and EC below 750 µS cm-1) were selected for greenhouse experiments with potted plants. The effect of the mixtures on plant quality was assessed. This resulted in a selection of the five most suitable mixtures. Growing media including materials with high N immobilization resulted in lower plant growth. We conclude that quality of composts and lignocellulosic materials for use in growing media could be successfully assessed based on these indicators. Compost was found to be an important source of P and K, while N could be a limiting factor, especially in mixtures based on organic recycled materials with strong N immobilization such as used coffee grit.