Substrate's end-of-life: environmental and horticultural considerations

The importance of soilless cultivation is steadily rising world-wide, leading to an increased waste problem of used media. At the end of its useful life, and in order to minimize environmental burden, the use of substrates should comply with the three R's: reduce, reuse and recycle. Unfortunately, reduction is not a viable option in most cases, as yield and plant resistance to stress are positively correlated with substrate volume. What about the other two R's? Reuse is the best approach in terms of its environmental impact and the results of life cycle analysis (LCA). It involves, however, the risk of pathogen proliferation and the possible deterioration of several physical, chemical and biological characteristics of the substrate. Among others, such changes might lead to compaction, increased bulk density and reduced air-filled porosity due to shrinkage; in organic media mineralization may lead to low level of dissolved O₂ in the rhizosphere, and other undesirable results. In many cases, however, reuse becomes possible following relatively simple remediation steps such as amending the substrate with bulking materials and disinfestation. In some cases (e.g., tuff) the same substrate can be used indefinitely, without negative effect on plant yield using only simple disinfestation means. Recycling is the last environmental-friendly approach, frequently possible by applying the used substrate as soil amendment or as an ingredient in media for less-demanding crops such as forest tree saplings. Examples for all of the above are presented and their horticultural and environmental benefits and drawbacks are discussed.