Characterisation of peat-free growing media to facilitate the transition to peat-free horticulture

With the growing necessity for a transition to peat-free horticulture, materials with equal or superior properties to peat, after formulation, are required. However, the diversity of peat-free substrate properties presents challenges for the design and prediction of growing media performance. Current peat-free substrates are based primarily on responsibly sourced materials from waste streams, including coir, pine bark, wood fiber, composted animal and green wastes – but there are many others. The composition and proportion of raw materials used, as well as temporal changes, affect the physical, chemical and biological properties of the end product. Physical properties vary according to differences in organic and inorganic fractions. Nonetheless, water retention, air exchange and plant support are necessary physical features. Optimum pH, EC, and nutrient provision are essential chemical properties. Alternative organic materials to peat typically contain a higher nutrient value, but understanding their dynamic biological and chemical properties, i.e. how nutrient profiles of different mixes change and interact with biology over time is key to identifying optimum formulations. On one hand, microbes affect plant-available nutrient pools through mobilisation, immobilisation, decomposition, and mineralisation, and are therefore central to questions surrounding storage conditions, shelf-life, and ultimate performance. On the other hand, pathogen load, disease suppression, and an abundance of favorable or unfavorable organisms in the substrate are also of importance to growers and media manufacturers. This article highlights the challenges and need for characterizing peat-free growing media, and reviews a range of physical, chemical and biological analytical techniques, with recommendations for a standard suite of analyses. Establishing early and practical characterisation protocols for peat-free growing media will ensure that the final substrate has suitable physical, chemical and biological properties that can be used to maximum advantage. This will increase confidence in peat-free growing media performance, and facilitate the transition to peat-free horticulture.