Relationship between dissolved organic carbon and oxygen uptake rate in growing media components

A key characteristic of growing media is the ability to maintain biological balance. The organic matter within a substrate should be the most recalcitrant against excessive degradation, even under wet conditions and with the abundance of nutrients as it occurs during its usage. This feature could be tested by measuring the oxygen uptake rate (OUR EN 16087-1:2012) The method is reliable and established for a wide range of materials (compost at different maturity stages, peat, coco coir, bark, etc.), but it is time consuming and laborious. As an alternative stability test, the Chemical Oxygen Demand (COD) method has been applied on the water extract (1:5 v/v) of substrates prepared for the determination of pH and EC. In this study the relation between OUR and dissolved carbon (calculated from COD) was investigated on 25 organic growing media components (13 coco coir, 2 peat, 2 compost, 2 digestate solid fraction, 1 almond shells, 1 wood fiber and 4 compost based growing media). A nonlinear correlation was found between the two variables; the exponential function was the best-fit equation (R2=0.8027). COD method gives an indication of the stability in short time and can be used as a tool for screening organic components of growing media. The OUR/COD ratio spotted a different behavior of digested or transformed materials from raw ones.