Using a simple mixing model to predict rooting media properties

A growing interest in peat alternatives, notably composts, resulted in the renewed interest for a model to predict the effects of mixing potting soil constituents. Our goal was to develop a mixing model which could predict the combination of 2 to 9 constituents. Parameters were EC, pH, nutrients including trace elements, water content at various suction pressures, air content at saturation and degradability. The outcome of the model and the results for measurements on the actual mixtures were compared. Most modeled values came to within 5% of the measured values which is considered acceptable for practise. The problem of interstitial filling is discussed and a parameter based on laboratory compacted bulk density is introduced. It is shown that pH and easily available water cannot yet be effectively modeled. It is argued that degradability and buffer capacity are new parameters of interest.