A mixing model to predict rooting media properties

With a mixing model it is possible to predict the properties of a mix of 2-9 constituents in any ratio. Predicted parameters were EC, pH, nutrients, trace elements, water content, air content and degradability.

A growing interest in peat alternatives, notably composts, resulted in the renewed demand for a model to predict the effects of mixing potting soil constituents. Our goal was to make such a mixing model which could predict the combination of 2-9 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 modelled values came to within 5% of the measured values which is thought acceptable for practise.

The effects of interstitial filling were included, interstitial filling being the filling of pores in one material with smaller particles from another material. PH as parameter could not be predicted but could be replaced with the pH buffer measurement. Easily available water however could not be effectively predicted by the model.

In addition, it proved possible to estimate how much of some additives needed to be added. The additives were lime, nitrate, fertilizers and iron chelates. Lime is administered to increase and stabilize the pH. Nitrate and iron are added to compensate the increased uptake of nitrate and iron by microorganisms. The fertiliser were added to reach the crop specific base dressing. When adding fertilisers the fertilisers already present in the substrate materials were subtracted.

Source: Wageningen University & Research

Publication date: Tue 11 Jul 2017