Hydraulic Performance of Horticultural Substrates—2. Development of an Evaluation Framework

Sustainable, environmentally friendly and resource-saving water and nutrient management in horticulture requires knowledge of the hydraulic suitability of horticultural substrates for each specific application. The aim of this study was to develop methods and a rating framework to assess the hydraulic performance of horticultural substrates. The hydraulic criteria related to high-quality horticultural substrates were defined as the amount of easily plant-available water (EAW) in the tension range between 10 and 100 hPa, the air capacity and the height of capillary rise. Limiting factors could be water repellency effects and shrinkage. The rating framework consisted of 5 classes between non-satisfactory and very good. The assessment of the hydraulic performance was split for the cultivation into 10-, 20-, and 30-cm-high containers. It was tested on 18 commercial substrates. More than 70% of the tested substrates revealed scores between very good and good. About 30% were evaluated as medium or satisfactory. The most critical aspect was the low air capacity in shallow containers. Shrinkage and water repellency sometimes strongly diminished the score. Both the measurement methods for quantifying substrate hydraulic properties and the evaluation procedure proved applicable. The impact of different ingredients and the composition of substrates of their hydraulic performance should be statistically analysed in further studies.