Hydraulic Performance of Horticultural Substrates—3. Impact of Substrate Composition and Ingredients

Horticultural substrates, also referred to as growing media, potting soils and gardening or soilless substrates, are widely used as a basis for vegetable and flower production in horticulture. They are created as a composition of different ingredients (bog peat, organic residuals, coir, perlite and other components). Hydraulic properties such as water storage capacity, air capacity, shrinkage behaviour, wettability or hydraulic conductivity are important variables for a comprehensive evaluation of the performance of horticultural substrates. A set of 36 commercial potting soils and substrates was selected and the hydraulic properties (water retention curve, unsaturated hydraulic conductivity function, capillary rise and shrinkage) were measured using the extended evaporation method (EEM). Additionally, the water drop penetration time was determined as a measure of wettability. The hydraulic performance of the horticultural substrates was evaluated. Generally, bog peat is the main component of horticultural substrates. Additionally, coir (raw coconut fibre), bark, different composts and mineral ingredients such as perlite, pumice, vermiculite, sand and others are used. The growing medium with the best hydraulic performance in this study revealed substrates composed of bog peat with added coir, perlite and organic residuals. Mineral ingredients in general decreased the content of easily available water but did not exhibit any significant effect on the other properties studied. However, the risk of a lack of air can be increased by the addition of clay. The presence of perlite had positive effects on the air content and the re-wettability. The presence of organic materials had significant and detrimental effects on the height of the capillary rise. We also found that some products declared as preferable for use in containers were better suited as substrates for bed cultivation. However, a comprehensive evaluation of the eligibility of horticultural substrates in horticulture requires not only hydraulic measurements but also growing experiments and an assessment of their chemical, biological and technological suitability.