Characterization of Hydrophobicity of Various Soil-Less Potting Mixtures.

Organic-based soil-less potting mixtures for greenhouse experiments are known to be difficult to re-wet once these mixtures dry.  Many commercial potting mixtures contain wetting agents or hydrophilic gels pre-mixed to help remediate this problem.  The objective of this study was to characterize the level of hydrophobicity of various organic-based soil-less mixtures.  Mixtures evaluated include three commercially available, peat-based mixtures that contain a wetting agent, and two custom-made mixtures that contain larger or smaller amounts of peat at 80% or 50% (v/v), respectively, without wetting agent treatment. Commonly used methods for evaluating hydrophobicity, such as the Molarity of Ethanol Droplet test (MED) or Water Droplet Penetration Test (WDPT), can be inconsistent for soil-less potting mixtures due to their lack of apparent structure, extremely low bulk density, and difficulty in packing.  Therefore, a pressure plate extractor was utilized to quantify volumetric water content (VWC) of the selected soil-less potting mixtures.  After saturation, samples were subjected to various pressures ranging from 10 to 1500 kPa for five days.  Our results suggest that there are significant differences in VWC among mixtures at all six pressure points.  At -10 kPa, VWC in the mixtures ranged from 34 to 43%. As root zone water pressure decreases, VWC of all mixtures reduced to 20% or less. Of the commercial mixtures tested, Fafard C-2 retained the least amount of water at each pressure except -1500 kPa, where Metro-Mix 360 retained 20% less water compared to Fafard C-2.  Pro-Mix BX had the highest water content at every pressure tested, and retained 19% more water at -1500 kPa than Fafard C-2.  Without wetting agent treatment, the custom mixtures retained a significantly lower amount of water at -1500 kPa than the commercialized mixtures, and only retained less than 10% VWC.  The experiment will be repeated and results will be discussed.