Physical and Chemical Characteristics of Hydroponic Media and Their Effects on Growth of Arugula Microgreens

We tested the effect of six different growing media on arugula (Eruca sativa Mill.) germination and growth to identify alternative substrates for growing hydroponic microgreens.  Chemical and physical properties were determined using cylinders containing perlite, fine-grade heat-expanded clay (HEC-F), 1:1 HEC-F:medium grade heat-expanded clay (HEC-FM) or 15:1:9 coarse grade coir:compost:perlite (coir-based mix).  Mixes were blended (by volume) in a cement mixer for five minutes and included nutrients and a wetting agent.  In addition, coir-based mix contained hydrogel.  A professional peat-based seedling mix and rockwool also were evaluated.  Round cylinders of 83 mL capacity were used for physical characteristic determinations and for the microgreen growth study.  Seeds were covered lightly with the same material used to fill containers except for rockwool-sown seeds, which were covered using tackifier.  Arugula was grown for three weeks using a hydroponic fertilizer solution and supplemental lighting (15 hours of light per day; mid-day readings of 78 to 171 µmol∙m^-2∙s^-1).  Total Porosity (TP) was highest for rockwool (92%) and peat-based seedling mix (83%) and lowest for HEC-F and HEC-FM (both 36%).  Predictably, those substrates with the highest bulk density (HEC-F and HEC-FM) had the lowest TP.  Aeration porosity (AP) values were lower for peat-based seedling mix, HEC-F and HEC-FM (5%, 7% and 10%, respectively) than for perlite, coir-based mix and rockwool (17%, 14% and 17%, respectively).  Determination of aeration porosity at an applied suction pressure of 7.5 kPa (AP_{-7.5 kPa}) indicated that peat-based seedling mix, HEC-F and HEC-FM had the lowest AP_{-7.5 kPa}  (7%, 12% and 17%, respectively) while perlite, coir-based mix and rockwool had the highest AP_{-7.5 kPa}  (26%, 24% and 26%, respectively).  Following microgreen growth study, final EC was similar for all treatments (320 to 398 µS∙cm^-1) and reflected the influence of the hydroponic solution saturation of media.  Compared to peat-based seedling mix, the use of perlite, coir-based mix and rockwool resulted in decreased shoot dry weight per seedling and number of seedlings; possibly due to aeration properties or to increased temperature exposure of seeds or seedlings resulting from the physical nature of the substrates as covering material.  Compared to peat-based seedling mix, HEC-F and HEC-FM had similar shoot dry weights per shoot and number of seedlings; thus, enhanced growth and germination occurred when using media with the lowest AP and AP_-7.5kPa..  In conclusion, heat-expanded clay media may be an adequate hydroponic media and suitable alternative to peat-based media for arugula grown as microgreens.