Abstract

The objective of this study was to determine the effect of arcillite, a calcined montmorillonite and illite clay, on the physical and chemical properties of a pine bark growth substrate and plant growth. To accomplish this, Cotoneaster dammeri ‘Skogholm’ and Rhododendron sp. ‘Sunglow’ were potted into pine bark substrate amended with 0, 27, 54, 67 or 81 kg/m³ (0, 45, 90, 112 or 136 lbs/yd³) arcillite. Plants were irrigated every l, 2, or 4 days. Container capacity, available water, and bulk density increased with increasing arcillite rate. Air space decreased with increasing arcillite rate. Total porosity and unavailable water were not affected by arcillite. Substrate NH₄, NO₃, P, K, Ca, and Mg concentrations increased with decreasing irrigation frequency. Substrate NH₄, P, and K concentrations increased with increasing arcillite rate suggesting that arcillite improved retention within the container substrate. Azalea ‘Sunglow’ shoot dry weight decreased linearly with decreasing irrigation frequency and increased curvilinearly with increasing arcillite with maximum dry weight occurring at 67 kg/m³ (l12 lbs/yd²). Cotoneaster ‘Skogholm’ shoot dry weight increased linearly with increasing arcillite rate at 2 and 4 day irrigations and curvilinearly at 1 day irrigation with maximum weight at 67 kg/m³ (112 lbs/yd²).