SUITABILITY OF COMPOSTS DERIVED FROM DAIRY SOLIDS, HORSE MANURE OR MUNICIPAL YARD DEBRIS FOR CONTAINERIZED AZALEA PRODUCTION

Composts are a potential substrate component for containerized plant production. Azalea is representative of broadleaf evergreen plants, the largest market segment of woody ornamental shrubs produced in the USA. It is an ericaceous plant suited to acid substrates that are low in soluble salts. Objectives of this study were to (i) evaluate compost as a minor soilless substrate component, and (ii) relate plant growth response to substrate characteristics. Six composts derived from dairy manure solids, horse manure, or municipal yard debris were mixed 33% (v/v) with Douglas-fir bark (DFB). The control treatment was a conventional nursery substrate: DFB + 20% (v/v) horticultural Sphagnum peat. Compost pH (7.5 to 9.1), pH buffering capacity (0.5 to 1.1 mol H+/kg compost-C/pH unit) and soluble salts (EC; 1.2 to 3.0 mS/cm) were high relative to peat. Five of 6 composts reduced azalea root and shoot dry weight and plant quality compared to the conventional substrate control. Compost addition increased pour-through substrate pH (1 to 2 pH units) and pour-though EC (1 to 2 mS/cm) vs. control. Preplant incorporation of pelleted elemental S into substrate mixes was ineffective in acidifying compost. Plant dry weight was negatively correlated with compost pH and pH buffering capacity. Compost buffering capacity was a more reliable indicator of plant growth response than was preplant compost pH. We conclude that the high pH and high pH buffering capacity of most composts is the major obstacle for use of compost with containerized azalea and other acid-loving plants. Determination of pH buffering capacity of compost can assist in determining the rate of acidifying amendment needed to reduce pH to the optimal range for azalea.