Suitability of Composts as Potting Media for Production of Organic Vegetable Transplants

Abstract

Composts, used alone or in mixtures with other materials, can serve as horticultural potting media in organic production systems. In this study, we evaluated the suitability of two locally available composts as media for lettuce and tatsoi produced organically in an unheated greenhouse. One of the composts was produced from food residuals with landscape wastes as a bulking agent, while the other was generated from used horse bedding. Although the two materials had relatively similar total N contents, C:N ratios, and bulk densities, they performed very differently as potting media. Net N mineralization, measured in laboratory incubations, was high in the compost derived from food residuals, but the horse-bedding compost showed net N immobilization, perhaps due to high salinity. Crop production in the food residuals compost was statistically similar to a control treatment consisting of a commercial peat-based potting medium with synthetic fertilizer. Crop growth in the medium consisting of horse-bedding compost, used at 100% or in a 50%/50% mixture with a commercial substrate of bark, peat, and sand, was unacceptable for commercial production. Although the cost per flat of the food residuals compost was slightly higher than that of the commercial peat-based medium, for organic production this additional cost may be insignificant since there are limited potting media options, and price premiums are typically available.