Lettuce (Lactuca sativa 'Webb's Wonderful') shoot and root growth in different grades of compost and vermicomposted compost

Immature compost can have negative plant and soil effects such as phytotoxicity, soil N immobilisation, and reduced plant growth, especially when used in horticultural applications, e.g. as a growing medium. Composting involves three main stages: sanitisation, stabilisation, and maturation. This study investigated the use of vermicomposting as a post-stabilisation method to increase the rate of maturation of composted horse manure. It also compared the effects of three grades (ungraded, >3 mm, and <3 mm particle size) and increasing rates (0, 10, 20, 50, and 75%, vol./vol., in a peat-based growing medium) of the compost and the vermicomposted compost on lettuce shoot and root growth. Compared with composting, vermicomposting reduced the pH and C/N, and increased the electrical conductivity and nutrient content of the end product. Mean shoot fresh and dry weights were significantly higher in plants grown with vermicompost, compared with compost. Shoot water content responded differently with increasing concentration of vermicompost or compost amendments. The different amendment grades only affected shoot dry weight significantly. Vermicomposting increased the rate of compost maturation, resulting in significantly larger plants with reduced conductivity stress and root/shoot ratio, especially at higher amendment concentrations.