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Composts in the production and performance of growing media for containers

Handreck, K.A.
Acta horticulturae 2014 no.1018 pp. 505-511
Pinus, bark, coconuts, coir, composts, containers, drainage, drawdown, energy, gardeners, growing media, hardwood, harvesting, hulls, nitrogen, organic matter, phosphorus, porosity, quality control, root diseases, sawdust, trees, water holding capacity, wettability
Australian production nurseries and gardeners have available to them excellent container media whose components are mainly organic and, in the south and east, predominantly composted pine bark. Coir fibre dust is often a minor component that enhances water-holding capacity. Where pine bark is not available locally, composted hardwood sawdust, composted green organics and chopped coconut husks can be major organic components. Media for production nurseries is typically ordered from one of a small number of specialist composters and growing media formulators who have in place quality assurance systems that have minimised batch-to-batch variation in the key properties air-filled porosity, wettability, pH, nitrogen drawdown and phosphorus and iron supply. Quality is assessed via the parameters set down in the Australian Standard for Potting Mixes, which was first published in 1989 and most recently updated in 2003. Routine use of the provisions of this Standard has overcome the often extreme variability in quality, notably nitrogen drawdown rate, experienced in the early days of the use of compost-based growing media. While a test for disease suppressiveness is not a part of this Standard, it is common experience (supported by research) that so long as a growing medium has excellent drainage and a high proportion of slowly-decomposing organic matter, root diseases are rare. Inoculation with selected bio-control organisms is not common. This happy situation should continue wherever bark from 30-40+ year old pine trees continues to be available to growing media producers, but downturns in building activity, diversion of bark to energy production and the increasing tendency to harvest younger trees is restricting supply of high-quality pine bark in some areas. Quality maintenance is more difficult with other composts, but possible with skilled management.