Organic growing media : constituents and properties

Organic growing media are essentially bulk products. Availability in large quantity allied to its excellent air and water retention, low pH and salinity, and freedom from pests and diseases has led to peat being the dominant organic constituent of growing media in many parts of the world for the last 50 yr. The unique microporous properties of Sphagnum peat and its resistance to degradation are matched by few other growing media constituents. Nevertheless, local scarcity of Sphagnum peat and the expense of transport has led to the use of other materials in growing media. Notable among these is coir, which unlike peat, a CO2 sink, is widely regarded as a rapidly renewable resource. Indeed, advances in processing and quality control in situ have led to a huge upsurge in the export and use of coir in growing media, particularly in Europe but also in the western United States. Locally available organic materials such as bark, composted materials including green (yard) wastes, municipal solid wastes, and even sewage sludge are also used in growing media. While possessing advantages such as the high air content of bark and nutrient supply of many composted materials, these media components may have disadvantages, from limited supplies due to bioenergy pulls and N lock-up in bark, to physical, chemical, and microbial contaminants in composts. Current innovative approaches involve increasing use of wood fiber in Europe, whole pine-tree thinnings in the United States, and realizing the use and transformation of composted wastes as next-generation constituents of growing media.