Container Substrates

Container substrates are important in that their characteristics determine the amount and frequency of irrigation and fertilization. Different media hold water and nutrients to different degrees. An ideal potting medium holds a large amount of water while at the same time being pore rich so that air pockets will be present in the plant root zone. Substrates with lower nutrient holding capacity will require fertilizer more frequently. Media with lower water holding capacity will likely require extra fertilizer and irrigation.

potting mixTypical container production media contain a mixture of sand, pine bark, and often small amounts of peat moss. Sometimes vermiculite and slow release fertilizer like polyon or osmocote may be incorporated into the media as well. The major component, pine bark, does not extensively hold nutrients.

Alternative media materials are currently being explored by researchers. In trials, these components have been effective at helping substrates retain nutrients and water - resulting in plant growth enhancement.

clayIndustrial clay mineral aggregates have been investigated with encouraging results. Calcined clays (like those used in cat litter) retain nutrients, encourage drainage, and enhance airspace while stabilizing pH. Clay is the product of extreme weathering that has broken large mineral rock structures down to particles less than 0.002 millimeters in diameter. Due to its large surface area and nutrient holding properties, clay is ideal for controlling nutrient leaching from pots and retaining water.

Research findings have shown that calcined clay usage in an 8:1 pine bark to clay mix can give the following benefits:

  • Reduction of irrigation by 200,000 ga/acre in a season
  • Increased available water for plants by 4%
  • Plants with calcined clay remained turgid 48 hours longer than plants without calcined clay
  • Container substrates contained 100% more phosphorus
  • Reduced phosphorous leaching by 60%
  • Some clay lots serve as fertilizers themselves by providing calcium and/or phosphorous
  • Containers are 20% lighter
The use of calcined clay in potting mixes has been shown to reduce water and nutrient loss. It serves as a form of slow release fertilizer while reducing environmental impacts of highly nutrified runoff water.


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