Chapter 10

Growing Media for Greenhouse Crops

Growing Media for Certified Organic Production

Various forms of seedling and potting substrates are used in the production of field transplants, in the growing of container plants, and in greenhouse crop production. Such substrate may be composed of a wide range of natural and synthetic materials. In certified organic production, there are limitations on the materials that may be used as base substrate or for supplemental fertilization as required by   National Organic Program’s Final Rule. Organic producers who choose not to mix their own growing media can purchase commercial off-the-shelf or custom potting mixes or can they can chose to mix their own media on the site. Either way finding appropriate growing media that is approved for use for organic production can be challenging.

Certifying Agents

To find out if a commercial product is acceptable for organic production it is recommended to check with your certifying agent that approved the organic system plan. The National Organic Program established a policy that each Accredited Certifying Agent (ACA, certifier) is responsible for conducting its own reviews of inputs for agricultural production, such as formulated pesticides and soil amendments. They must review both active and inactive (inert) ingredients for compliance. All certifying agents must verify that materials used by certified organic operations comply with the USDA organic regulations, including the National List of Allowed and Prohibited Substances, and any annotations provided therein. (See 7 CFR 205.601-606).

Ingredients Allowed for Organic Media

Contrary to what some critics say, organic growers are permitted to use a wide array of materials in growing media. The challenge is more a matter of ensuring consistent quality of ingredients than in finding enough of them. The section that follows features a brief description of some of the materials commonly used in organic growing media and discusses some of the issues that surround them.

Soil

Clean commercial topsoil is an acceptable natural ingredient, but you have to be certain that it has not been treated with prohibited ingredients to kill microbes and weed seeds. Soil contaminated with pesticides, prohibited fertilizers, or environmental pollutants may not be used. Certifiers might require that any soil used must come from land in certified organic production.

Compost

Compost is perhaps the most common potting-mix ingredient among organic producers. Cheaper than traditional components such as peat moss, compost holds water well, provides nutrients, and can be made right on the farm.

The National Organic Standard (NOP) is very explicit about compost making. Compost piles must maintain a temperature between 131 and 170 degrees F (55 to 77 °C) for at least three days in a static or enclosed vessel system, or at least 15 days in a windrow system, with at least five turnings. Unless these criteria are met, the resulting product is not—in the eyes of the National Organic Program—considered compost. Rather, it is simply a pile of raw materials. If one of those raw materials is manure, it can make a big difference in how it may be used in crop production.

Sphagnum Peat Moss and Other Forms of Peat

Sphagnum peat moss is the most commonly used soilless medium, because it is widely available and relatively inexpensive. Peat moss is a very stable organic material that holds a great deal of water and air and does not decompose quickly.

Newspaper

Ground-up newspapers can be used as a substitute for peat moss in growing media. Newsprint should not be more than 25 percent by volume of the mix. Avoid the inclusion of glossy paper or paper with colored inks, as these are prohibited.

Sawdust

The quality of sawdust used as media depends on the wood. Cedar, walnut, and redwood sawdust can be toxic to plants. Oak, hickory, and maple are reputed to tie up soil nitrogen more readily than sawdust from evergreens. Sawdust from treated or painted lumber is not allowed in organic production.

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