Although it can be done, few herbs are raised in soil inside greenhouses. More often, they are raised in a soilless medium. (Some farmers' markets now require this.) Certified organic greenhouse-grown herbs must be raised in a certified organic potting mix. Commercial potting mixes typically contain wetting agents and synthetic fertilizers and are not allowable, requiring organic growers either to mix their own or to purchase a certified organic mix. (See the ATTRA publication Potting Mixes for Certified Organic Production.) Most organic potting mixes are based on good-quality compost amended with peat moss and perlite or vermiculite and supplemented with organic fertilizers like bone meal, feather meal, and kelp. Such a mix would be suitable for herb production, with one variation. Since most herbs are native to regions having neutral or slightly alkaline soils, the optimum pH for herbs is 6.0-7.0.(8) Most soilless mixes have a pH somewhere between 5.0 and 6.0, so they will have to be amended with lime before use. For more information on soilless media, amendments, and suggested mixes, see the ATTRA publications Organic Potting Mixes for Certified Production, Plug and Transplant Production for Organic Systems, and Organic Greenhouse Vegetable Production.
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