Substrates and fertilizers for organic container production of herbs, vegetables, and herbaceous ornamental plants grown in greenhouses in the United States

Abstract

The value of organic greenhouse production in the United States is $112 million, and organic floriculture production has increased 40% in greenhouses since 2008. The U.S. National Organic Program (NOP) guidelines allow organic production in containers. One of the primary concerns for current and potential organic container growers is managing container substrate and plant fertility. This manuscript provides a review of current literature on organic substrates and fertilizers for container grown herbs, vegetables and herbaceous ornamentals according to NOP guidelines. Organic-approved substrates can be purchased from commercial suppliers or produced in house. Many organic substrate components are similar to those used in conventional production (peat moss, pine bark, perlite, vermiculite, rice hulls, and whole pine tree) but lack a synthetic wetting agent. Growers who mix their own substrates often add compost at percentages ranging from 20 to 50%. Compost has high water holding and cation exchange capacity. It is often locally or regionally produced and purchased, which impacts variability and quality. For this reason, growers often need to manage substrates with compost more carefully. Single source or blended organic fertilizers may be incorporated in substrates prior to planting. These are derived from a variety of plant and animal based sources. A small number of mined components may be added to substrate-incorporated fertilizers (i.e. limestone). These fertilizers will typically last for four to five weeks, and are often supplemented with liquid fertilizers for longer-term crops or species with high nitrogen requirements. Liquid fertilizers also tend to be more quickly plant available compared to substrate-incorporated fertilizers. With careful management, it is possible to grow container plants in organic production systems that are comparable to those produced in conventional production systems.