Fig. 1. Coconut coir is the most widely used organic substrate in hydroponic crop production. Coir is often shipped in a compressed form, such as the brick shown here, to minimize freight costs.
Photo: Christopher J. Currey

For hydroponic crop production, stone wool and phenolic foam are two of the most popular substrates used in recirculating water systems for leafy crops such as nutrient-film technique (NFT) and deep-flow technique (DFT) systems, as well as high-wire slab or bag systems used for vining, fruiting crops. These are time-tested and production-proven substrates for hydroponic production systems. However, there is an increasing interest in using organic substrates for hydroponic production.

When talking about organic substrates, I am referring to “little o” organic, not “big O” organic. Certified Organic (the “big O” organic) refers to substrates that are Certified Organic according to the guidelines set forth by the United States Department of Agriculture. Alternatively, “little o” organic refers to products that are derived from once-living organisms, including plants. While some organic substrates may certainly be certified Organic, the two terms are not mutually inclusive, and the focus of this article is going to be on the broader class of organic substrates.

There are a few reasons why producers are interested in organic substrates for producing food crops hydroponically, but they all center around sustainability. One of the biggest reasons why organic substrates are increasing in popularity is due to disposal options, as nearly all organic substrates are able to be composted. Some are best-suited to industrial composting facilities, which may still require transportation offsite. Alternatively, those that are easily composted can be especially appealing for the possibility of onsite disposal and processing. Another aspect is related to the perception of sustainable manufacturing processes. While both inorganic and organic substrates require energy for processing, using plant-derived materials that are usually agricultural bi- or coproducts can appear less energy-intensive compared to other substrates.

The most widely recognized and used organic substrate in hydroponic production is coconut coir (Fig. 1). Coconut coir, coconut fiber or sometimes simply “coir” are different names for the same product. Coconut coir is the fibrous pith in the coconut husk, a coproduct of the coconut production industry. Once extracted from the husk, the coir is then further processed into different sizes from larger chunks or chips, to finer ground material. Grow bags or slabs are comprised of different combinations of these materials. Slabs comprised of all ground coir have the highest water-holding capacity, but the least air space. As coarser coir is included and the proportion increases, water-holding capacity decreases and air space increases. These different formulations provide opportunities to accommodate different crops and growing styles, as the requirement for wet and dry cycles varies among crops, but can also be used to “steer” between vegetative and reproductive (generative) growth of crops including tomato. While the crop time for vines such as tomato can be long — up to nine months — the high lignin content of the coir resists decomposition by microbes and the substrate maintains its physical properties.

In addition to coir used in slabs or bags for growing vine crops, coconut coir is also available in smaller block and plug forms for seedling propagation. The fiber is shredded and ground, similar to slabs. However, unlike the traditional plastic wrap used to stabilize slabs, the loose material for blocks and plugs is bound physically, with a net or paper wrapping, or chemically, with a biopolymer binder, to minimize the loss of substrate into the nutrient solution and hydroponic system.

Coconut coir products are frequently shipped in a compressed form to make shipping economical. In order to make them suitable for greenhouse use, they need to be decompressed prior to using them; this can be achieved by rehydrating them. Another step to preparing coconut coir substrates is to thoroughly leach them to get rid of unwanted sodium that may be present.

Sphagnum peat moss is another organic component that can be used as a substrate for hydroponic production, though there are a few caveats. First, like the loose fibers of coconut coir, ground peat moss needs to be stabilized with a physical or chemical binder to keep an excessive number of particles from entering into the recirculating nutrient solution. Second, unlike coconut coir, peat moss can degrade easier and, over time, the degrading peat will lose pore space, diminish air exchange to the roots and compress.

There are a few other types of organic substrate that are used for microgreen production, sold as mats or rolls of sheeting. Wood fiber is the most common organic component for making microgreen substrates. However, other products made from jute (Corchorus olitorius or C. capsularis) or industrial hemp (Cannabis sativa) or are also available. These are single-use products and are disposed of or composted once the microgreens are harvested.

There are a variety of inorganic and organic substrates that are suitable for hydroponic crop production, and both types can be used with great success. If you are interested in trying to minimize the end-of-life environmental impacts of your growing substrate, consider some of the organic materials currently available on the market.

Christopher (ccurrey@iastate.edu) is an Assistant Professor in the Department of Horticulture at Iowa State University.