Media and Fertilizer Options for Growing Organic Greenhouse Vegetables

Organic greenhouse vegetablesAccording to Berger, an international supplier of growing media, greenhouse vegetable growers looking to make the switch from conventional to organic production will need to make adjustments in several areas, including growing media, fertilizer, pest control, plant growth regulators, irrigation, and environmental controls.

Berger offers a technical worksheet that outlines several of the factors growers must consider when it comes to growing media and fertilizers. Here are some of the highlights.


Certain materials are not permitted for organic production, including synthetic fertilizers and wetting agents. However, most primary components can be certified or are authorized for organic production. Even if the growing media appears to be similar to traditional substrates, it may have been modified to cater to organic requirements.

When incorporating fertilizer into growing media for organic production, the composition can be variable, and the availability of nutrients is dependent on several factors. It will also be difficult to determine true values for nitrogen, phosphorus, and potassium.

• In conventional production, growers can use a non-ionic wetting agent, which often has a high efficiency and a long shelf life. In organic production, they must use a Yucca-based wetting agent, which is not only less efficient, but often more expensive.

• When it comes to coir, traditional production involves a buffered coir that is washed and treated with calcium and magnesium to remove the potassium and sodium. In organic production, the washing of the coir will tend to rob the media of calcium and magnesium.

Composting is a critical part of plant nutrition in organic production. At Berger, compost is always produced with the same sources of nitrogen and carbon. Piles are monitored throughout the composting process for temperature, carbon-nitrogen ratio, and other factors. Once matured, the piles are covered to protect them from contaminants and to preserve the quality of the compost. They are then sieved with a starscreen system, and runoff water from compost production is treated to protect the environment.

The primary difference between chemical and organic fertilizers is the source of fertility and the need for microorganisms. Synthetic fertilizers are man-made inorganic compounds, usually derived from byproducts of the petroleum industry. Organic fertilizers are substances containing nutrients derived from the remains or byproducts of plants or animals. In addition, while chemical fertilizers can provide plants with an immediate supply of nutrients, in organic fertilizers, the release of the nutrients is highly variable and depends on the microbiological activity in the soil.

The availability of nutrients is largely based on biological activity in the mix, and this process takes time. If there isn’t an adequate microflora present in the growing media or if the conditions aren’t adequate for their activity, nutrients won’t be available regardless of how much organic fertilizer was added. The microflora can be replenished by inoculating the growing media with certain products such as compost tea.

A turn-key organic growing media may not be the best solution. Too little fertilizer means the crop will suffer from nutrient deficiencies, while too much fertilizer means the electrical conductivity levels may become too high, the material may start composting, and/or the biological activity could release ammonia and other organic compounds that may cause injury or reduce crop growth.

It is best to start supplemental fertilization with liquid fertilizers soon after transplanting and continue with regular feeding even if the mix has compost or worm castings. Keep in mind that certain organic liquid fertilizers are not compatible with all irrigation systems.

Plant growth regulators aren’t accepted for organic production. However, other techniques can be used, such as brushing, manipulation of day-night temperatures, avoiding overcrowding with proper spacing, and avoiding low light intensity.