Organic Rooting Medium: Composting Woods

Fresh wood by-products, such as pine bark, wood chips, and coarse sawdust, are suitable for use in soilless organic rooting media mixes. Pine bark is a common ingredient in many peat moss-based formulations. However, these wood by-products contain tannins that are toxic to plants.

Therefore, composting is essential in order to remove this chemical as well as other unwanted oxidizable substances. The time and method of composting vary with the product and composting conditions.

Composting

For freshly gathered pine bark, wood chips or coarse sawdust, composting for at least a year under warm climatic conditions is required. Composting is best done outdoors, with periodic turning for equal exposure of the material to the climatic elements. Heat generated in the composting process may require cooling by watering during low rainfall periods, thereby keeping the compost pile from catching fire.

Composting also stabilizes the physio-chemical properties as the easily oxidizable components are eliminated, leaving a bio-stable organic structure. Following composting, the material will require milling and sieving in order to obtain a uniform, particle-sized product as well as removing the fines not decomposed during composting.

The useful particle size range is between 0.59 to 2.3 mm with the coarser material used for long-term plant production mixes, and the finer as an ingredient in seed germination and seedling formulations.

Composting is also a concentrating process for elements that may reach a plant toxic level. Elemental accumulation will depend on what elements were in the initial composting materials. For example, manganese is present in pine bark at fairly high concentrations, and with a relative increase due to composting, can potentially reach a toxic level.

Therefore, before use, the pine bark needs to be laboratory tested to determine its pH and elemental content. This test procedure is by water-equilibrium extraction (Jones, 1998), a test method not provided by all soil testing laboratories. Adjusting the water pH of pine bark to between 5.6 to 5.8 using dolomitic limestone is recommended.

Since pine bark, wood chips and sawdust are natural plant products, they will contain some content level of all of the essential plant nutrientelements, some in sufficient quantity to provide that needed by a plant rooted in it. Having the material assayed before use as described above, nutrient element supplementation can be done selectively by taking advantage of what elements already exist in sufficient plant-available quantities.

I have used composted milled pine bark as a rooting medium in both greenhouse and field growing of tomato, lettuce and other garden vegetables with great success. One of the interesting observations is that crop performance seems to improve with continued use, suggesting that stabilization of its physiochemical properties is a long-term composing-use process.

This method of growing works best using the sub-irrigation method. Before use, dolomitic limestone and 10-10-10 chemical fertilizer are mixed into the pine bark (Table 1). Similarly prepared pine bark as a rooting medium placed in pots for the growing of tomato and cucumber with the delivery of water by drip irrigation has been used with excellent results.

If a nutrient solution formulation is delivered by drip irrigation, the pine bark will require only the addition of dolomitic limestone (see Table 1). The water-equilibrium extraction test results will determine if any of the micronutrients should be included in the nutrient solution formulation.

An advantage to the use of these wood by-products is that they fit the designation as being natural organic substances that are bio-degradable. In addition, they have properties that make them suitable for inclusion in soilless organic mixes for a wide range of growing applications.

The essential requirement being that they are adequately composted and prepared for use by milling and sieving, and supplemented with the essential elements based on the water-equilibrium testing procedure.

Making pine bark soilless mix

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Table 1. Ingredients to make 1-cubic yard of pinebark soilless mix for growing of tomato and other garden vegetables using the sub-irrigation method.

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Amount Ingredient

9 bushels composted milled pinebark

1 lb. dolomitic limestone (agricultural grade)

1 lb. 10-10-10 commercial fertilizer

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Note: It is not necessary to add any of the micronutrients as there should be a sufficient amount for one crop cycle. Thoroughly mix the pinebark and dolomitic limestone, adding sufficient water to make the mixture moist (not wet). Let stand for 30 days with occasional mixing.

After 30 days, add 10-10-10 fertilizer, thoroughly mixing. The soilless mix is ready for use. Have the final mix tested using the water-equilibrium extraction method. If additional elements need to be added, follow the recommendation given.

Nitrogen supplementation may be necessary during mid-season based on a leaf analysis just as tomato fruits are being set. From 15 plants, collect the end leaflet from a maturing leaf at the same position as a setting fruit cluster. Submit to a Plant Analysis Laboratory and follow the assay recommendations given.