J. Benton Jones, Jr.
Dr. J. Benton Jones Jr. has 50 years of experience growing plants hydroponically. He is an Emeritus Professor at the University of Georgia, Athens and has authored eight books and written articles for magazines that deal with hydroponic issues. He currently has his own consulting company, Grosystems, Inc. Dr. Jones currently lives in Anderson, South Carolina.
Although fresh wood by-products like sawdust and pinebark are suitable for use in soilless organic rooting media mixes, most types contain elements that are toxic to plants. Therefore, composting wood by-products is essential. Dr. J. Benton Jones Jr. explains...
Fresh wood by-products, such as pinebark, wood chips and course sawdust, are suitable for use in soilless organic rooting media mixes. Pinebark is a common ingredient in many peatmoss-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 varies with the product and composting conditions. For freshly gathered pinebark, wood chips or course 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 courser 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 pinebark at fairly high concentrations, and with a relative increase due to composting, can potentially reach a toxic level. Therefore, before use, the pinebark 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 pinebark to between 5.6 to 5.8 using dolomitic limestone is recommended.
Since pinebark, wood chips and sawdust are natural plant products, they will contain some content level of all of the essential plant nutrient elements, 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 pinebark as a rooting medium in both greenhouse and field growing of tomato, lettuce and other garden vegetables with great success (Jones, 2005). One of the interesting observations is that crop performance seems to improve with continued use, suggesting that stabilization of its physio-chemical properties is a long-term composing-use process. This method of growing works best using the sub-irrigation method (Jones, 2011, pp. 33-35). Before use, dolomitic limestone and 10-10-10 chemical fertilizer are mixed into the pinebark (Table 1). Similarly prepared pinebark as a rooting medium placed in pots for the growing of tomato and cucumber with delivery of water by drip irrigation has been used with excellent results. If a nutrient solution formulation is delivered by drip irrigation, the pinebark 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.
References:
Jones, Jr., J. Benton. 2001. Laboratory Guide for Conducting Soil Tests and Plant Analyses. CRC Press, Inc., Boca Raton, FL.
Jones, Jr., J. Benton. 2005. Hydroponics: A Practical Guide for the Soilless Grower. 2nd Edition. CRC Press, Inc. Boca Raton, FL.
Jones, Jr., J. Benton. 2011. Hydroponic Handbook: How hydroponic growing systems work. GroSystems, Inc., Anderson, SC.
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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 in 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.
Organic products are not the only way to go green in the garden. Making your system more efficient, while challenging, is one of these ways…
Today, the challenge for the hydroponic grower is to determine what can be done to make his growing system more efficient in its use of items needed to grow successfully– that is, to make the growing procedure conform to green principles. It’s not an easy task, as little is being researched today to assist growers in ways to improve the efficiency of commonly used hydroponic growing systems.
Great care is needed in the formulation and use of nutrient solutions so the elements applied are better utilized by the growing crop. For those growing systems requiring a rooting medium, minimizing the accumulation of salts in the rooting medium and reducing the requirement to periodically leach with water are green issues. What are needed are nutrient solution formulations that fit the plant species and stage of growth, as well as having use factors that result in full utilization.
Some growers add enough solution to create an effluent, which serves as a leaching of the rooting medium and partially removes that which accumulated from the previous irrigations. However, the effluent must be collected and disposed of. At one time, I was advising a greenhouse tomato grower who was using his spent nutrient solution as irrigation water for his commercial vegetable garden. It seemed like a win-win situation. But he soon noticed that the elements in the spent nutrient solution do not match the nutrient element needs of the vegetable plants Continued applications could significantly alter the fertility status of the soil, creating an imbalance among the essential plant nutrient elements. After one season, the hydroponic grower looked for another means of disposing the spent nutrient solution.
There is a way to reuse this nutrient solution effectively, however: reconstitution. Reconstitution of a nutrient solution is little practiced, as testing is required to determine its pH and elemental content (and thus what will be needed to adjust the pH and what quantity of reagents will be needed to restore the elemental content to that of the original). For reconstitution, the nutrient solution will require filtering to remove suspended debris and sterilization in order to kill microbial organisms. Nonetheless, reconstitution can be cost-effective. It saves in water and reagents, as well as the cost associated with disposal if a spent nutrient solution is identified as being a hazardous waste.
Aside from nutrient solution, what is to be done with the rooting medium at the end of the growing period? Can it be recycled or put to some other use? A greenhouse tomato grower switched his rooting medium from perlite to composted milled pinebark. At the end of the growing season, a local nursery purchased the pinebark for use as a potting soil for bedding and woody ornamental plants. The grower recovered the initial cost for the pinebark and reagents added during the growing season – a very profitable means of disposal. I have used spent perlite as a soil amendment in my vegetable garden, as it adds plant nutrient elements and—with continued application—is making my clay textured soil more friable.
Most hydroponic growing systems also require a reliable source of electrical power. Each time an electrically powered pump comes on to deliver a nutrient solution to the rooting medium, the cost is based on the time required to deliver the nutrient solution. Some growers use a set time method for nutrient solution delivery whether the plants have need for water or not. Having a means of determining the water needs of the plant and applying only when needed can result in significant savings in electrical power, as well as reduced treatment requirements of the nutrient solution. Placing a water-sensing device in the rooting medium, or using a program that predicts plant water use based on measured energy inputs, makes sure water is applied only when needed.
Growers need to be aware of the real costs associated with disease and insect control—in particular, that prevention is more cost-effective (green) than having to treat for a disease or insect outbreak. It is matter of knowing what disease organisms and insects are likely to appear, and then following those procedures that will prevent unwanted outbreaks.
In general, most hydroponic growing systems are inefficient in their use of water and reagents, and have fairly high electrical power requirements. Making you current hydroponic growing systems green is a significant challenge with no easy answers. However, as quality water becomes scare and costly, significant changes will have to be made in how water is used to efficiently deliver the essential plant nutrient elements to the rooting medium for root absorption with a minimum of lost to the environment. The ideal would be no loss– a totally green method for growing—and it’s a goal worth pursuing.
Knowing the elemental content of your organic fertilizer is imperative to plant health. However, as J. Benton Jones, Jr. points out, this information isn’t as easy to discover as one would think…
