We're a small CSA farm just starting our third year of production. Our soil tests last year indicate a shortage of boron, which showed itself in some hollow stems of our broccoli. What would you recommend for incorporating more boron into our soil?

Camela Decaire



We asked The Rodale institute field researcher Dave Wilson to tackle this one. As always, Dave had a lot to say:

Boron soil nutrition is influenced by many factors. The most important are soil texture, organic matter content, and pH. Available boron is readily leached out of the soil by excessive rainfall or irrigation. This is especially true of course-textured (sandy) soils. Because less leaching occurs in fine-textured soils, silt and clay soils are not usually as boron deficient as are sandy soils.

But, boron deficiencies occur over a much wider range of soils and crops than do deficiencies of any other micronutrient element and, with some crops, there can be a close range of boron deficiency and boron toxicity (with over-application of boron leading to toxicity).

Here are the best ways to ensure more boron without overdoing it:

Soil organic matter: The borate ion bonds to organic matter in the soil. Organic matter is a major storehouse for boron and it provides one of the primary sources of available boron for crop use. Boron is released from organic matter by microbial action. Crops grown in soils low in organic matter content usually need more frequent boron application.

Manures: Manures are a source of the plant needed micronutrients including zinc, boron, iron and copper. But boron in manure is usually very low, ranging from 0.02 to 0.12 pounds per ton. At the highest concentration, a rate of 20 tons per acre would just barely meet the boron needs where boron deficiencies are known.

Compost is a great source of organic matter and, depending on the feedstocks in the compost, it can be a source of boron. Municipal leaves are also a source of these micronutrients, including boron. A compost mix of animal manures and leaf litter is an excellent soil amendment and source of both macro and micronutrients. Compost application and subsequent build-up of the soil organic matter is the "long-term" approach to remedy boron deficiencies in the soil. The increased organic matter level will help "tie-up" and retain applied boron and keep it from leaching from the soil as easily.

Cover crops for nutrient recycling: Growing cover crops in the vegetable beds over winter will help to capture and recycle some of the soluble boron that otherwise would be leached away with the fall, winter and early spring rains and snow melt. The boron will be tied up in the plant tissue of the cover crop, conserved over winter and made available again in spring when the cover crop is turned under and soil microbes breakdown the material.

Kelp Meal (dehydrated chopped seaweed) is another source of micronutrients including Boron. Liquid fish and liquid kelp are also sources of soluble nutrients, including boron, that can be used as a foliar supplemental spray. The boron concentration in these products varies and would have to be verified.

Compost tea is another source of soluble boron. In 2004, we had our compost tea analyzed for macro and micro nutrients: It contained 0.36 ppm of boron. Since plants can be fed through their leaves, this can be another source of boron through foliar application of compost tea. The concentration of boron in compost tea will depend upon the feedstock's used in the compost and the other recipe ingredients used to make the compost tea. The compost tea can also be fortified with liquid boron if a boron deficiency is present. Typically, foliar applied nutrients have the benefit of being anywhere from 4 to 30 times more efficient than soil applications.

Boron applications: In the short term approach, boron applications may be needed.

NOTE: OMRI (the Organic Materials Review Institute) List does list boron-based products for use in organic production, however they are listed in the "Restricted Use" category. If a CSA is "certified organic," then the farmer needs to check with his or her certification agency to see what the restrictions are on the use of the specific boron product. Typically you would have to have a soil test and/or a plant-tissue test from your crop to document low levels of the micronutrient. Once this "boron deficiency" is established, the certification organization may authorize the use of boron as a broadcasted material before planting and also as a foliar spray to be used during the growing season. Some of the OMRI approved boron products (Under the “Restricted Use” category) are: Biomin (Boron 3 percent), Fertibor, Granubor, Phyto-Plus (boron 3 percent), Solubor and Solubor DF.

The vegetable crops that need high boron levels are: asparagus, broccoli, cabbage, cauliflower, celery, eggplant, horseradish, peppers, squash, sweet corn and tomatoes. In addition, beets, carrots, rutabaga, sweet potatoes and turnips also respond to boron. Because a continuous supply of boron is required by plants from germination to maturity, it is often necessary to split soil applications of boron or to apply boron as a foliar spray.

In a Boron deficient soil where the soil test analysis levels are below 0.70 ppm, depending on the particular crop and the exact soil test level, recommended broadcast rates of boron range from 0.5 to 5 lbs per acre, broadcast before planting. For instance, at that soil test level for broccoli, 1.5 to 3 pounds of boron is recommended broadcast and foliar spray.

WARNING: Boron can reduce germination when it comes in direct contact with the seed, therefore, broadcast applications instead of “in-row" application treatments are recommended. The broadcast application should be made one to two weeks before seeding. If the soil boron level is very low, it is recommended to broadcast a boron source before planting these crops and then use a foliar spray applied during the growing season. A single foliar application of boron should not exceed 0.25 to 0.5 pounds per acre.

When crops are grown with irrigation, it is important to split boron applications to compensate for leaching looses of this micronutrient (and boron can be applied with the irrigation water).



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