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?
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
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
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
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