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
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).
