By John Harapiak
Canola on sandy, northern soils is best candidate.
By John Harapiak
here has been a great deal of debate concerning the validity of some of the
recommendations made for the application of micronutrients on prairie crops.
In many situations, the research and extension based communities suggest that
the need for applying these relatively expensive micronutrients has been 'over
sold' and quite often, this practice is simply not profitable for the growers
involved. However, to be fair, there are crop consultants and growers who might
disagree with this assessment.
|Boron deficiencies in Canola typically occur in quite small patches.
The soil in this field is quite sandy and was formerly covered with pine
trees. Photo Courtesy of Lyle Cowell, SWP.
Traditional research unable to confirm boron responses
Over the years, there has been a great deal of agronomic debate about the need
for applying the micronutrient boron (B) within western Canada. In fact, Rigas
Karamanos of Westco has published summary reports based on all of the traditional,
replicated small plot trials that have been carried out in this region. He correctly
concludes that, based on the results he collected from all of these science
based research experiments, there is little, if any evidence to support the
widespread usage of B fertilization.
Recognizing limitations of traditional trials
However, a new landscape-based approach to assessing the need for application
of B may be lending an improved understanding of the conditions that might trigger
a response to an application of B, especially for canola production. In these
trials, the response to B application is separately assessed in each landscape
segment. Unfortunately, the size of the B deficient area in many fields is usually
smaller than the amount of area required for conducting a traditional field
Conditions favouring a B deficiency
A wide range of soil factors, including moisture stress, soil pH, soil texture
and soil organic matter content can influence the B supply that is available
to crops. Quite naturally, the soil-based characteristics can vary significantly
within relatively short distances, especially in some of the formerly forested
soils. In these areas, the spatial variability can also be further influenced
by surface topography, especially in sandy, glacially modified soils.
Soil tests are unreliable for identifying B deficient
Problem: Soil tests for B are not that reliable, at least partially due to the
fact that it is difficult to collect a representative soil sample. This is because
of the high degree of spatial soil variability that exists in soils that tend
to be deficient in boron (see 'Conditions favouring boron deficiency'). While
B deficiencies have been confirmed in more than 70 countries, many of which
are located within more tropical regions, soil test information from these areas
may not be useful. These soils are typically low in organic matter content.
The prairie soils are relatively high in organic matter content, a soil component
that adsorbs B and is critical to retaining the B within the rooting zone.
Sandy, formerly forested soils most vulnerable
the prairie region, soil boron supplies tend to be lower in sandy, formerly
forested soils. Targetting for higher canola yields by increasing rates of N
fertilizer application may stress soil B supply in marginal soils that failed
to respond to B in the past. Indications of probable responses of canola to
B applications has been reported in the Peace River region, in the area of sandier
soils in the Wainwright-Vermilion region, on the sandier soil phases of soils
in the Shellbrook-Prince Albert-Choiceland-Carrot River regions as well as within
the Swan River Valley.
Over-application of S can increase B stress
In a canola S fertilizer demo trial located on a sandy soil near Vermilion,
application of higher S rates resulted in purple coloured leaves, in which the
midrib on the underside was broken. This feature can be indicative of a B deficiency
in canola. Excessive application of S fertilizer has been shown to decrease
the B content of plants. It appears that the presence of an excess supply of
sulphate ions at the root surface can suppress plant uptake of the less abundant
borate ions. There is also evidence that some canola varieties are more sensitive
to marginal levels of B than are others.
Options for application of boron
Seedrow or near seedrow applied B can damage root development, the upper safe
limits for which have not been well established; therefore, only low rates should
be applied in this manner. Broadcast and incorporated B tends to be the most
common method of placement, although band applications have been most effective
in some trials. While foliar applications tend to be somewhat less effective,
they represent the only option if a crop deficiency is discovered after the
crop is growing.
Consider utilizing manure to supply boron
The B content of animal manures ranges between 0.03 and 0.05 pounds per ton
of manure, therefore manure can provide an alternative to the application of
commercial B fertilizers. Application of manure could be a particularly effective
option for spot treatment of the relatively small areas of a field that show
a potential for being deficient in boron. Typically, these relatively small
patches of soil are sandy in nature and low in organic matter content. The addition
of manure will increase the organic content of the soils and thus provide a
mechanism (i.e. adsorption/ chelation) for reducing the risk of B leaching.
Tissue testing for boron
Plant boron concentrations are not uniformly distributed throughout the plant,
with the highest concentrations occurring within the leaves. For B tissue analysis,
it is suggested that the youngest, fully expanded leaf be sampled. Older leaves
tend to have higher accumulations of B, especially along the outer perimeter
of the leaves. For this reason, older leaves should not be sampled for tissue
Boron deficiency symptoms
The growth of new cells in the growing point of canola plants can be adversely
affected by a boron deficiency. The first visual symptom is a cessation of terminal
bud growth, followed by the death of young leaves. Additional symptoms of a
B deficiency include the leaves becoming pale green, while losing more colour
at the base than the leaf tip. A deficiency of B can also manifest itself by
the thickening, twisting, wilting and purpling of the leaf tissue. Late season
symptoms include impaired floret sterility, reduced seed set and yield loss
(see Boron deficiency symptoms).
Responses in canola
It should be noted that canola responses to foliar B applications have been
reported in fields that had not demonstrated any deficiency symptoms. These
may be associated with the occurrence of drought stress at the flowering or
seed set growth stage. There have also been a few grower reports that a foliar
application of B appeared to benefit a canola crop recovering from hail induced
damage. This might be in keeping with the importance that B plays in the development
of new plant tissue (e.g. leaves, flowers and seedpods).
As growers continue to crop their fields, micronutrient deficiencies will become
more common. However, the vast majority of prairie soils still contain adequate
levels of micronutrients and deficiencies are the exception rather than the
norm. By all means, monitor your fields for potential signs of micronutrient
problems. However, only include micronutrients in your fertilizer program when
you have clear evidence of a need and an opportunity for profit. Using test
strips in combination with a yield monitor equipped combine is the best approach
for assessing the benefits of using micronutrients on a specific farm.
Conditions favouring boron deficiency
- Soils with low organic matter content.
- Sandy soils that are acidic.
- Soils with a pH in the range of 8.5.
- Soils which have been over-limed.
- Drought/moisture stress when moisture flow to the roots is reduced
as is root extension.
- Higher rainfall because boron is highly mobile within the soil and
subject to leaching.
Boron deficiency symptoms
- Slowed and stunted terminal growth.
- Death of terminal buds.
- Development of 'rosette' effect.
- Effects often 'spotty' within fields.
- Leaves more chlorotic at the base than at the tip.
- Leaves tend to thicken and curl.
- Flower buds fail to form or are misshapen.
- Stems may crack as well as midrib of leaf.
- Poor pollination and seed set.
- Shortened internodes.
- Pink to whitish-yellow spots at leaf base.
John Harapiak has more than 40 years of western Canadian based
fertilizer related experience. He continues to contribute stories to Top
Crop Manager and he can be contacted by e-mail at: firstname.lastname@example.org.