Micronutrients

Micronutrients are those nutrients required in extremely small quantities (less than 100 parts per million in plant dry weight). Unfortunately, the basic functions of micronutrients are less understood than macronutrients. Also, there is very limited knowledge about the forms and mechanisms of micronutrient transport in the xylem and phloem. Micronutrient deficiencies in canola are much less common than macronutrient deficiencies. However, canola yields can be severely depressed when micronutrient deficiency occurs. This section will review the various micronutrients and canola responses.

Boron (B)

Boron is a micronutrient that occasionally limits canola yield in western and eastern Canada. Unfortunately, current soil test methods using hot-water boron extraction do not consistently predict economic responses to boron fertilizer in canola 1. Boron deficiency is rare but when it occurs, it usually is on sandy areas of fields. This may be due to aluminum toxicity to roots, which can occur in acidic sandy soils 2. Of the known micronutrient deficiencies, boron deficiency is the most widespread globally. However, boron deficiency is rare in western Canada.

Role of boron in canola plants

Boron’s role in plant nutrition is the least understood of all the nutrients. Boron is not an enzyme constituent nor does it seem to directly affect enzyme activities. Most of our understanding about boron arises from symptoms observed during deficiency. Possible roles for boron include:

  • sugar transport and carbohydrate metabolism
  • cell wall synthesis and structure
  • RNA metabolism
  • respiration
  • hormone metabolism
  • stomatal regulation
  • membrane function

Cell walls are dramatically affected by boron deficiency. This shows up as cracked, hollow or corky stems. The cell wall diameter and proportion of plant dry weight increase under B deficiency. Most plant boron is complexed with organic compounds in the cell walls, apparently serving a nonspecific structural role.

One of the first plant responses to induced boron deficiency is decreased root elongation, however, there is a lack of understanding how this occurs. Boron deficiency also restricts pollen tube growth. This is why boron demand is higher during the reproductive stage than vegetative stage. Boron also affects fertilization and seed set by increasing pollen production and viability.

Boron supply from soil and canola uptake

Daily boron uptake is greatest around 50 per cent flowering 3. Boron is classified as immobile within plants but mobility through phloem has been measured within canola plants and other species that transport sucrose as their primary photoassimilate 4.

Boron transporters within plants have been found to regulate boron uptake based on boron conditions. Plants can sense internal and external boron conditions and regulate deficiency or toxicity problems by regulating boron transporters to maintain boron balance within plant tissue 5. In the future, identification of key genes and processes involved in boron modulation within the plant may allow for the development of cultivars more tolerant of boron stress.

Boron uptake is affected by other nutrients. High levels of calcium and potassium have been shown to increase boron deficiency symptoms but these antagonisms are not well understood.

There is genetic variation for boron efficiency in canola, with reports from China showing one major dominant gene 6 as well as multi-gene effects. Genetic differences in boron efficiency were related to differences in root uptake or plant utilization. Research from Pakistan reported that Brassica napus was more sensitive to boron deficiency than mustard (Brassica juncea), but needed relatively less boron fertilizer for optimum grain yield. This area merits further research with western Canadian cultivars and conditions as many of these findings were observed on soils with less than 0.8 per cent organic matter.

Canola response to boron

On western Canadian Prairie soils, no significant yield increases were seen in 22 trials in seven separate experiments with soil- or foliar-applied boron. In these experiments, boron in canola tissue was correlated to the rate of boron applied, but no yield response was observed. In addition, the hot-water boron soil test extraction method was not an effective diagnostic tool for determining the boron status of soils 1.

On sandy soils in northeastern Saskatchewan, boron fertilizer did not provide a consistent yield increase on soils thought to be deficient in boron. The response to boron depended on site, year, cultivar and boron fertilizer rate, time and method of application, indicating that boron deficiency in a field probably occurs in isolated patches. Boron fertilization had no impact on the amount of severity of sclerotinia stem rot, blackleg or alternaria pod spot 7.

A 40-site trial from Western Canada found that canola did not respond to boron application, even on soils containing up to 0.15 milligrams per kilogram (parts per million) of boron, which is considered low. Research by Agriculture and Agri-Food Canada at Melfort, Saskatchewan did not find a boron response on four soils testing low in boron, and one soil was similar to the responsive site in a previous study. Two years of foliar and soil applied boron trials on a sandy Black soil in central Alberta testing very low in boron did not find any yield response with either B. rapa or B. napus canola. The Battle River Research Group did not find a boron response after two years of trials in central Alberta. Research in Washington and Idaho on three soils testing low in boron did not find a canola yield response to boron fertilizer either. Irrigated canola research conducted by Alberta Agriculture Food and Rural Development near Lethbridge did not find a yield response to micronutrients, including boron, over six site-years.

However, there have been cases where boron fertilizer did show a response under controlled conditions. Agriculture and Agri-Food Canada potted studies with B. rapa rapeseed on Alberta Gray Wooded soils showed that boron deficiency symptoms and poor seed set were alleviated with added boron. Under field conditions, the Canola Council of Canada recorded a significant yield response to boron at its Crop Production Centre site near St. Claude, Manitoba in 2000.

Boron fertilization of canola has not consistently improved seed yield, kernel weight, protein or oil content. Where boron deficiency does occur in western Canada, it probably is in small field patches. A 1990s study in northeastern Saskatchewan, an area where a reduction in canola yields was thought to have been due to boron deficiency, found no benefit from boron fertilizer, regardless of seed row or foliar application methods. Even after conducting soil and plant tissue analyses this study concluded it was still difficult to predict a profitable yield increase from boron fertilization.

A northern Quebec study of boron on canola did find an economic response from foliar applications of one kilogram per hectare (one pound per acre) of boron in fields where boron deficiency had previously been reported in barley.

Foliar boron applied at flowering has been tested on canola in Ontario as a way to prevent blossom blast during summer heat waves. Four years of grower field studies from 2008-11 found inconclusive results. In the trials, foliar boron applied at flowering improved yields marginally by three per cent on average, but rarely resulted in an increase in return.

Ontario research noted the greatest yield improvement occurred in 2010, a year with higher temperatures at flowering than in 2008 and 2009 when temperatures were cooler than normal. In 2011 conditions were cooler than 2010 but extremely dry.

Boron deficiency is more likely to occur in:

  • Sandy soils with low organic matter. Boron tends to leach in these soils. In general, sandy soils and soils with less than one per cent organic matter have lower micronutrient availability. Grey Luvisols have also shown a higher likelihood of boron deficiency.
  • High pH soils (8.0 or higher). Generally, boron becomes less available as pH increases above 6.3 to 6.5. At higher pH, the borate anion is likely adsorbed to clay and organic particles.
  • Drought. Under drought conditions, boron deficiency can occur due to reduced mass flow to roots as well as polymerization of boric acid. Boron uptake rates increase as the level in the soil water increases. Boron moves through the soil with mass flow, not diffusion, so a rain event can alleviate most boron deficiencies. Also, soil organic matter is the primary source of boron in western Canadian soils and under drought, mineralization (release of boron from soil organic matter) is slowed.
  • Saturated fields. Under high rainfall conditions boron can be leached in sandy textured soils.
  • Fields with high levels of calcium and potassium. These antagonisms have been shown to increase boron deficiency symptoms, but they are not well understood.

Identifying deficiencies

Canola is more sensitive to low soil boron during the reproductive stage than the vegetative stage, so if deficiency symptoms show up at the vegetative stage this is a sign of potential serious deficiency, which could have an effect on yield.

Boron deficiency symptoms in canola first appear in new growth due to the intermediate mobility. Symptoms range from:

  • New leaves are stunted and may roll up along the length of the leaves around the mid vein. Leaves may look generally deformed and be rough skinned leaves with torn margins.
  • Interveinal chlorosis. Veins are green but the rest of the leaf is light green. Chlorosis can look like yellow to brown spots in the interveinal areas of leaves.
  • Red to brown-purple coloured new leaves.
  • Early leaf drop.
  • Shortened stems.
  • Cell walls are dramatically affected by boron deficiency. This shows up as cracked, hollow or corky stems. The cell wall diameter and proportion of plant dry weight increases under boron deficiency. Most plant boron is complexed with organic compounds in the cell walls, apparently serving a nonspecific structural role.
  • Long and unproductive flowering period, flower sterility and poor pod set. Boron is involved in pollen tube fertilization, and with boron deficiency, pods may not form. The plant keeps flowering and flowering producing new bud clusters from leaf axils, trying to get fertilization and pod set to occur.
  • Short, stunted root hairs and poor rooting. One of the first plant responses to induced B deficiency is decreased root elongation, however, there is a lack of understanding how this occurs.

To identify a boron deficiency:

  • Ensure that poor crop growth is not the result of a macronutrient deficiency (visual symptoms can easily be confused with sulphur deficiency symptoms), drought, salinity, disease or insect problem, herbicide injury or some physiological problem.
  • Check around to see whether boron deficiency has been identified in a particular crop or soil type in the area.
  • Examine the affected crop for symptoms described above.
  • Take separate soil samples from both the affected and unaffected areas for complete analysis, including micronutrients. Unfortunately, even with a soil test result that shows deficiency, there is not a consistent indicator to predict profitable canola yield response to boron fertilizer. The problem with all boron soil tests is that alone they are a weak indicator of boron availability. Most boron supplied to the crop comes from the breakdown of organic matter, a soil test has to consider soil organic matter percentage, and given that boron is very mobile in the soil, you really need soil texture as well. Boron, like nitrogen and sulphur, is subject to leaching.
  • Send plant tissue samples from both the affected and unaffected areas for complete analysis, including micronutrient levels. Plant analysis at early flowering may help to identify boron deficiency. The youngest open leaf is shown to be the most reliable tissue to identify boron deficiency in Brassica napus, but be sure to follow the labs requirement for plant parts submitted, how many to submit, and how to choose a proper sample.
  • If all indications point to a micronutrient deficiency, apply the micronutrient to a specific, clearly marked out affected area of land to observe results in subsequent seasons.

Even after observing boron deficiency symptoms and conducting soil and plant tissue analyses, prediction of a profitable yield response is difficult. Therefore, in situations of suspected deficiency, apply boron fertilizer to a small affected area of the field in a carefully marked test strip. Visual observations combined with yield measurements from the treated and untreated areas should help determine if a measurable response occurred. If a positive response is measured, boron fertilizer could be applied in future canola crops on these areas.

Fertilizer sources and methods of application

The most common boron product is sodium borate, broadcast and incorporated in the spring at rates of 0.5–1.5 pounds per acre or foliar applied at 0.3 to 0.5 pounds per acre. Other methods of applying boron include:

  • Broadcast-incorporated. This is the safest method, as boron can be toxic to canola seedlings. Application rates should not exceed 1.7 kilograms per hectare (1.5 pounds per acre) on soils with a pH less than 6.5 to avoid boron toxicity problems.
  • Banding. Do not place more than 1.7 kilograms per hectare (1.5 pounds per acre) in close proximity to the seed row.
  • Foliar boron fertilization appears to be effective up to the early flowering stage. Ensure foliar applications do not exceed 0.3 kilograms per hectare (0.3 pounds per acre) to avoid toxicity problems. For all applications, extreme care must be taken to apply the correct amount uniformly to avoid toxicity.
  • Seed placement is not recommended as concentrated boron can be toxic to seedlings. Rates of sodium borate that exceed one pound per acre in the seed row can kill canola seedlings.

Copper (Cu)

Knowledge about copper fertility of western Canadian soils has increased over the past three decades. Previously, copper deficiency was thought to be limited to organic or peat soils. More recent research has identified that Black, transitional Gray-Black and sandy Dark Brown soils may be copper deficient for cereal production. Although copper deficiency and fertilizer response has been documented with cereals under field conditions, canola has only shown some response when soil test showed very low levels of copper at less than 0.2 parts per million 8.

Role of copper in canola plants

Copper is a transition element that forms stable complexes in the plant and soil, and is capable of electron transfer activites (energy processes). Copper’s role in plant functions is mainly as a reactive constituent of enzymes that catalyze oxidation-reduction reactions. Some examples of copper containing enzymes include:

  • plastocyanin (needed for energy capture through photosynthesis)
  • superoxide dismutase (needed for detoxification of oxygen radicals)
  • many different types of oxidases (enzymes that degrade or change compounds together with oxygen)

One of the phenol oxidases is involved with lignin synthesis.

Due to copper’s role in photosynthesis, deficiency leads to low carbohydrates levels, at least during the vegetative stage. The low carbohydrate content in copper-deficient plants contributes to impaired pollen formation and fertilization. The reduced lignification in copper-deficient plants also affects pollen fertility since lignification of anthers is needed to release pollen.

Copper supply from the soil and canola uptake  

Copper is a metallic nutrient that originates from minerals in the soil. The total copper content in western Canada soils usually falls in the range of five to 50 parts per million. Approximately 1/2 to 1/4 of the total copper exists within minerals and is unavailable to plants. Copper associated with oxides and organic matter has been found to be an important source of plant available copper, probably by replenishment of dissolved and exchangeable copper. The oxide and organic fractions increase with the clay content, which explains why copper deficiency is more likely on sandier textures. Exchangeable copper ranges from 0.1 to 10 per cent of total soil copper. Only very small amounts of copper exist as soluble Cu+2 in the soil water. Research on the Prairies has found that DTPA extractable copper is highly variable across cultivated and native fields. This means that larger numbers of soil samples are needed to obtain a precise estimate of the true soil average.

Very little information exists on the mechanisms of copper uptake by plant roots. The driving force for copper uptake is the electrical chemical gradient across the root cell membranes. Since free copper levels inside the cell are kept low to avoid harmful reactions, and the membranes have a large negative potential, this creates a large force for copper uptake. Therefore, there is no need for active copper uptake systems. It has been suggested that calcium channels likely also allow passage of other ions such as copper (Cu+2).

Within several weeks of emergence, daily copper uptake rapidly rises, and peaks near mid-flowering 9.

Copper remobilization is much higher in old leaves and is related to nitrogen remobilization. Despite the intermediate mobility of copper, deficiency symptoms in sensitive crops during the vegetative stage first appear in new growth. However, canola does not display strong copper deficiency symptoms. Potted experiments with extreme copper deficiency have reported canola symptoms of:

  • interveinal chlorosis shortly after emergence
  • larger than normal leaves
  • wilting leaves
  • delayed flowering with a shortened flowering stem

Evidence suggests that canola growth may be affected by imbalances between copper, molybdenum (Mo) and manganese (Mn) levels. Manganese: copper ratios (DTPA extractable) greater than 15 may result in a copper deficiency. Molybdenum also may antagonize copper. However, the molybdenum antagonism is in turn affected by sulphur levels. Sulphur additions were found to lower molybdenum contents in canola plants, reducing molybdenum antagonism with copper, and copper deficiency was alleviated without adding copper fertilizer.

Canola response to copper

Black, transitional Gray-Black and Dark Brown soils may be copper-deficient for cereal production. Canola is not as sensitive as cereals to copper shortages, but canola can still show a response to copper application in deficient soils.

Response to copper applications are generally low, unless soil tests show extremely low copper levels (e.g. below 0.2 milligrams per kilogram or parts per million). On those soils, cereals should be showing clear benefits from copper applications. Canola has shown to be more tolerant of soil copper deficiencies than wheat, barley, oats and flax. A compilation of research data from Saskatchewan and Alberta on mineral soil suggests that 0.30 milligrams per kilogram (parts per million) diethylenetriaminepentaacetate (DTPA) extractable copper may be the critical level for canola. Since the critical copper level is much lower for canola than cereals, copper fertilization programs for deficient soils should focus on application to the cereal rotation phases.

Copper deficiency is more likely to occur in:

  • Sandier soils with low organic matter. Copper oxide and organic fractions increase with the clay content, which explains why copper deficiency is more likely on sandier textures.
  • Organic or peaty soils. Soils that have very high levels of organic matter (greater than 30 per cent organic matter to a depth of 30 centimetres) often have low micronutrient availability.
  • Peat soils with a manganese:copper ratio (DTPA extractable) greater than 15.
  • Molybdenum also may antagonize copper. However, the molybdenum antagonism is in turn affected by sulphur levels. Sulphur additions were found to lower molybdenum contents in canola plants, reducing molybdenum antagonism with copper, and copper deficiency was alleviated without adding copper fertilizer.

Identifying copper deficiencies

Canola does not display strong copper deficiency symptoms. Potted experiments with extreme copper deficiency have reported canola symptoms of:

  • interveinal chlorosis shortly after emergence
  • larger than normal leaves
  • wilting leaves
  • delayed flowering with a shortened flowering stem

Due to copper’s role in photosynthesis, deficiency leads to low carbohydrate levels, at least during the vegetative stage. The low carbohydrate content in copper-deficient plants contributes to impaired pollen formation and fertilization. The reduced lignification in copper-deficient plants also affects pollen fertility since lignification of anthers is needed to release pollen.

Despite the intermediate mobility of copper, deficiency symptoms in sensitive crops during the vegetative stage first appear in new growth.

To diagnose a copper deficiency:

  • Ensure that poor crop growth is not the result of a macronutrient deficiency, drought, salinity, disease or insect problem, herbicide injury or some physiological problem.
  • Find out if a micronutrient deficiency has been identified before in a particular crop or soil type in the area. If copper fertilizer provides an economic benefit for cereals in the area, then it is more likely to also help canola in the area. If cereals are not deficient, canola will not be either.
  • Examine the affected crop for specific micronutrient deficiency symptoms.
  • Take separate soil samples from both the affected and unaffected areas for complete analysis, including micronutrients. Soil test methods (DTPA extraction) show that copper is highly variable across fields. This means that larger numbers of soil samples are needed to obtain a precise estimate of the true soil average.
  • Send plant tissue samples from both the affected and unaffected areas for complete analysis that includes tests for micronutrient levels.
  • If all indications point to a copper deficiency, apply the micronutrient to a specific, clearly marked out affected area of land to observe results in subsequent seasons.

Fertilizer sources and methods of application

Copper sulphate or copper oxide can be broadcast and incorporated at rates of 3.4 to eight kilograms per hectare (three to seven pounds per acre) of copper in the form of copper sulphate is recommended for deficient mineral soils. Copper oxide at similar rates is also effective, but not in the year of application. Increase broadcast and incorporated rates of copper fertilizer to 11-17 kilograms per hectare (10-15 pounds per acre) when applied on peat soils, but be cognizant of the manganese:copper interaction that can result in manganese deficiency. Soil application rates should be effective for up to 10 years.

Chelated forms of copper can be effective in the year of application, but there is no residual benefit (see below for more on chelates). Chelated copper can be broadcast and incorporated at 0.6 kilograms per hectare (0.5 pounds per acre) of copper, seed placed at 0.3-0.6 kilograms per hectare (0.25-0.5 pounds per acre) or foliar applied at 0.2-0.3 kilograms per hectare (0.2-0.25 pounds per acre).

Foliar application on mineral and organic soils can be an effective rescue, unless deficiency is severe. Foliar rates of 0.1-0.3 kilograms per hectare (0.1-0.3 pounds per acre) are recommended. Chelates are negatively charged organic molecules that create a protective bond with positively charged micronutrients. This prevents micronutrients from binding with soil particles and becoming insoluble and unavailable to the plant. Iron, zinc, copper, manganese, calcium and magnesium can be chelated. Chelates are especially useful for micronutrients applied to alkaline soils. High pH soils are more likely to have ions that will bind with micronutrients to become insoluble substances. Plant roots take up the chelated nutrient, and the micronutrient is released inside the plant. Chelated micronutrient fertilizers are more expensive, but are more efficiently taken up by the plant, hence the lower rates.

Iron (Fe)

Role of iron in canola plants

Iron is a component of ferrodoxin, which acts as an electron transmitter in nitrate and sulphate reduction, nitrogen fixation, and energy production. Iron is needed for chlorophyll synthesis, so low chlorophyll content of young leaves (interveinal chlorosis or yellowing) is the most obvious visible symptom of iron deficiency. In young growing leaves, about 80 per cent of the iron is located in the chloroplasts. Iron is also thought to be involved with protein synthesis and root tip growth.

Iron concentration in whole plant tissue is highest in young canola plants, and declines in concentration as the plants mature 10.

Canola response to iron

Western Canadian soils developed from parent materials rich in iron. Therefore, there have been no reports of iron deficiencies in field crops or responses to iron fertilizer on the Prairies. Also, there has been no work to calibrate soil test values for iron on the Prairies.

Presence of lime in the soil may induce iron chlorosis, which is common in some Northern Great Plains soils. Iron chlorosis is common in gardens grown on calcareous soils and with iron-sensitive crops, such as strawberries. Iron chlorosis is also observed with some broadleaf shrubs and trees.

Iron is needed for chlorophyll synthesis, so low chlorophyll content of young leaves (interveinal chlorosis or yellowing) is the most obvious visible symptom of iron deficiency. However, yellowing of young leaves can have many causes, and iron deficiency would not be a likely cause. When diagnosing a potential deficiency:

  • Ensure that poor crop growth is not the result of a macronutrient deficiency, drought, salinity, disease or insect problem, herbicide injury or some physiological problem.
  • Find out if a micronutrient deficiency has been identified before in a particular crop or soil type in the area.
  • Examine the affected crop for specific micronutrient deficiency symptoms.
  • Take separate soil samples from both the affected and unaffected areas for complete analysis, including micronutrients.
  • Send plant tissue samples from both the affected and unaffected areas for complete analysis that includes tests for micronutrient levels.
  • If all indications point to a micronutrient deficiency, apply the micronutrient to a specific, clearly marked out affected area of land to observe results in subsequent seasons.

Fertilizer sources and methods of application

Foliar application of iron sulphate on garden vegetables, fruits and shrubs is an effective means of correcting Fe deficiencies. Consult manufacturer label for instructions for use on canola.

Manganese (Mn)

Manganese is a metallic micronutrient that is occasionally deficient in western Canadian organic, high pH soils. Although oats can be affected by manganese deficiency in cold organic soils (gray speck of oats disease), there have been no documented deficiencies with canola in western Canada.

Weathering of manganese containing soil minerals is the source of plant available manganese. The main manganese form that exists in soil solution or adsorbed to soil colloids is Mn+2, which is also the form absorbed by roots. Manganese availability decreases when pH increases above 6.2 in many soils. Low temperature and high organic matter can also decrease manganese availability. Rhizosphere microbes play a role in manganese availability by either oxidation or reduction. These microbes can either increase or decrease manganese availability. The rhizosphere acidification by canola roots likely increases manganese availability and makes this crop relatively tolerant of low soil manganese.

Manganese concentration in canola tissue initially rises in the first two to six weeks after emergence, and then declines as canola matures 10.

In some crops, seed manganese content has been shown to be important for initial manganese nutrition and plant growth, as well as disease resistance.

Canola response to manganese

The chance of getting a canola response to a manganese application is low, and lower for canola than for other crops. Rhizosphere acidification by canola roots likely increases manganese availability and makes this crop relatively tolerant of low soil manganese.

Manganese deficiency is more likely to occur in:

  • Soils with higher pH. Manganese availability decreases when pH increases above 6.2 in many soils.
  • Soils that have very high levels of organic matter, especially when cool. Generally, soils with greater than 30 per cent organic matter to a depth of 30 centimetres have low micronutrient availability.
  • Manganese:copper ratios above 15 (in the plant) may lead to copper deficiency while ratios below one may lead to manganese deficiency.

Manganese toxicity, or too much available manganese in the soil, can also occur in canola. It was documented on a field of canola near Lac La Biche, Alberta, in 2010. Manganese toxicity causes chlorosis of leaf margins and cupping of leaves in canola.

Identifying deficiencies

Manganese deficiencies are not well documented in canola but could be similar to manganese deficiency in other crops, characterized by interveinal chlorosis while veins stay green.

To identify a manganese deficiency:

  • Ensure that poor crop growth is not the result of a macronutrient deficiency, drought, salinity, disease or insect problem, herbicide injury or some physiological problem.
  • Find out if a micronutrient deficiency has been identified before in a particular crop or soil type in the area.
  • Examine the affected crop for specific micronutrient deficiency symptoms.
  • Take separate soil samples from both the affected and unaffected areas for complete analysis, including micronutrients.
  • Send plant tissue samples from both the affected and unaffected areas for complete analysis that includes tests for micronutrient levels.

If all indications point to a micronutrient deficiency, apply the micronutrient to a specific, clearly marked out affected area of land to observe results in subsequent seasons.

Fertilizer sources and methods of application

Manganese sulphate could be broadcast-incorporated in the spring at 50-80 pounds per acre of manganese, although this is not usually economical. It can be seed-placed at four to 20 pounds per acre.

Chelated manganese can be foliar applied at 0.5 to one pound per acre of manganese.

Potash (potassium chloride, KCl) has been shown to enhance manganese uptake by several crops.

Manganese toxicity

Several cases of manganese toxicity were identified in northern Alberta in 2010 in areas of the field that had very strongly acidic soils. The symptoms were similar to sulfur deficiency with chlorosis of leaf margins and cupping of leaves in canola. To correct the manganese toxicity problem, liming would be one tactic aimed at increasing soil pH. However, the cost can be prohibitive especially with debatable returns given the lack of information on yield loss due to manganese toxicity in canola in western Canada.

Molybdenum (Mo)

Molybdenum is a transition element needed in extremely low amounts; only nickel has a lower requirement. All the Brassica species appear to be sensitive to low molybdenum supply and can exhibit peculiar symptoms (for example ‘whiptail’ of cauliflower).

Role of molybdenum in canola plants

Only a few enzymes are known to contain molybdenum as a cofactor:

  • the enzyme that helps to change nitrate to organic nitrogen in the plant (nitrate reductase)
  • the major enzyme involved in nitrogen fixation in legumes (nitrogenase)
  • an oxidase/dehydrogenase enzyme involved in changing the products of nitrogen fixation
  • likely an enzyme involved in sulphate metabolism (sulphite reductase)

Molybdenum functions are, therefore, closely related to nitrogen metabolism and nitrogen fixation.

Once absorbed by roots, molybdenum is readily mobile in both the xylem and phloem transport systems, likely as MoO4-2.

Canola response to molybdenum

Molybdenum is needed in extremely low amounts, yet all Brassica species appear to be sensitive to low molybdenum supply. Molybdenum functions are closely related to nitrogen metabolism and nitrogen fixation.

Molybdenum deficiencies in canola have not been documented in western Canada. A deficiency may be more likely to occur in acid mineral soils with a large content of reactive iron oxide hydrates. Some interaction occurs between molybdenum uptake and levels of phosphorus and sulphur. Plant molybdenum uptake is usually enhanced by soluble phosphorus and decreased by sulphate. Both MoO4-2 and SO4-2 compete strongly for root uptake.

Molybdenum deficiency tends to occur more often in acid (low pH) soils. This is an exception. Other micronutrients become less available as pH rises.

Identifying deficiencies

Specific information on canola is limited, but brassicas deficient in molybdenum show cupping of leaf margins of younger leaves, interveinal chlorosis and, in later stages, a twisting of leaves around the central mid-rib.

To identify a molybdenum deficiency:

  • Ensure that poor crop growth is not the result of a macronutrient deficiency, drought, salinity, disease or insect problem, herbicide injury or some physiological problem.
  • Find out if a micronutrient deficiency has been identified before in a particular crop or soil type in the area.
  • Examine the affected crop for specific micronutrient deficiency symptoms.
  • Take separate soil samples from both the affected and unaffected areas for complete analysis, including micronutrients.
  • Send plant tissue samples from both the affected and unaffected areas for complete analysis that includes tests for micronutrient levels.

If all indications point to a micronutrient deficiency, apply the micronutrient to a specific, clearly marked out affected area of land to observe results in subsequent seasons.

Fertilizer sources and methods of application

Sodium or ammonium molybdate can be applied by soil or foliar. Given that molybdenum is needed in such trace amounts, a seed treatment may also work.

Liming to raise soil pH can help to make more soil molybdenum available to crops.

Molybdenum is unique among the micronutrients since there is a wide range between deficiency and toxicity.

Zinc (Zn)

Role of zinc in canola plants

Zinc exists only in the Zn+2 form in plants, and is not involved with redox reactions. Zinc has an ability to form complexes with nitrogen, oxygen and particularly sulphur and performs catalytic and structural roles in enzymes. Many enzymes contain zinc as a structural, catalytic or cofactor component. Protein synthesis, hormone (auxin) and carbohydrate metabolism also require zinc. Membrane stability also relies on zinc, and the most obvious zinc deficiency symptoms (such as leaf chlorosis and inhibited stem elongation) probably arise from membrane breakdown.

Uptake of zinc by canola

Weathering of soil minerals is the primary source of plant available zinc (Zn+2). Weathering removes zinc faster than other metals, except for copper. Zinc deficiency commonly occurs in acidic, highly weathered soils (typically tropical). Plant available zinc exists as exchangeable Zn+2, dissolved Zn+2 in soil water, adsorbed zinc to manganese oxide and organically bound zinc. Soil test labs often use a critical level of 0.5 parts per million DTPA extractable zinc, but research in western Canada has found this level too high for predicting cereal response and that DTPA is an unsuitable extractant. No further work has been conducted to find a more suitable extractant and calibration for cereals and oilseeds on the Prairies.

The mechanism for zinc uptake by roots is not well understood and may involve both active and passive processes. Once absorbed by the roots, zinc is likely complexed with small organic molecules similar to other metallic micronutrients. Copper and other cations compete for root zinc uptake.

Daily zinc uptake is greatest during canola flowering 10.

Canola response to zinc

In western Canada, sporadic zinc deficiencies have been identified in fields of alfalfa, flax and beans, but not canola.

Zinc deficiency is more likely to occur where:

  • The pH of soil is high. Zinc availability increases as soil pH decreases (becomes more acidic). Deficiencies are more likely on high pH, calcareous soils due to zinc adsorption to lime particles.
  •  High rates of phosphorus have been applied. Application of phosphorus fertilizer may increase zinc deficiency. High phosphorus levels can induce zinc deficiency by inhibiting zinc translocation within the plant rather than affecting root uptake.

Identifying deficiencies

Zinc deficiency symptoms range from:

  • leaf chlorosis and inhibited stem elongation. Membrane stability relies on zinc, so these symptoms probably arise from membrane breakdown.
  • purpling on new emerging leaves
  • brown spots on cotyledons
  • interveinal chlorosis
  • cupping of leaves

To identify a zinc deficiency:

  • Ensure that poor crop growth is not the result of a macronutrient deficiency, drought, salinity, disease or insect problem, herbicide injury or some physiological problem.
  • Find out if a micronutrient deficiency has been identified before in a particular crop or soil type in the area.
  • Examine the affected crop for specific micronutrient deficiency symptoms.
  • Take separate soil samples from both the affected and unaffected areas for complete analysis, including micronutrients.
  • Send plant tissue samples from both the affected and unaffected areas for complete analysis that includes tests for micronutrient levels.
  • If all indications point to a micronutrient deficiency, apply the micronutrient to a specific, clearly marked out affected area of land to observe results in subsequent seasons.

Fertilizer sources and methods of application

Zinc sulphate can be broadcast and incorporated in spring or fall at 3.5 to five pounds (of zinc) per acre. Alternatively, zinc oxysulphate can be broadcast and incorporated in fall at five to 10 pounds (of zinc) per acre. Additionally, chelated zinc can be foliar applied at 0.3-0.4 pounds (of zinc) per acre, or broadcast incorporated at one pound (of zinc) per acre.

Other micronutrients

There are other micronutrients and beneficial elements than those discussed above. However, deficiencies of the remaining nutrients are limited to certain plant species other than canola or deficiencies are infrequent anywhere in the world.

Chlorine (Cl) is found in abundance in nature as chloride salts. Choride (Cl) is highly mobile in soil and plants and is readily absorbed by roots. Chlorine is involved in photosynthesis, charge balance, enzyme activation, stomatal regulation and disease resistance. Cereal (winter wheat) responses to chlorine fertilizer have occurred on the prairies, apparently due to disease suppression or improved water relations rather than chlorine nutritional needs. Canola responses to chlorine have not been reported.

Nickel (Ni) has recently been established as an essential nutrient. In higher plants, urease is the only known nickel containing enzyme. Other nickel roles include iron absorption, seed viability, nitrogen fixation and reproductive growth. The plant available form is Ni+2. Root uptake likely follows similar patterns as other micronutrient metals. Nickel appears to be readily mobile in both xylem and phloem. Nickel deficiency in field grown crops has not been reported.

Silicon (Si) is the second most abundant element in the earth’s crust and is a beneficial nutrient for a few wetland plant species such as rice. In non-wetland species, silicon can counteract zinc deficiency induced by high phosphorus. Since silicon is so abundant in nature, proving its essentiality is very difficult. Silicon may affect plant stability by influencing lignin biosynthesis as well as through deposition in cell walls. Increased leaf rigidity has been reported in cereal and cucumber crops. Silicon may also contribute to disease and insect resistance. Silicon may decrease toxicity from high levels of manganese, iron and aluminum.

Sodium (Na) is an essential nutrient for some plant species that use the C4 photosynthetic pathway. Canola uses the C3 pathway and, therefore, sodium is not a beneficial nutrient for this crop.

Footnotes

  1. Karamanos, R. E., Goh, T. B., &Stonehouse, T. A. 2003.Canola response to boron in Canadian prairie soils. Can. J. Plant Sci., 83, 249–259.
  2. Blevins, D.G., & Lukaszewski, K.M. 1998. Boron in plant structure and function. Annual Review of Plant Physiology and Plant Molecular Biology, 49, 481-500.
  3. Karamanos et al. 2003 unpublished
  4. Stangoulis, J., Tate, M., Graham, R., Bucknall, M., Palmer, L., Boughton, B., & Reid, R. 2010. The Mechanism of Boron Mobility in Wheat and Canola Phloem. Plant Physiology, 153, 876-881.
  5. Miwa, K. & Fujiwara, T. 2010.Boron transport in plants: co-ordinated regulation of transporters. Annals of Botany, 105, 1103 – 1108.
  6. Shi, L., Wang, Y. H., Nian, F. Z., Lu, J. W., Meng, J. L., & Xu, F. S. 2009.Inheritance of boron efficiency in oilseed rape. Pedosphere, 19(3), 403–408.
  7. Malhi, S. S., Raza, M., Schoenau, J. J., Mermut, A. R., Kutcher, R., Johnston A. M., & Gill, K. S. 2003. Feasibility of boron fertilization for yield, seed quality and B uptake of canola in northeastern Saskatchewan. Can. J. Soil Sci., 83, 99–108.
  8. Karamanos R.E., Kruger, G.A., & Stewart, J.W.B. 1986. Copper deficiency in cereal and oilseed crops in northern Canadian prairie soils. Agron. J., 78, 317-323.
  9. Karamanos et al 2003 Unpublished data
  10. Karamanos et al. 2003 Unpublished data

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