Micronutrients in Crop Production

Western Canadian crops require 17 essential nutrients to grow normally. Carbon, hydrogen, and oxygen derived from the air comprise more than 90 per cent of the fresh plant tissue. Macronutrients derived from the soil and needed in large amounts are nitrogen (N), phosphorus (P), potassium (K), sulphur (S), calcium (Ca), and magnesium (Mg). Legumes are the exception because they fix N from the air. With a few exceptions, Ca and Mg are not limiting in Saskatchewan because of the nature of the soils. The soil supply of N, P, K, and S is often supplemented by fertilizer and manure.

The remaining essential nutrients derived from the soil are referred to as micronutrients, because they are needed in small amounts. They are boron (B), chloride (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni) and zinc (Zn). Micronutrients are important for plant growth, as plants require a proper balance of all the essential nutrients for normal growth and optimum yield.

Soil factors that affect the availability of micronutrients:

Organic matter

• Mineral soils very low in organic matter, such as Gray soils, may be deficient in micronutrients.
• Peaty and muck soils are likely to show micronutrient deficiencies. For example, peaty soils in the Carrot River and Meadow Lake areas of Saskatchewan may be deficient in Cu and Mn.

Soil texture

• Sandy soils are more likely to show micronutrient deficiencies than clay soils.

Soil pH

• Micronutrient availability generally decreases as the soil pH increases, with the exception of Mo.

Management, climatic and spatial variability

• Soil moisture and temperature are important. For example, cold, wet soils restrict root growth, reducing the volume of soil explored by the roots. Saskatchewan soils can be cool and wet in the spring, at which time micronutrient deficiencies may show up, but disappear when the soils warm up.
• Deficiency of one of the macronutrients (N, P, K, or S) may also restrict the ability of the plant roots to explore for other nutrients. For example, P is important for early root formation and growth.
• Micronutrient deficiencies generally appear as patches in fields because micronutrient levels can vary across landscapes. For example, Cu deficiencies in northeastern Saskatchewan occur on peaty and sandy patches and eroded knolls, whereas Cu deficiency can be widespread on sandy soils.
• Land application of livestock manure can increase the amount of available Cu and Zn.

Micronutrient Deficiency

The best ways to measure the performance of a micronutrient treatment is to determine if the yield increases; or, for some crops like potatoes an improvement in quality will cover the costs of the micronutrient application and return some profit.

Diagnosing micronutrient deficiencies in the field by assessing crop symptoms is difficult, even for trained agronomists. Look for "multiple evidence" before recommending a micronutrient for a whole field. A combination of crop symptoms in the field, tissue tests, soil tests, test strips, cropping history and other techniques can be used to confirm micronutrient deficiencies and economic yield responses.

Take soil and plant tissue samples from the affected and unaffected areas within the same field for a complete comparative analysis. This service is available from most soil testing laboratories. Call the laboratory for sampling details for a complete comparative test.

Many factors, such as macronutrient deficiency, drought, salinity, disease, insect, herbicide injury or other physiological problems can cause poor or stressed plant growth. Stressed growth may show symptoms similar to micronutrient deficiencies.

Crop nutrient deficiency symptoms may be specific to a crop. Some deficiency symptoms can look similar between micronutrients.

Management tools to help with decision-making

Keep good field records; know which fields have had suspected or confirmed problems with micronutrients; soil test annually; and, monitor each crop for symptoms. The amount of micronutrients needed varies by crop. Geo-reference micronutrient deficient areas within a field to make site-specific management easier. Micronutrients tend to be expensive in comparison to macronutrients, so site-specific management makes economic sense.

If all indications point to a micronutrient deficiency, then foliar apply a plant available form of the micronutrient in strips across the affected field at the appropriate crop stage to see if the micronutrient fertilizer corrects the deficiency. Alternatively, soil apply the micronutrient to a test strip across the field in question at the beginning of the next crop season and monitor crop response over more than one season. Assess the yield of treated and untreated areas to see if the yield response is economic. As over applying micronutrients can lead to toxicity levels resulting in yield loss, caution is necessary, especially with the micronutrient B.

What should you do when your soil test shows a marginal level for a micronutrient? A marginal level for a composite sample would imply patches in a field may be deficient. A marginal level should be treated as a flag to monitor the field more closely for the micronutrient deficiency. A measure of need may be made by proving an economic yield response to the application of a micronutrient. The best suggestion is to apply a test strip to verify whether a micronutrient is going to give a positive yield response and to verify whether the returns are economical. If a producer decides to apply a micronutrient to an entire field, leaving a "no micronutrient applied" check strip will be beneficial in determining whether there was an economic response.

If a micronutrient recommendation based on a soil and/or a tissue test is made for a field that has no history of a micronutrient deficiency, then further investigation, including crop scouting and another soil and tissue test is advisable.

Crop symptoms occur when micronutrient deficiencies are moderate to severe.

Micronutrient deficiencies that do not display symptoms but reduce the yield of a crop are referred to as "hidden hunger." Know the field when assessing for "hidden hunger." If soil tests over a number of years indicate that a micronutrient level is decreasing into the marginal range for that crop, then consider applying the micronutrient in test strips first to see if there is a positive yield response and if that yield response is economical. On the other hand, applying micronutrients when they are not needed may reduce yields and/or economic returns.

Forms of micronutrient fertilizer

Sulphate (salts)

The sulphate form of micronutrients such as Cu, Zn, Fe and Mn represent a water-soluble form that is plant available. Borate is the equivalent plant available form for B. Sulphates are the most commonly used form for field crops. Sulphates can be applied to the soil or foliage. Sulphate products, applied at agronomically recommended rates, can provide long term residual value.

Oxysulphate

An oxysulphate is an oxide of a micronutrient that has been partially reacted with sulphuric acid. In the year of application, the oxide portion is not nearly as available as the sulphate portion. The amount of sulphate in the product varies by product. Water solubility of oxysulphates can vary greatly. It is generally accepted that a minimum of 50 per cent water solubility is required for the micronutrient to be a readily available nutrient source. In general, the higher the water solubility portion, the better. Residual value is similar to sulphates.

Oxide

Micronutrient elements (Cu, Zn, Fe, and Mn) bonded with oxygen form oxides. The bonds with oxygen are very strong, meaning these products are not soluble in water and are not in plant available form. An oxide of a micronutrient needs to be converted to a plant available form in the soil before being taken up by the plant. Oxides represent the final form to which other forms are eventually converted under western Canadian soil conditions, and may then be slowly converted back to plant available form. For crop response during the growing season, plant available forms (water-soluble forms) of micronutrients need to be used. Pure oxide forms are less commonly used under western Canadian conditions and may be of residual value.

Chelate

Micronutrients such as Cu, Fe, Mn, and Zn are held within ring-type compounds. Chelated micronutrients remain in plant-available form longer because the chelated structure slows the micronutrient reaction with soil minerals. There are a large number of chelating agents. For example, a synthetic chelating agent is EDTA, and a natural chelating agent is citric acid. Chelated micronutrient products are not all equally available to the plant. Chelated micronutrients can be soil or foliar applied. Chelates are generally many times more expensive than the sulphate or oxide forms on a per pound actual micronutrient basis, but this is partly compensated for in the low recommended rate of chelate product needed to supply the micronutrient. Chelated products applied at label rates have limited residual value. Follow label rates and directions. Chelates are more commonly used now than in the past.

Manure

Livestock manure can be a source of micronutrients such as Cu and Zn, especially since these nutrients are often added to the feed rations. Repeated applications of manure have been shown to increase the content of available Cu and Zn in Saskatchewan soils.

Other forms

Carbonates and nitrates and mixtures with elemental forms are examples of other forms, but are seldom used.

Soil and tissue sampling

Soils should be sampled to a depth of zero to six or zero to 12 inches using proper soil sampling techniques. A soil plus tissue test for affected and unaffected areas is recommended. The comparative test helps confirm whether the problem is nutrient related.

Plant tissue sampling guidelines

Plant samples must be clean, but do not wash. Soil, roots, dirty hands, soiled tools, etc. can contaminate a plant tissue sample. Place samples in clean bags, preferably those provided by the laboratory. Follow laboratory instructions carefully. Tissue samples from 15 to 50 plants or 50 to 75 leaves are required, depending on the stage of the crop. It is essential that tissue samples are from the same plant parts as indicated by the laboratory, and plants must be in the same stage of development. Contact the laboratory for sampling guidelines. Scout fields early so that corrective action can be taken if necessary. Tissue samples taken during late stages of plant development may also provide information for corrective action for the next crop.

Boron (B)

Boron is mobile in the soil and is subject to leaching, like nitrate and sulphate. Organic matter is the main source of B in western Canadian soils. Most Saskatchewan soils contain enough organic matter to supply B for crop needs.

Boron deficiencies have been suspected in alfalfa and canola on sandy and eroded sandy soils in the Gray soil zone. Boron may be limiting to seed production of alfalfa in these soils. Symptoms that appear in spring under cool and wet conditions tend to go away when soil conditions become warm and drier. Apply B in test strips to confirm economic yield response. Additions of high rates of B on soils where B is not required can result in toxicity and a reduction in yield. There is a narrow range between deficiency and toxicity, so extreme care must be taken to avoid overlap when B fertilizer is applied.

The first symptoms appear in the new growth, as Boron impacts cell development, sugar and starch formation and translocation. Stunted and small plants with misshapen, thick, brittle leaves are common symptoms. Cell wall development, elongation of pollen tubes, and pollination are affected by boron. Boron is not transferred easily from older to younger (upper) leaves, so younger leaves show symptoms first. In canola, the yellowing of the youngest leaves can be confused with sulphur deficiency. In alfalfa, symptoms include rosetting, yellow top, poor flowering, death of the terminal buds and poor seed set.

Boron toxicity is indicated by yellowing, followed by browning of the leaf margins and tips with sharp boundaries between the yellow and/or brown and unaffected green area.

Borate and borax forms of B fertilizer are most commonly used. Where B deficiencies have been proven, broadcast rates should not exceed 0.5 lb. actual B/ac. for cereals and 1.5 lb. actual B/ac. for canola. Do not seed-place B when using narrow openers. Boron is toxic when in contact with seed. Use low rates (not to exceed 0.3 lb. actual B/ac.) when foliar applying. Follow label directions for water volumes. Boron is toxic at low levels, resulting in reduced yield. Therefore, extreme care must be taken to apply B uniformly and to avoid overlap.

Chloride (Cl)

Chloride is a mobile nutrient in the soil. Large variations in soil Cl levels can occur over short distances. Large year to year variations also occur.
Chloride in the plant is involved in controlling water loss, maintaining turgor, transportation of K, Ca and Mg within the plant, and photosynthesis. Recent research has indicated that Cl assists in reducing the incidence of root diseases as well as helps reduce the incidence of some leaf spotting diseases of winter wheat. Chloride also impacts on N uptake. Canary seed has shown to be responsive to Cl applied as KCl (potash) for soils in Saskatchewan testing low in Cl.

For 0 to 24 in. soil samples*:

* Based on criteria for winter wheat from North Dakota State University.

Potash (0-0-60, 0-0-62) is the most common source of Cl.

Copper (Cu)

If you suspect a Cu deficiency in wheat, barley or canary seed (crops most sensitive to Cu deficiency) or flax, alfalfa (less sensitive than wheat) based on a soil or tissue sample, consider a foliar application on a test strip. If there is a Cu deficiency in that field, the result will be an economic yield response.

Copper is relatively immobile in soil.

Solubility and plant availability of Cu is highly dependent on soil pH. Copper solubility increases approximately 100 fold for each unit decrease in soil pH.

Copper deficiencies will most likely show up first in wheat, barley, oats or canary seed, as these crops are highly sensitive to Cu deficiency. Canola, rye, flax, and forage grasses are much less sensitive to Cu deficiency. Crop cultivars can differ widely in sensitivity to Cu deficiency. Sandy soils in the Black and Gray soil zones and peaty soils are most likely to be deficient in Cu.

Where Cu and Zn are both deficient, they both must be applied to obtain a yield increase.

Copper deficiencies usually occur in irregular patches within fields.

• High levels of soil P can also depress Cu absorption by plant roots creating the Cu deficiency.
• Avoid blending Cu sulphate fertilizer with other fertilizers. The blend readily absorbs moisture.
• On very P deficient soils, addition of Cu and Zn without adding P fertilizer may negatively affect growth.

Copper is involved in several enzyme systems, cell wall formation, electron transport and oxidation reactions. Copper is not readily transferred from older to younger leaves. In cereals, older leaves remain green and healthy with the newer leaves yellowing and wilting, and the leaf tips pigtailing. Excessive tillering, aborted heads, delayed maturity, prolonged flowering period and poor grain filling are also symptoms. These symptoms appear in irregular patches within fields. These patches have a "drought-like" appearance. Copper deficiency is often associated with increased incidence of root rot, stem and head melanosis (purpling, appears as brown patches in the field at maturity) and possibly may increase the incidence of ergot.

Options for applying Cu under 0-till systems

The best option is to broadcast and incorporate three to five lb. Cu/ac. during a year when strategic tillage takes place (a tillage operation during long-term zero-till). This will usually correct the Cu deficiency for several years in the following rotational crops.. At lower rates (less than two lb. Cu/ac.) broadcast and incorporation of granular Cu fertilizers may not be effective in increasing the yield of wheat in the year of application. However, two lb. of liquid Cu/ac. sprayed on the surface and incorporated was effective in correcting the Cu deficiency in the year of application, as well as for several years after.

Seedrow or side-band 0.25 to 0.5 lb. liquid Cu/ac.

Foliar apply Cu each time a Cu sensitive crop is grown in the rotation. If the Cu deficiency is severe, more than one application may be needed to correct the deficiency.

The most effective method of alleviating a Cu deficiency is by soil application of Cu sulphate. Foliar application may not be sufficient on severely Cu deficient soils to obtain optimum yields. The least effective method is to place the granular Cu in a concentrated band. Copper sulphate and chelated forms are more readily available than the oxide form. The effectiveness of oxysulphate products depends on their solubility. To correct a severe Cu deficiency, broadcast copper sulphate at three to five lb./ac. actual Cu (or higher rates for very deficient soils) or 0.5 lb./ac. of actual Cu as chelate, then incorporate. At this rate, the Cu sulphate is effective for many years. There is no residual effect from the Cu chelate at the 0.5 lb. Cu/ac. rate. Applying Cu in the affected patches within fields is cost effective.

The oxide form of Cu is released more slowly and is often not available to the plants in the year of application. Fall application of some Cu oxide products may release some plant available Cu for the following year. Copper from seed placed oxide and oxysulphate products may not become available for a number of years.

Foliar applied Cu chelates or Cu sulphate can be used to correct Cu deficiencies in the growing season. They are best applied after elongation starts (first node visible) to flag leaf fully emerged stage to be effective in restoring seed yield. Foliar rates should be no less than 0.18 and no greater than 0.3 lb. actual Cu/ac. Copper applied at the four leaf stage or at heading was ineffective in restoring yield. Copper sulphate is very corrosive to application equipment.

It is suggested that Cu deficiency may increase the incidence of ergot. However, the presence of ergot does not necessarily imply a Cu deficiency.

Pastures low in Cu may never be fertilized enough to bring up the Cu levels in the forage in order to meet the diet needs of cattle. On the other hand, sheep have a low tolerance to Cu, storing excess in their liver over a number of years until the Cu reaches lethal levels. A recommended practice is to supplement feed rations to correct micronutrient deficiencies identified by feed testing.

Livestock producers must also be aware of the Cu by Mo balance in forages to avoid problems that arise when Mo levels increase out of balance with Cu, causing poor Cu utilization in cattle. The availability of Mo to plants increases with increasing soil pH, making the balance of these nutrients an issue for high pH soils of east central Saskatchewan.

Iron (Fe)

Iron deficiencies in most field crops are rare in Saskatchewan. The exception is soybeans, which are rather inefficient users of Fe. Soils of high pH, poor drainage and saturation, and with high nitrates, carbonates and salts aggravate Fe deficiencies. Fe deficiencies may also occur for fruit trees, shrubs, ornamentals and strawberries especially when grown in high pH and calcareous soils. Other factors like very high P, cold and wet conditions, high lime and genetic differences in crops may result in the expression of Fe deficiency symptoms. Some varieties, for example with soybeans, are more sensitive to Fe deficiency than others. Iron deficiency sensitivity ratings are often provided for soybean varieties.

Iron is a catalyst to chlorophyll formation, acts as an oxygen carrier, and aids in respiratory enzyme systems. Iron is not translocated within the plant, so deficiency symptoms first show up on the younger leaves. The classic Fe symptom is interveinal chlorosis, a pale green to yellow leaf with sharp distinction between green veins and yellow interveinal tissue. This is termed iron deficiency chlorosis or IDC.

Foliar applied Fe fertilizers are most effective in correcting crop deficiencies in the growing season and are perhaps the best solution for high value crops like fruits grown in high pH soils. Soil applied iron was not effective in countering Fe deficiency in soybean. For soybean, selection of IDC tolerant varieties is the best defence when IDC is a concern, but foliar Fe may be a suitable rescue treatment. Lowering the pH of soils in Saskatchewan to increase the availability of Fe does not make economic sense. For fruit trees and ornamentals, a combination of foliar applied Fe chelates and soil applied iron fertilizer may be effective for several years.

Manganese (Mn)

Manganese deficiencies have been reported in oat and barley grown on organic (peat) soils across the northern grain belt in Saskatchewan. Manganese is immobile in soil. Peat soils deficient in Mn and having a high pH and/or good drainage, may respond to Mn fertilization. There seems to be little or no response to soil applied Mn fertilizer on mineral soils.

Where Cu and Mn are deficient, both these micronutrients need to be corrected in order to obtain a yield increase due to the strong Cu by Mn interaction. Fields in the Gray soil zone, and peaty areas where Cu has been corrected, may show Mn as the next limiting micronutrient. Consequently, scout these fields for Mn deficiency symptoms.

Manganese is a component in enzyme systems. Manganese activates several important metabolic reactions, aids in chlorophyll synthesis, accelerates germination and maturity, and increases the availability of P and Ca. Manganese is not translocated in the plant, so symptoms first appear on younger leaves. There appears to be some translocation of Mn in oat. Yellowing between the veins is the main deficiency symptom and can be confused with iron deficiency. Gray speck of oat is the most common symptom, with the gray specks appearing in interveinal areas. Severe Mn deficiency in oat can cause significant loss in yield.

Manganese toxicity can occur due to high soluble Mn in soils of very low pH which increases Mn solubility. Isolated instances of Mn toxicity are suspected in some sandy, acidic soils in central Saskatchewan.

Seed-placed Mn sulphate, at two to 10 lb. actual Mn/ac. (based on soil test recommendations) is recommended as the most effective means of correcting Mn deficiency in peaty soils. However, foliar applied Mn sulphate at one pound per acre is also effective. Broadcast applications are generally not economical.

Molybdenum (Mo)

Molybdenum deficiency has not been identified in Saskatchewan. Molybdenum is needed in very small amounts, so treating the seed is probably the most common way to correct this deficiency should it occur.

Livestock producers must also be aware of the Cu by Mo balance in forages to avoid problems that arise when Mo levels increase out of balance with Cu, causing poor Cu utilization in cattle. The availability of Mo to plants increases with increasing soil pH, making the balance of these nutrients an issue for high pH soils of east central Saskatchewan.

Zinc (Zn)

Zinc is relatively immobile in soil. Zinc deficiencies may occur on calcareous, high pH, sandy texture, high P, and eroded soils. Zinc deficiencies usually show up under cool, wet conditions in early spring when root growth is slow. Poorly drained soils may also be deficient. Badly eroded soils and eroded knolls may be low in Zn. High variability in soil available Zn  may be encountered over short distances. Soil test to be sure. Deficiency symptoms will most likely show up first in dry bean and lentil has been identified as potentially responsive to Zn fertilization. Very high rates of P may induce Zn deficiency in flax.

Zinc is involved in enzyme systems and metabolic reactions, and is necessary for production of chlorophyll and carbohydrates. Zinc is not generally translocated within the plant (but is partly mobile in wheat and barley), so the first symptoms appear on the younger leaves. Symptoms differ from one species to another. In wheat and barley, the leaves may have light blotches or chlorosis between the veins. Younger leaves will be smaller. In flax, grayish brown spots appear on the younger leaves with shortened internodes appearing stunted.

Zinc fertilizer can be soil applied or foliar applied. Higher rates of Zn soil applied (2-5 lbs Zn/acre) should provide benefits into the following years. This should provide several years effectiveness. Chelates are foliar applied to correct Zn deficiency during the growing season but have little residual value. Oxide forms of Zn may have limited effectiveness in the year of application, but may be used to provide residual effect. Oxysulphate forms may provide some immediate plant need as well as a residual effect. The higher the percentage of sulphate (soluble and plant available) fraction, the more Zn will be in the plant available form.

Band two to five lb. actual Zn/ac. in the sulphate form. Foliar apply 0.3 lb. actual Zn/ac. chelated form. Severely Zn deficient bean crops may require two foliar applications to correct the deficiency. Zinc fertilization can increase the zinc content of the grain and therefore its human nutritional value where needed.

Table 1. Recommended methods of application of generalized categories of micronutrient products.*

1 Although foliar applications of copper sulphate are effective, the product is extremely corrosive.
2 Broadcast and incorporated rates of manganese are generally uneconomical.
* Based on research in western Canada.