Micronutrients
Micronutrients include boron, copper, iron, manganese, molybdenum and zinc. Plants use these elements in smaller amounts than the major nutrients, but they are still very crucial to a plant's overall nutrition.
Micronutrients are usually found in much lower levels in the soil than macronutrients. Soil pH, organic matter, clay content and mineralogy can influence the micronutrient soil test. This makes estimating micronutrient availability less reliable than macronutrient evaluation.
Response of Crops to Micronutrient Fertilizers
Crops vary in their response to micronutrients. Highly responsive crops will often respond to additional micronutrients if the soil micronutrient concentration is low. Medium-responsive crops are less likely to respond and low-responsive crops do not usually respond to fertilizer additions even at low soil micronutrient levels. A high rating does not mean the crop always needs additional micronutrients. Response to any given nutrient only occurs when the soil is already low in that nutrient.
Below is a table of established vegetable and grain crops and their relative response to micronutrients. While this information is not available for most specialty crops, closely related crops provide a good starting point for specialty crop producers. Always monitor specialty crops closely over the season for nutrient deficiency symptoms including yellowing or discolouration, interveinal chlorosis, or tip-burn. When symptoms develop, collect tissue or soil samples from affected plants and compare them with unaffected plants in another area of the field. It is important to also examine plants and roots closely for insects or diseases, because these can also lead to similar symptoms.
Response of Crops to Micronutrient Fertilizers1
| Crop | Mn | B | Cu | Zn | Mo | Fe |
| Asparagus | Low | Low | Low | Low | Low | Med |
| Beans, snap | High | Low | Low | High | Med | High |
| Beets, table | High | High | High | Med | High | High |
| Broccoli | Med | High | Med | High | High | |
| Cabbages | Med | Med | Med | Low | Med | Med |
| Carrots | Med | Med | Med | Low | Low | |
| Cauliflower | Med | High | Med | High | High | |
| Celery | Med | High | Med | Low | ||
| Cucumbers | High | Low | Med | |||
| Lettuce | High | Med | High | Med | High | |
| Oats | High | Low | High | Low | Low | Med |
| Onions | High | Low | High | High | High | |
| Parsnips | Med | Med | Med | Low | ||
| Peas | High | Low | Low | Low | Med | |
| Peppers | Med | Low | Low | Med | ||
| Potatoes | High | Low | Low | Med | Low | |
| Radishes | High | Med | Med | Med | Med | |
| Rye | Low | Low | Low | Low | Low | Low |
| Spinach | High | Med | High | High | High | High |
| Sugarbeets | High | Med | Med | Med | Med | High |
| Sweet corn | High | Med | Med | High | Low | Med |
| Tomatoes | Med | Med | High | Med | Med | High |
| Turnips | Med | High | Med | Med | ||
Mn = Manganese B= Boron Cu = Copper |
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1 Taken, with permission, from the Michigan State University Cooperative Extension Publication E486 Secondary and Micronutrients for Vegetables and Field Crop (Table 3, Relative response of selected crops to micronutrient fertilizers.) |
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Boron
Boron plays an important role in the structure of cell walls, fruit set and seed development. It is also a component of protein and carbohydrate metabolism.
Boron deficiency symptoms vary widely between crops. In rutabaga and turnips it causes brown watery areas or hollow centres in the roots. Boron deficient celery plants develop cracked stems, often with brown cat scratches, and blackened hearts. In brassica crops the deficiency appears as hollow stems and brown curds. Sugarbeets, table beets and spinach all develop yellow leaves with cracking on the roots.
Boron toxicity may occur when sensitive crops are planted in a rotation where boron has been applied (or over applied). The toxicity symptoms include: spot like, striped or blotchy yellowing on the leaves. This leads to the death of the tissue, usually beginning at the leaf tips and margins of older leaves. Under severe conditions, this will eventually develop over the whole plant.
There is no OMAFRA-accredited soil boron test. Some soil tests will report a soil boron value. Soil levels are often less than 1 ppm making it very difficult to get an accurate measurement.
Some crops are very sensitive to boron, even at low levels. Consider the boron sensitivity of all rotational crops when applying boron. A soil pH between 5.0 and 7.0 provides the best conditions for boron uptake. Boron deficiencies are more likely to occur on soils with low organic matter, and on exposed or eroded subsoils. Boron availability decreases during periods of drought.
Copper
Copper plays a role in chlorophyll production. It may also have a role in the suppression of some diseases.
Copper deficiencies are most common in crops grown on organic (muck) soils. In carrots it causes pale roots, while in onions it causes dieback and curling of the leaf tips. Copper deficient onions may also have pale bulbs with thin scales. Copper deficient lettuce leaves lose firmness and develop bleached yellow stems.
There is no OMAFRA-accredited soil copper test. Soil tests are unreliable on Ontario soils. Plant analysis is a more useful tool. Copper availability may be lower on coarse sands with very low organic matter and organic or muck soils. Copper availability decreases as soil pH increases.
Iron
Iron is needed for chlorophyll formation, plant respiration and the formation of some proteins.
Iron deficiency first appears as yellowing between the veins of the newest growth. The veins will remain green except in extreme cases.
There is no OMAFRA-accredited soil iron test. An iron soil test does not correlate well with plant uptake or fertilizer response in Ontario. Plant analysis is a much more reliable indicator of iron availability.
Iron deficiencies are rare in Ontario. Factors associated with iron deficiency in other parts of the world are extreme imbalances with other metals such as molybdenum, copper or manganese. Excessive phosphorus in the soil may also contribute to iron problems. Soil with low organic matter or a combination of high pH, high lime, and wet cold soils can induce iron deficiency symptoms in sensitive crops.
Manganese
Manganese is involved in photosynthesis and chlorophyll production. It helps activate enzymes involved in the distribution of growth regulators within the plant.
Manganese deficiency causes yellowing between veins of young leaves. Leaves gradually turn pale-green with darker green next to the veins.
Manganese toxicity can occur on soils with a low pH. It causes brown spots or yellow mottled areas near leaf tips and along the leaf margins. It usually develops on older leaves. Brown spots may also develop on veins, petioles and stems.
Manganese availability is greatest at a soil pH of 5.0 to 6.5. Soil-applied manganese may be useful in acidic (low pH) sandy soils; however, on high pH and muck soils, foliar applications are often more effective than soil applications. It is important not to add more lime than is needed to correct soil acidity. High organic matter levels decreases manganese availability
If a manganese deficiency is confirmed, apply foliar sprays when the plants are about ⅓ grown or sooner. Two or more sprays may be necessary at 10-day intervals.
The OMAFRA-accredited manganese soil test uses a "Manganese Availability Index". This index evaluates manganese availability based on soil manganese level and soil pH. The table below outlines manganese requirements for vegetable crops. The manganese requirements of most specialty crops are unknown, but closely related crops may provide a good starting point.
Manganese Requirements
| Manganese Index1 | Manganese (Mn) Required2 - kg/ha | |
| Onions, Lettuce, Beets | Other Vegetable Crops | |
| 0-7 | 2 | 03 |
| 8-15 | 2 | 0 |
| 16-49 | 0 | 0 |
| 50+ Above Normal | 0 | 0 |
1 Manganese Index = 498 + 0.248 (phosphoric acid extractable Mn in mg/L of soil) - 137 (soil pH) + 9.64 (soil pH)2 |
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2 Manganese should be applied as foliar spray of manganese sulphate. Soil applications are inefficient. |
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3 The manganese soil test is low, however manganese deficiency is not expected on this crop. If deficiency symptoms appear, make a foliar application of 2 kg Mn/ha in 200 L water (1.8 lb Mn/acre in 18 gal water). |
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Molybdenum
Molybdenum plays an important role in nitrogen metabolism within the plant. It is involved with nitrogen fixation in legumes. It also plays a role in pollen viability and seed production.
Early deficiency symptoms are similar to nitrogen or sulfur deficiency. Affected plants become stunted and lack vigour. Leaves may turn brown along the margins. Molybdenum deficiency causes cupping (whiptail) in cauliflower. Deficiency in legumes leads to poor nodule formation
There is no OMAFRA-accredited soil molybdenum test. Deficiencies are best confirmed with a plant analysis. Molybdenum availability improves with increasing soil pH. Liming can improve the availability on acid soils. Deficiencies may also occur under low soil moisture conditions.
Zinc
Zinc is important in early plant growth and in grain and seed formation. It also plays a role in chlorophyll and carbohydrate production.
Zinc is relatively immobile within the plant. Deficiency symptoms appear first on younger leaves. Young leaves become mottled and show interveinal chlorosis, striping or banding.
Zinc deficiencies are most often seen on sandy soils with high pH levels. Heavily eroded knolls may also have deficiency problems. Large applications of phosphorus may aggravate zinc deficiencies. Livestock manure is often an excellent source of zinc.
The OMAFRA-accredited soil zinc test reports a "Zinc Index Value". This index evaluates zinc availability based on soil zinc level and soil pH.
Zinc Soil Index Interpretation
| Zinc Soil Index1 | Suggested Treatments |
| Greater than 200 | Contamination of the sample or of the field is likely. |
| 25 to 200 | Soil zinc availability is adequate for most field-grown crops. |
| 15 to 25 | Zinc availability is adequate for most field crops. If the field sampled is uneven in soil texture, pH, or erosion, some areas may respond to zinc applications. |
| Less than 15 | Zinc is likely to be deficient and should be applied in the fertilizer. |
| Zinc Index = 203 + 4.5(DTPA extractable zinc in mg/L soil) - 50.7(soil pH) + 3.33(soil pH)2 | |
| 1 These values are indices of zinc availability based on extractable soil zinc and soil pH. | |
