What About Micronutrients?

Crop nutrition is never simple, but sometime it gets cloudier still when the topic of micronutrients comes up. What should you know about micronutrients before you talk to the salespeople?

Zinc

• Availability is reduced at high soil pH levels, as zinc forms insoluble compounds in these conditions.
  
• Other areas likely to have zinc deficiency are sandy soils, eroded knolls where subsoil is exposed, organic (muck) soils.
  
• Unlikely to have zinc deficiency in fields that receive manure applications.
  
• Deficiency symptoms appear on new growth; include stunting, shortened internodes, discolouration of new leaves.
  
• There is an OMAFRA-accredited soil test for zinc, but when sampling, galvanized metal sampling equipment or pails must not be used. Plant analysis is also useful.
  
• Zinc deficiency is likely to be patchy, so sample problem areas and healthy areas separately and compare the results to help pinpoint the problem.
  
• OMAFRA-accredited zinc soil tests report a zinc index. This index considers soil available zinc and soil pH to make a more accurate determination of potential crop need for zinc.
  
• Corn, onions, spinach, and edible beans are the vegetable crops most likely to respond to zinc applications if analysis shows a need.
  
• Zinc fertilizer can be applied to the soil or the foliage.

Manganese

• Availability is reduced at high soil pH levels or high organic matter levels. Problems may also be seen in wet seasons.
  
• Deficiency symptoms generally begin on new leaves; include interveinal chlorosis. Eventually symptoms can extend to the rest of the plant.
  
• There is an OMAFRA-accredited soil test for manganese. Deficiency is likely to be patchy, so sample problem areas and healthy areas separately and compare the results to help pinpoint the problem.
  
• OMAFRA-accredited manganese soil tests report a manganese index. This index considers soil available manganese and soil pH to make a more accurate determination of potential crop need for manganese.
  
• Edible beans, cucumbers, lettuce, onions, peas, potatoes, radishes, red beets, spinach, sugarbeets, and corn are the crops most likely to respond to manganese applications if analysis shows a need.
  
• Manganese fertilizer is most effective if applied foliarly. Apply as a foliar spray of manganese sulphate or chelate. Use a spreader sticker with manganese sulphate.
  
• Manganese toxicity can also occur, especially in waterlogged or poorly aerated conditions, or on sensitive crops such as cabbage or Delicious apples, if the soil pH is 5.0 or less.

Boron

• Boron deficiencies are more likely with sensitive crops, in dry conditions, high (over 7) or low (under 5) pH soils, low organic matter soils, areas where subsoil is exposed.
  
• Reliable soil tests have not been developed for boron. Tissue analysis comparing problem areas to healthy areas may help diagnose the problem.
  
• Many crops will experience boron toxicity if boron was applied in excess to other crops in the rotation. Avoid banding boron.
  
• Broccoli, cauliflower, celery, and red beets are the vegetable crops most likely to respond to boron applications.
  
• Boron can be applied to the soil or the foliage.

Copper

• There is no reliable soil test for copper in Ontario conditions. If a problem is suspected, tissue analysis comparing poor and healthy areas of the field may assist in diagnosis.
  
• Copper deficiency is most likely to occur in organic (muck) soils or on very sandy soils. Copper is less available as soil pH increases.
  
• Vegetable crops most likely to respond to copper, if analysis shows a need, include lettuce, onions, red beets, spinach, and tomatoes.

Molybdenum

• There is no reliable soil test for molybdenum in Ontario conditions.
  
• As soil pH levels increase, molybdenum availability increases. Phosphorus enhances molybdenum absorption. Deficiencies are most likely when soils are dry.
  
• Broccoli, cauliflower, lettuce, onions, red beets, and spinach are the crops most likely to respond to molybdenum applications if tissue/foliar analysis shows a need.
  
• Keeping soil pH within crop-optimal levels should address any concerns with molybdenum.

Iron

• Iron deficiency may become a problem in high pH soils.
  
• Symptoms are interveinal chlorosis of new leaves.
  
• Currently there is no accredited soil test for iron in Ontario. Confirm a suspected deficiency with a foliar analysis.
  
• Iron chelates can be applied as foliar sprays. Generally, soil applications of inorganic iron sources are not effective in supplying iron to the crop.

General Considerations

Applying micronutrients in excess can cause more damage than from deficiencies. Starter fertilizers containing micronutrients will be more likely to cause toxic effects than the same fertilizers without micronutrients.

Micronutrients are often considered "cheap insurance" in case of "hidden hunger", however the cost can be substantial. Find a way to do a test strip so you can see if you get a benefit (or injury). Remember, the fertilizer salesperson will always get a benefit from micronutrients, but will you?

For more information about micronutrients get your copy of OMAFRA Publication 611 Soil Fertility Handbook and Best Management Practices: Managing Crop Nutrients by visiting Service Ontario Publications at www.publications.serviceontario.ca or the OMAFRA office nearest you.