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Fertiliser 4: Micronutrients for arable crops

Course: Fertiliser management | Last Updates: 7th October 2015

Professor Philip White
Ecological sciences
James Hutton Institute
Biography >>

Plants require at least 14 mineral elements to complete their lifecycle. Of these, there are six macronutrients – nitrogen, phosphorus, potassium, sulphur, calcium and magnesium – that are required in large amounts, and eight micronutrients – boron, chlorine, manganese, iron, zinc, copper, molybdenum and nickel – that are needed in much smaller quantities.

In addition, although not considered to be essential, cobalt and sodium are required by some plants and it’s also been shown that boosting silicon and selenium levels can be beneficial in certain circumstances.

Lack of any of these essential elements will slow plant growth and development, and reduce crop yields. So where yields are coming in below expectations, it’s usually worth taking a closer look at the crop to determine whether further nutrition is required.

Often a warning comes with the onset of characteristic visual symptoms that reflect the role of a particular nutrient. Another point to note is that the location of deficiency symptoms usually reflects the plant’s ability to redistribute the mineral element from older to younger tissue.

Measuring for deficiencies

Both soil and leaf analysis are used to diagnose micronutrient deficiencies. However, because soil concentrations of elements aren’t always available to the plant and mineral elements can become “locked up” and difficult for the plant to extract, foliar analysis (otherwise known as tissue testing) is often the more important of the two.

Where leaf testing is being used, it’s important to choose your leaf carefully. Not all leaves contain the same concentrations of any given nutrient and leaf age can determine the reading that you get. As a rule of thumb, it’s usually best to take the youngest, fully expanded leaf.

Common deficiencies

Manganese deficiency is the most common micronutrient deficiency seen in UK crops. Sugar beet, cereals and peas are all considered to be susceptible, especially where they are grown on high pH soils.

The first sign of a manganese problem in cereals is pale green, limp foliage. In sugar beet, plants develop interveinal mottling on the leaves and the leaf margins tend to curl inwards. Peas have internal discolouration, which shows up when they are split in half, otherwise known as marsh spot.

Boron deficiency is seen in alkaline and low organic matter soils, as well as highly leached sandy soils, and can affect sugar beet, brassica crops and carrots.

With iron, zinc, copper and manganese, levels in soil may not be available to the plants. Although iron deficiency is rare in field crops, it often affects fruit crops growing on calcareous soils.

Zinc deficiency is seen in 10-15% of field crops, often showing on sandy soils with both a high pH and phosphate status. Copper deficiency is rare, but may occur regionally on light, shallow soils in cereals and sugar beet.

Nickel levels are usually sufficient for UK crops and molybdenum deficiency is associated only with acid soils, where it can cause a condition known as “whiptail” in cauliflowers.

Symptoms of deficiency

Chlorine, zinc and molybdenum are readily translocated within the plant, which is why deficiency symptoms are first seen in older leaves. But for iron, copper, manganese and boron, retranslocation from older to younger leaves is a slower process, with deficiency symptoms initially being observed in younger leaves.

Correcting deficiencies

Make sure that your soil pH has been checked and corrected, if necessary, before you embark on micronutrient applications. In recent data from Lancrop Laboratories, 20% of soil was below the optimum pH.

Remember that it can be difficult to address a micronutrient deficiency during the growing season, as it’s often too late by the time visual symptoms have been seen.

A better approach is to know which fields are prone to deficiency and try to pre-empt the problem.

Boron can be applied as either soluble borate salts, to the soil or foliage. Where borate salts are applied to the soil, they are either broadcast or banded, according to the crop and soil conditions.

Iron will quickly become unavailable in many soils, so applying inorganic iron fertilisers is often ineffective. Fortunately, the application of iron chelates to soils does work. Alternatively, foliar application of soluble iron salts is effective and these can be used as an emergency measure to combat iron deficiency.

Zinc, copper and manganese deficiencies can all be corrected by applying soluble fertilisers to soil or in foliar sprays. Molybdenum requirements are low and its mobility in plant tissue is high, so it can be applied by soil, foliar or seed treatments.

Micronutrient formulations

There’s a vast and often bewildering product range to choose from. Whatever your purchasing decision, it’s essential that the micronutrients in your chosen product are in a form that makes them available to the plant.

For fertiliser products, this means that they mustn’t get locked up in the soil. While most micronutrients will remain in the soil after application, both manganese and boron will leach in certain circumstances.

With foliar sprays, the nutrient must be able to get into the plant. As a result, the elements must be soluble so that they can be taken up across the leaf surface. Another important consideration is that they are non-toxic and safe for crop use.

  • Boron – both techniques should be used together. Soil testing before planting is recommended and a value less than 0.8mg/litre of dry soil is considered to be a risk. Foliar tests also provide a good guide to potential shortfall, with a value below 5-100mg/kg of dry matter, depending on the crop, indicating a possible deficiency.
  • Chlorine – deficiency can be diagnosed by soil or leaf analysis. However, chlorine is rarely deficient and a crop’s requirement can usually be met by chlorine in the soil and in rainfall.
  • Manganese – soil testing can be variable and should be backed up by foliar analysis to confirm a diagnosis, with a value lower than 20mg/kg dry matter indicating a possible problem.
  • Zinc – leaf analysis is also the most useful diagnostic guide, where less than 15mg/kg may indicate a shortfall.
  • Nickel – foliar analysis is again the best guide, with leaf concentrations less than 0.1 mg/kg dry matter highlighting a possible deficiency.
  • Copper – leaf analysis is not a reliable indicator of a plant’s copper status, so soil analysis is the solution. A value lower than 1mg cu/litre of dry soil is a warning sign.
  • Molybdenum – deficiency can be diagnosed by either method.
  • Iron – neither soil testing nor foliar analysis is especially useful
  • Boron – death of growing tips and a stunted, bushy habit. In sugar beet, blackening at the leaf base and beneath the crown is characteristic, while carrots often have a darkening on the root surface.
  • Chlorine –wilting leaves, and as the deficiency becomes more severe, leaf curling and shrivelling, together with a spotty necrosis.
  • Manganese – pale, limp foliage, with spotty necrosis, reflecting the role of this element in antioxidant processes.
  • Iron – up to 90% of leaf iron is found in the chloroplasts, so deficiency symptoms include interveinal yellowing in younger leaves.
  • Zinc – the most common sign of zinc deficiency is interveinal chlorosis and “little leaf”, which is thought to be caused by altered auxin metabolism.
  • Copper – chlorosis in younger leaves, often followed by distortion of the leaf.
  • Nickel – nickel deficient plants accumulate toxic concentrations of urea, causing necrosis at leaf tips.
  • Molybdenum – without sufficient molybdenum, plants can’t convert nitrates to ammonium, so they lose their colour and suffer from reduced vigour.
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