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Saturday 23 September 2017

Tracing best elements

Aim for a balanced supply of each nutrient at growing to prevent drops in yield

Manganese deficiency is one of the most encountered trace element deficiencies in both winter and spring cereal crops
Manganese deficiency is one of the most encountered trace element deficiencies in both winter and spring cereal crops

Mark Plunkett

Trace elements have a vital role to play in the production of high-yielding cereal crops on an annual basis. Cereal crops have higher requirements for major nutrients such as nitrogen (N), phosphorus (P) and potassium (K).

These nutrients are usually required at kilogrammes per hectare rates, whereas trace elements are normally required in grams per hectare rates. It is critical that there is a balanced supply of each nutrient throughout the growing season as temporary shortage at critical times of the year can result in a yield reduction.

In this article, I will discuss the main trace element deficiencies affecting winter and spring cereal crops in Ireland.

Manganese

Manganese (Mn) deficiency is one of the most encountered trace element deficiencies in both winter and spring cereal crops. Deficiencies tend to be due to low soil Mn availability for crop uptake rather than an absolute deficiency in the soil. Manganese is essential in the early stages of crop development, especially in root and tiller development. Crops with sufficient levels of Mn tend to be less prone to take-all infection.

There are several soil conditions that reduce the availability of soil Mn, including:

  • Recently limed or over-limed soils;
  • High soil organic matter or peaty soils;
  • Poorly consolidated or puffy seedbeds;
  • Light/sandy, well-drained soils with a pH greater than 7.0.

In addition, other seasonal and weather factors can play a part, such as low soil temperatures plus wet/dry soil conditions that may reduce soil Mn availability and aggravate a Mn deficiency on a wide range of soils (in the early stages of crop development).

Mn deficiency is easily detected, as it can be associated with parts of a field where there will be patches of pale yellow, floppy plant growth. The younger foliage is first affected, turning a pale green colour.

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Dark green lesions appear on the older leaves of oats and these fade to a dull, grey-green colour. Because of the presence of these lesions, Mn deficiency is often described as 'grey speck' in oats. In barley, a series of dark brown purple dots appear along the leaf veins in older leaves. In wheat, these dots are tan coloured. In severe cases, plants will be stunted, leaves will wither and the plant will die.

Leaf analysis is the most reliable way to confirm a deficiency of Mn rather than soil analysis, and the youngest leaves should be sampled rather than the whole plant.

Cereal crops grown on deficient soils should receive a split application of foliar Mn with the first application at the 2-4 leaf stage, followed up 2-3 weeks later with a second application.

Zinc

Zinc (Zn) deficiency is quite common here and was first identified in Co Louth in 1985.

Good, deep tillage soils with a high percentage of stones are most at risk of Zn deficiency. These are the shale soils that are found in Louth and Meath. Soils with low levels of Zn are also found in Carlow, Laois, Donegal, Wexford, Kilkenny, Tipperary, Cork and Waterford.

Soils with low levels of organic matter, commonly found on intensive tillage soils, will tend to have low levels of Zn. Soils with high soil P levels and high soil pH (>7.0) will reduce the availability of Zn and induce a Zn deficiency. The application of N and magnesium improves the response to Zn sprays.

Zn deficiency generally shows up in the early stages of growth, when the plant is only a few inches high, appearing first as yellow streaks in the young leaves with a white to yellowish tip. White spots often appear in the leaves or along the edges and a portion of the marginal area may die. Frequently, the entire plant is stunted due to shortening of the internodes.

Soil analysis is a reliable guide to diagnosis or prediction of Zn deficiency. When soil Zn levels are less than 1mg/l, a severe yield reduction has been found in barley and wheat. On deficient soils, treat crops at the 2-4 leaf stage and again at mid-tillering to stem extension (growth stage 25-30).

Copper

Copper (Cu) deficiency occurs mainly on coarse-textured, sandy and organic soils derived from acidic and igneous rocks. Soils low in Cu are common in the old red sandstones soils of Cork, soils derived from granite in Wicklow and east Kildare, and sandy and gravely soils of Carlow and south Kildare.

The symptoms of Cu deficiency are not as specific as those for most other trace elements. In the early stages (2-4 leaf stage to early tillering), plants are stunted, pale green in appearance and the tips of the youngest leaf becomes papery/ white and leaf spiralling may occur. Towards the end of tillering, plants are drawn and pale green in colour with white tips. The internodes are long and the plants become flaccid and are prone to lodging. Symptoms may not appear until flag leaf or even ear emergence. Affected ears are white and 'rat-tailed' in appearance, with blind grains in the ear. Therefore, it remains essential to check soil copper levels and treat crops in the early (growth stage 12-14) stages of development to overcome any crop yield reduction.

On deficient soils, an application of copper sulphate (10-20kg/ha) will give long-term control of Cu deficiency and will be effective for many years. This should be applied before sowing and cultivated into the seedbed as Cu is immobile in the soil. It is best to apply a foliar to the crop for two years after a soil application.

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