ARCHIVE: Fertiliser Manual (RB209)

Micronutrients (trace elements)

Micronutrients or trace elements are those crop nutrients required in small amounts for essential growth processes in plants and animals. Some micronutrients that are essential for animals are not required by plants but the animal usually acquires them via the plant. In practice only a few micronutrients are known to be present in such small amounts in soil in England and Wales that there is a risk of deficiency in plants and animals. Deficiency is most frequently related to soil type, soil pH, soil structural conditions and their effect on root growth, and crop susceptibility.

Visual symptoms of a deficiency of a specific micronutrient can be confused with those produced by other growth problems. Consequently visual diagnosis of a micronutrient deficiency should, where possible, be confirmed by plant and/or soil analysis.

Deficiencies affecting crop growth

Boron (B): Deficiency can affect sugar beet, brassica crops and carrots on light textured soils with a pH above 6.5, particularly in dry seasons. Symptoms include death of the apical growing point and growth of lateral buds. In sugar beet, there is blackening at the leaf base and beneath the crown (‘heart-rot’). Carrots can show a darkening of the root surface (‘shadow’). Soil analysis prior to growing a susceptible crop is recommended. When extracted with hot water, a value less than 0.8 mg B/litre dry soil is associated with a risk of deficiency. Leaf analysis is also a useful diagnostic guide and a value lower than 20 mg B/kg dry-matter may indicate deficiency (although the deficiency value varies between crop species).

Copper (Cu): Deficiency is not widespread in crops but can occur mainly in cereals on sands, peats, reclaimed heathland and shallow soils over chalk. Sugar beet may also be affected. In cereals, symptoms are yellowing of the tip of the youngest leaf followed by spiralling and distortion of the leaf. Ears can be trapped in the leaf sheath and those that emerge have white tips. Barley awns can become white. Soil analysis is useful for identifying whether deficiency is likely. When extracted with EDTA, a value lower than 1 mg Cu/litre dry soil indicates possible deficiency. The copper content of the leaf does not reliably indicate the copper status of the plant.

Iron (Fe): Deficiency occurs commonly in fruit crops grown on calcareous soils, but is not a problem in annual field crops. Symptoms in fruit are yellowing of the young leaves with veins remaining green. Deficiency cannot be reliably diagnosed using soil or plant analysis.

Manganese (Mn): Manganese is the micronutrient most commonly deficient in field crops. Deficiency occurs in many crops on peaty, organic and sandy soils at high pH but can occur less severely on other soils when over-limed. Susceptible crops include sugar beet, cereals and peas. In cereals, deficiency often shows as patches of pale green, limp foliage. Sugar beet leaves develop interveinal mottling and leaf margins curl inwards. Dried peas show internal discolouration when the pea is split (‘marsh spot’). Leaf analysis provides a reliable means of diagnosis with a value lower than 20 mg/kg dry matter indicating possible deficiency. Soil analysis is not a reliable guide to deficiency.

Molybdenum (Mo): Deficiency is associated with acid soils and is not generally a problem in limed soils. Cauliflower may be affected and symptoms include restricted growth of the leaf lamina (‘whiptail’). Soil or plant tissue analyses may be used to diagnose molybdenum deficiency. Soil analysis is usually by extraction with acid ammonium oxalate (‘Tamms reagent’). The leaf and curd content, in dry material, is around 2.0 mg Mo/kg in normal cauliflower plants and around 0.35 mg Mo/kg in deficient plants.

Zinc (Zn): Deficiency is rarely found in field crops. In the few cases where deficiency has been found, it has been on sandy soils, with a high pH and phosphate status. Top fruit and forest nursery stock are most likely to be affected. Leaf analysis is the most useful diagnostic guide and, in susceptible crops, less than 15 mg Zn/kg dry matter may indicate deficiency. Soil analysis usually is by EDTA extraction; for susceptible crops, a value less than 0.5 mg Zn/kg indicates a risk of probable deficiency while less than 1.50 mg Zn/kg indicates possible deficiency.

Deficiencies affecting livestock performance

The availability of cobalt, copper and selenium does not restrict grass growth, but too little in grazed crops can led to deficiency in some animals. Where a deficiency has been correctly diagnosed, treatment of the animal with the appropriate trace element is usually the most effective means of control, though application of selenium and cobalt to grazing pastures is effective.

ADLib logo Content provided by the Agricultural Document Library
© University of Hertfordshire, 2011