Zinc deficiency
Molybdenum deficiency can occur on lighter acid soils, especially red, loamy acid soils. Boron, zinc, manganese and copper (especially on heavier clay soils at pH higher than 7).
Symptoms of trace element deficiencies in lucerne include:
• Manganese (Mn) deficiency — reduced growth, strong interveinal yellowing confined to the youngest leaves, veins remain green. Note that Mn toxicity may occur in acid soils — older leaflets become yellowed around the margins and cup downwards.
• Zinc (Zn) deficiency — reduction in size and upward curling of youngest leaves followed by appearance of brown-to-bronze spots on the upper surface of top leaves but not on the youngest leaves.
• Molybdenum (Mo) deficiency — occurs mainly on acid soils. Because Mo is essential for strong nodulation of roots by rhizobia, deficiency symptoms are similar to those of N deficiency.
Copper deficiency
• Boron (B) deficiency symptoms — leaves become yellow or reddish yellow and plant tops are bunched. Later, the yellowing extends to the interveinal tissues and the upper leaf surface yellows and turns red to reddish yellow. Lower leaves remain a healthy green colour.
• Copper (Cu) deficiency — wilting of younger leaves, a striking curvature of younger leaf petioles causing leaflets to bend back against the lower part of the petiole. Yellowing does not develop. Growing point may die followed by death of leaflets and petioles. Plants may appear drought stressed.
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Table 6.2 Minor elements needed by lucerne
a Availability depends on soil pH — Mn may become toxic at pH below 6.0.
b At soil pH below 7, 0.5 mg/kg is required and where soil pH is above 7, 0.8 mg/kg is the critical level.
c Mo requirement is best determined using test strip sprays, and deficiencies are normally expected only in soils with pH lower than 6.0-6.5.
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