Molybdenum deficiency (plant disorder)

Molybdenum deficiency is a plant disorder that occurs when plants do not have access to sufficient molybdenum, an essential micronutrient. Molybdenum plays a crucial role in nitrogen metabolism, particularly in the enzymes nitrogenase (involved in nitrogen fixation in legumes) and nitrate reductase (involved in nitrate assimilation in all plants). It is also involved in sulfur metabolism and hormone metabolism.

Symptoms:

The symptoms of molybdenum deficiency can vary depending on the plant species and the severity of the deficiency. Common symptoms include:

• General chlorosis (yellowing): Often starting in older leaves due to molybdenum's mobility in the plant. Unlike nitrogen deficiency, the entire leaf may become uniformly chlorotic rather than just showing interveinal chlorosis initially.
• Stunted growth: Reduced overall plant growth and development.
• Nitrogen deficiency symptoms: Plants may exhibit nitrogen deficiency symptoms even if adequate nitrogen is present in the soil, as molybdenum is needed to utilize nitrogen properly.
• Cupping or rolling of leaves: Leaf margins may curl upwards or inwards.
• Necrotic spots: Dead or dying tissue may appear on leaves, especially in severe cases.
• "Whiptail" in Brassicas: A characteristic symptom in some Brassica species (e.g., cauliflower, broccoli) where the leaf blade fails to develop properly, resulting in a long, narrow, and distorted leaf resembling a whiptail.
• Poor flower formation and seed production: Reduced yield due to impaired reproductive development.

Causes:

Molybdenum deficiency is most likely to occur in:

• Acidic soils: Molybdenum availability decreases with decreasing soil pH.
• Sandy soils: Soils with low organic matter content have a lower capacity to retain molybdenum.
• Soils with low molybdenum content: Some soils are naturally deficient in molybdenum.
• High sulfur content: Excessive sulfur can interfere with molybdenum uptake.

Diagnosis:

Diagnosis can be confirmed through soil and plant tissue analysis. Soil tests will measure the level of available molybdenum in the soil, while plant tissue tests will determine the molybdenum concentration in the plant leaves.

Treatment:

Molybdenum deficiency can be corrected by:

• Soil amendment: Applying molybdenum-containing fertilizers to the soil. Sodium molybdate and ammonium molybdate are common forms.
• Foliar application: Spraying molybdenum solutions directly onto the plant leaves. This method is effective for rapid correction of deficiencies.
• Liming: Increasing the soil pH through the application of lime. This improves molybdenum availability in acidic soils.

Prevention:

Preventive measures include:

• Soil testing: Regularly testing soil pH and nutrient levels to identify potential molybdenum deficiencies.
• Liming acidic soils: Maintaining an appropriate soil pH range (typically between 6.0 and 7.0) to ensure adequate molybdenum availability.
• Applying molybdenum fertilizer: Applying small amounts of molybdenum fertilizer to soils known to be deficient, especially before planting molybdenum-demanding crops like legumes and brassicas.
• Using molybdenum-coated seeds: Coating seeds with molybdenum can provide seedlings with an initial supply of the nutrient.