Manganese

Manganese is required for the oxygen-evolving complex of photosystem II, the part of photosynthesis that splits water molecules to release the oxygen plants produce. It also activates enzymes involved in respiration, lignin synthesis for cell wall strength, and antioxidant defence against oxidative stress.

Manganese availability responds strongly to soil pH and drainage, in the opposite direction to how these factors affect deficiency in most other micronutrients: manganese becomes more available as soils become more acidic or waterlogged, and less available as they become more alkaline or well aerated.

• High-pH mineral soils are prone to manganese deficiency
• Sandy soils and peat or other organic soils are also commonly deficiency-prone
• Strongly acidic, poorly drained soils are prone to the opposite problem — manganese toxicity

Manganese deficiency appears as interveinal chlorosis with a fine, still-green venation pattern, sometimes accompanied by small necrotic (dead) spots. It is most often seen on high-pH mineral soils and on peat or other organic soils where manganese availability is naturally low.

Manganese toxicity is common on strongly acidic, poorly drained soils, where the soluble Mn²⁺ form accumulates to excess. Symptoms include brown speckling and crinkling of leaves and stunted root growth. Raising soil pH through liming is the standard corrective approach in principle, though the appropriate rate depends on the soil.

Manganese competes with iron, zinc, and copper for uptake, so an excess of one can affect the availability of the others. Because manganese availability is so closely tied to soil pH, liming decisions made to manage other nutrients can shift a soil between manganese deficiency and toxicity risk.