Manganese deficiency accurs most often on high organic matter soils, on soils with neutral to alkaline pH and soil that are naturally low in Manganese content.
Plants show yellowing, veins remain green. Mature leaves may be 'mottled.' First signs of deficiency often show up as small pin-head sized brown specks on the leaves. Flower formation is reduced. In cotton the young leaves are yellow (lower leaves are less affected) sometimes grey to reddish-grey between veins. The veins remain green. Mn deficiency will delay maturity.
Functions in plant:
Aids in chlorophyll formation: serves as a catalyst in enzyme reactions such as breakdown of carbohydrates, formation of vitamins and nitrogen transformation.
Mobility in plant: Relatively immobile.
Mobility in soil: Relatively immobile (Soluble forms are usually fixed into insoluble forms within 1 year.)
Influence of soil pH: The availability of Mn drops off sharply as the pH goes from 5.0-7.0.
Factors affecting level: High pH; low organic matter usually will give low Mn levels; some sandy soils are low in Mn; poorly drained, calcareous silts and clays are often low.
Factors affecting utilisation: pH; high levels of Fe, Cu and Zn may reduce uptake of Mn; problems show up more during dry years-apparently because of better drainage, thus more oxidation and less solubility of Mn.
Adequate level in plant: 23 ppm in soybeans, 25-30 ppm in cotton, sugar cane 40-250 ppm, wheat 10-300 ppm.
Broadcast 5-20 kg per hectare (rate depends on soil pH and particular crop sensitivity); spray- 0.5-0.75 kg in 2-400 litres water per hectare. Mn sulphate is preferred over Mn oxide unless the pH of the fertilizer is very low.
The more acid forming the fertilizer, the more available the Mn. Mn toxicity occurs quite often below pH 5.0 plants vary widely in their response to Mn.