Manganese (chemical symbol Mn) deficiency occurs in a wide range of crops with onions, beetroot, parsnip, cabbage, cauliflower, tomato and pumpkin the most susceptible.
This deficiency is most common on alkaline soils (high pH), particularly if the irrigation water contains high levels of bicarbonate. It is found on soils of the Cottesloe and Karrakatta associations when high rates of phosphate fertiliser are used.
Manganese is needed for a number of plant functions including chlorophyll synthesis. It is a partially mobile element in the plant so symptoms may first appear in the youngest or oldest leaves.
In general, affected crops are pale green and growth is reduced. Specific symptoms may first appear on the youngest or oldest leaves and vary from species to species.
The most common symptom is for leaves to turn pale green between the veins, with normal coloured areas next to the veins. As the deficiency progresses, the area between the veins becomes paler, enlarges and may brown and die.
In cabbage, the interveinal chlorosis symptom is replaced by a general mottled yellowing of the leaves.
Beetroot shows triangular or spear-shaped leaves with the edges curled forward, as well as yellow mottling with small dead patches which give the leaf a typical speckled appearance. These symptoms are so distinctive in this species they are called ‘speckled yellows’.
In onions and sweetcorn, the interveinal chlorosis appears as yellow stripes on the leaves.
In tomatoes, the veins remain green, while the tissue between the veins becomes increasingly yellow as the deficiency becomes worse. This causes a net-like pattern to appear on the leaves.
Manganese deficiencies are most often observed on well drained neutral or calcareous soils. However, other soils may cause manganese deficiencies, particularly as a result of heavy fertiliser usage. It can also be induced on these soils by heavy applications of lime.
In practice, manganese deficiency in vegetables does not occur on acid swamps except after they have been heavily limed, but is common on alkaline marl-based swamps. It is also common on sands containing limestone.
Manganese deficiency is controlled by using manganese sulphate (MnSO4 .7H2O) as a soil applicant or a foliage spray. Chelated forms of manganese can also be used as a foliar spray although this treatment is more expensive.
Soluble manganese quickly reacts with the soil to produce less available forms. Application in a band minimises such reactions and is therefore more efficient. Rates can be lower than broadcast application.
For a broadcast application, apply 50kg/ha of manganese sulphate or 10 to 20kg/ha if applied in a furrow or band.
Sometimes it has been difficult to control manganese deficiency by soil applications, but good control has been obtained through foliage spraying.
Foliage spraying is usually the best way of correcting manganese deficiency as relatively low rates are as effective as high rates of soil application.
A 0.8% spray (8g/L) applied at 500L/ha supplies 4kg/ha manganese sulphate. Add a wetting agent for better leaf coverage. A second or third application may be needed.
The spray is most successful when plants are fairly young but good responses have been obtained when plants are more than halfway through their growing period.
Deficiency symptoms in most species are associated with leaf levels less than 20mg/kg with particularly severe symptoms at less than 10mg/kg. Healthy plants normally contain 50 to 200mg/kg of manganese although levels up to 1500mg/kg have been recorded where fungicides containing manganese, such as Mancozeb®, have been applied.
The original version of this material was authored by M Hawson.