Role in grapevine function
Iron plays an important role in chlorophyll formation, energy trapping and transfer in photosynthesis and respiration, and cell strengthening.
Mobility and availability in soils and the vine
Soils that are truly deficient in iron are rare and are generally leached or wind-blown sands.
Most soils contain adequate iron, but deficiency in vines can result from:
• A bicarbonate inhibition of uptake of iron ions at the root surface. High levels of bicarbonate are found in alkaline soils, due to free lime or sodicity, and are aggravated by poor aeration due to soil compaction and/or waterlogging. If soil alkalinity alone is inducing iron deficiency, the severity of symptoms will not change much during the season. Variations in tolerance will influence the usefulness of some of the rootstock cultivars (derived from less tolerant American Vitis species) in such conditions.
• In waterlogged soils, symptoms of iron deficiency often disappear if irrigation is withheld. Vines and weeds can gradually reduce the quantity of water in the root zone and improve aeration.
• Iron can also become more available as soils warm up later in the season. Iron is immobile within the vine.
Symptoms of deficiency
Iron deficiency results in general yellowing (chlorosis) of young leaves and new growth. Insevere cases, total shoot chlorosis can occur making these tissues appear almost white. This total shoot chlorosis can then be followed by the development of necrotic spots between the veins.
When deficiency is not severe, veins retain their green colour. Iron deficiency is most likely to occur in vines on soils with an abnormally high pH.
‘Looks like’
Iron and manganese de?ciencies can be confused, however iron deficiency always shows up on the youngest parts of the shoots, whereas manganese deficiency is seen on older leaves. Symptoms similar to iron deficiency can also be caused by excessive uptake of the herbicide simazine. Symptoms of iron toxicity are rarely seen in grapevines.
Nutrient management
Leaf analysis for iron is not a reliable indicator of iron status in the vines. Soil tests are also unreliable. Generally, the observation of deficiency symptoms on vines will provide the best estimate of iron status. Any test results should always be considered in conjunction with obvious symptoms and overall vigour of vines.
Fertiliser application
It is not possible to correct iron de?ciency easily. Soil conditions that lead to iron deficiency usually quickly render soil-applied iron compounds insoluble. Iron fertilisers are usually based on either a chelated or sulfate form of iron.
Only some chelated iron products contain a material that is able to protect the iron from bicarbonate-induced inhibition of uptake when applied to alkaline or high calcium soils. The best known is Fe-EDDHA.
EDTA based iron chelates are not suitable for use in calcareous soils.
A need for use for these products should be clearly established as they can be expensive. If deficiency is an issue, apply iron to vines immediately.
Foliar treatment with iron sulphate or iron chelates is only successful if good coverage is provided because iron does not move far from its point of entry to the vine, e.g. stomata, and re-greening of leaves can be patchy.
Excess concentrations of iron(>150g iron in 1000L spray) in foliar sprays can cause leaf burn.
Model iron fertiliser requirements
For pre-planting and young vines
The installation of a drainage system may be necessary if waterlogging is likely to be a problem in a planned vineyard development, because iron deficiency can be related to waterlogging. If deficiency symptoms are severe growers should apply iron chelates.
In mature vineyards
Maintenance of appropriate soil pH and drainage are the best ways of ensuring adequate availability of iron (and other trace elements) to grapevines. Irrigation control to limit waterlogging is the best preventative and corrective measure for iron deficiency if waterlogging is the cause. Otherwise, or if symptoms are severe and deficiency clearly established, growers should apply iron chelates as per manufacturer’s recommendations.
FURTHER INFORMATION
Product or service information is provided to inform the viticulture industry about available resources, and should not be interpreted as an endorsement.
Further detail on trace element, nutrition, deficiency symptoms and management, and petiole testing can be found in the Grapevine Nutrition: Research to Practice™ training manual, Cooperative Research Centre for Viticulture, Adelaide 2005.
