One of the most common nutrient deficiencies seen on citrus is the yellowing of the leaf with dark green veins.
- Yellow leaves with dark green veins on older leaves indicates magnesium deficiency, and is corrected using Epsom salts (Magnesium sulphate).
- Yellow leaves with dark green veins on young leaves indicates iron deficiency, and is corrected using Iron Chelate.
Citrus are heavy feeders and are prone to nutrient deficiencies in autumn when they’re fruiting heavily and maturing their fruit, and magnesium deficiency is a common occurrence with citrus during this period.
Magnesium deficiencies can occur also when the soil pH is too acidic (pH 5.5 or lower) but this is rather uncommon in Australia as this phenomenon occurs in acidic sandy soils where magnesium leaches readily.
When magnesium deficiency first appears in citrus, the yellowing of the leaf between the green veins begins at the tip and edges of the leaf, and moves down towards the leaf stem (petiole). With prolonged deficiency, these areas can turn completely yellow, leaving a dark green inverted V-shape at the base of the leaf.
Iron deficiencies tend to occur in soils that are too alkaline, as a high soil pH makes the iron in the soil less available to plants. High soil pH conditions are a more common occurrence, as heavy applications of garden lime or mushroom compost (which is loaded with garden lime) can make the soil excessively alkaline. In addition to using iron chelate in the short term, such conditions are best corrected in the long term with the application of sulphur to lower the soil pH.
How to Differentiate Magnesium Deficiency from Other Nutrient Deficiencies
Magnesium (Mg) is a secondary macronutrient which is mobile in the tree, it is readily translocated from old leaves to new growth, so magnesium deficiency occurs only on mature leaves which were previously normal and healthy in appearance. Magnesium deficiency symptoms can appear on branches bearing a heavy crop, but not on other branches on the same tree with little or no fruit.
The micronutrients Iron (Fe), Zinc (Zn), Manganese (Mn) and Copper (Cu) are all immobile, they are not translocated from old leaves to new growth, so the symptoms of these micronutrients deficiencies only develop on new growth.
By looking at which stage of the leaf growth the nutrient deficiency occurs, we can easily rule out the unlikely cause. Having said that, it’s also entirely possible for a citrus tree to have both an iron and magnesium deficiency at the same time, and in such cases, we treat for both.
How to Use Epsom Salts to Treat Magnesium Deficiency
You can buy Epsom salts (Magnesium sulphate) from a garden supply centre or garden nursery, and it’s exactly the same Epsom salts that you can purchase from your supermarket and use in your bathwater for a relaxing hot bath, so either can be used for correcting magnesium deficiency.
- For fruit trees and large shrubs, apply 20g (4 teaspoons) of Epsom salts (Magnesium sulphate) per square metre (square yard), spreading evenly around the drip line of the tree or shrub, then water in well. Wash off any granules that have landed on plant foliage. Also, don’t apply any closer than 10cm (4”) to stems or trunks.
- Others recommend dissolving 10g (2 teaspoons) of Epsom salts (Magnesium sulphate) in a litre of water and applying at a rate of 1 litre per square metre of garden bed with a watering can.
- For a quicker result, some gardeners apply Epsom salts as a foliar spray. Dissolve 10g (2 teaspoons) of Epsom salts (Magnesium sulphate) in a litre of water and spray over the leaves. Some sources discourage application onto foliage, as the amount of magnesium that can be absorbed through the leaves is quite limited, and a magnesium foliar spray may burn the leaves.
Applications of Epsom salts can be repeated monthly.
Repeat applications may be necessary as the form of magnesium in Epsom salts is highly mobile in soil and washes out easily.
How to Use Iron Chelate to Treat Iron Deficiency
Iron chelates are compounds made up of iron attached to an organic (carbon-containing) molecule to make it usable by plants, as plants can’t absorb elemental iron or simple iron compounds very easily.
You can buy iron chelate from your garden supply centre or garden nursery, it comes as either an iron-lignosulfonate chelate or an iron-EDTA chelate, it’s always in a small bottle and isn’t cheap, but you only need a small amount with each application.
To correct iron deficiencies, iron chelate is mixed with water and applied as a foliar spray over the leaves, or as a soil drench, watered in around the roots. Follow product instructions for how to apply, how much to use and when to best apply.
Applications of iron chelate can be repeated every 2-4 weeks.
In warmer weather the leaves should green up within a week, if not, reapply as necessary.
Similar But Reversed Leaf Symptoms
If the symptoms are the opposite of the one’s discussed here, then see the the article – citrus leaves are green with yellow veins for an explanation.