Autumn is a good time to check your citrus trees for magnesium deficiency. Citrus have a high magnesium requirement and magnesium is essential for the formation of chlorophyll (green colour) in leaves. Without enough magnesium plants will not be able to make sugars and starches, and growth will be poor.
Magnesium deficiency often shows up in citrus in autumn because magnesium is also required for developing fruit and many citrus species produce fruit over the cooler months.
Because magnesium is very mobile in plants, a shortage of this essential element results in magnesium being drawn from the older leaves to new growth. Deficiency shows as pale leaves, beginning with inter-vein yellowing of the outer edges of the oldest leaves, so that a green V remains with the point of the V at the leaf tip, and widest part of the V closest to the stem. In extreme cases, entire leaves may yellow.
Magnesium deficiency can occur in several ways. If soil is too dry roots can’t absorb magnesium, so regular watering of citrus is necessary. It is more common where soil is quite acidic and this can be remedied by watering in some dolomite, which will supply magnesium, plus calcium to raise the pH. If soil pH is in a suitable range for citrus, magnesium deficiency can also occur where heavy rain has leached it from soil, or where excess potassium has been added to soil – this includes use of wood ash, or overuse of seaweed fertilisers, which can be very high in potassium.
In these situations, a quick remedy to save this year’s crop is to dissolve some Epsom salts in a small amount of warm water, then dilute it in a full watering can of cold water, and water it into the soil under the outer part of the foliage canopy. You will need about 250 g of Epsom salts for a very young tree, and up to 2 kg for a fully-grown tree.
Magnesium is also important for sweetness of fruit. If your citrus fruits are not as sweet as you would like, it could be due to magnesium deficiency.
However, a general yellowing or paleness of all leaves (chlorosis), while only the veins remain green, could be the result of iron deficiency. Iron deficiency begins in the youngest leaves. This can occur where soil is too alkaline for the tree to absorb iron. If the alkalinity occurred through an accidental overdose of lime or dolomite, the pH can be lowered by adding elemental sulphur to the soil around the tree. If soils are generally alkaline, including some well-rotted cow or horse manure that has NOT had lime added (under mulch, but not dug in) as part of your fertiliser will help reduce the pH by replacing some of the calcium ions in soil with hydrogen ions as it decomposes. To prevent crop losses, it is worthwhile checking soil pH around citrus trees each spring, and correcting it, if necessary.
See also: Feeding citrus