While my lime and orange trees have dark green foliage, my Eureka Lemon leaves have been a sickly, uneven green despite nesting in deep, damp loam and receiving some modest applications of complete trace element powder and decomposing sugar cane mulch over time.
It would be nice if nature had colour-coded deficiencies in the major plant nutrients (Nitrogen, Phosphorous and Potassium) in the color of foliage, however it is not so simple.
Firstly, it is the lesser known trace elements (Iron, Magnesium, Manganese and Zinc) that are frequently the cause.
Secondly, the result is always a yellowing of the foliage, requiring a careful cross-checking against pictures to look at the location and extent of the yellowing, along with reading about secondary symptoms, to determine which nutrient is deficient.
So here is a link with the pictures for the task along with descriptions of secondary symptoms from the NSW DPI website.
I also have color photo copies of nutrient deficiency pictures from Allen Gilbert's Citrus book available at Brunswick Library at (shelf location 634.304 GIL, ISBN 9781864471038) which is quicker and easier to use in the garden than browsing on a smartphone.
|Manganese deficient leaves (NSW DPI)|
According to the link above, "Manganese deficiency more acute when Nitrogen levels low" so this supports my tentative theory. As also mentioned in the link, the problem is indeed worse on the south side of the tree (how specific is that!), and this only occurs with Manganese and Iron and I have ruled out Iron deficiency for the moment.
So, I will start treatments with Nitrogen and Manganese.
Treat how I hear you ask?
Nitrogen is in all fertilisers/manures in varying degrees and trace element powders are available at nurseries/big hardware stores, although Manganese Sulphate is very hard to get hold of. Fortunately, the Richgro Complete Trace Elements mix has 3.1% Manganese which I will apply only after having first treated the Nitrogen problem using Sulphate of Ammonia (I like to keep my garden organic and won't use pesticides but I don't mind moderate use of inorganic nutrients because I want to get this plant strong again asap as citrus leaf miner has also been hurting the tree).
How to avoid this problem in the future?
Well, citrus, particularly lemon, is notorious for being greedy, and all ever-greens need plenty of Nitrogen, so switching sugar cane mulch to a permanent mulch of nitrogen rich lucerne hay should keep the hungry beast happy and good Nitrogen levels should improve the uptake of Manganese.
If you want a more advanced discussion of this topic dealing more with soil pH and the advantages of foliar rather than soil feeding, see this link (I did test the pH of the soil around the lemon and it is pH 6 which is fine for citrus).
Interestingly, the leaves on our figs and one of the pomegranates have similar patterns as my lemon, so this article may apply to more than just citrus.
(from Paul - Coburg Community Gardening Member)