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Citrus deficiencies by Malcolm Campbell

There aren't many home gardens in South Australia or in the West without a few citrus trees growing to perfection.
I know the vagaries of "Fruit fly" in the Eastern states, make their growth less-than trouble-free and many wouldn't bother growing Citrus because it becomes a constant battle with insects. For those that grow them like weeds in the Mediterranean areas of Australia and even in the sub-tropical regions, you will soon discover that they have a few deficiencies which appear when grown in fairly alkaline soils. Alkaline soils are those with limestone in the lower profiles. In Mediterranean regions the soil profile generally gets more alkaline with depth, even if you mulch with acidic composts and leaf-litter on the surface.

The ideal conditions for most Citrus are warm, frost-free sites on free-draining soils. On alkaline soils in the pH range of 8 - 9, Citrus crops frequently develop lime-induced-chlorosis, which shows up on the leaves as a green vein pattern against a pale green leaf. This is a common condition for many native plants and acid-loving ornamentals for gardeners who work alkaline soils. The problem is that although iron is often available in these soils, the high pH or degree of alkalinity in the lower soil profile, makes the Iron uptake very slow. The remedy is quite simple on a small scale, but extensive use of Iron Chelates (pronounced 'kellates') has only a short-term benefit. More efficient and longer-lasting is Iron Sequestrine, but it is very expensive to use, on other than valuable container-grown plants. Just a word of caution. If you use Sequestrine on container-grown plants water it in over a garden bed because the liquid looks like burgundy and it will stain your cement paths. Also of use is the fairly cheap Flowers of Sulphur, but the sulphur needs to be added frequently. The organic ' solution' is to compost green matter and rely on the leaching of humic acid into the lower soil profile which certainly works, but most gardeners want a quicker response.

An even slower solution that does make the soil more acidic over time and then results in the uptake of available iron, is to bury old iron filings or pieces of iron off-cuts under your Orange, Mandarin or Grapefruit trees. As the iron oxidises it breaks down into forms that can eventually be used by your Citrus trees, but I stress that the process is a pretty slow one. Both organic and chemical remedies rely on warm soils and in Winter and early Spring even when the correct acidifying agents are added, they work very slowly. The message is, persevere with the problem until soil temperatures rise.

Another pretty common problem on alkaline soils is the lack of uptake due to the high soil pH of several minor trace elements, such as Zinc, Manganese and Magnesium. Their lack of uptake by most plants would go unnoticed except that Citrus have hungry appetites for nutrients and in particular, these three. The alkaline soils also make their uptake as difficult as for Iron. Even when you add a complete Citrus fertilizer to your soil, the presence of Potassium locks these minor trace elements up, so that they are not readily available to your Citrus trees.

That's why the experienced Riverland Citrus blocker or Sunraysia blockies apply the minor trace elements of Zinc, Manganese and Magnesium as foliar sprays, several times during the warm weather. Even this is not as straight forward as you would expect it to be. These three compounds cannot be sprayed at the same time, because they precipitate and settle out, thereby depriving your favourite Navel Orange of its due reward. The Zinc, can be applied as Zinc Sulphate and mixed with Manganese Sulphate then sprayed in late afternoon on a day when no rain is expected for at least three days. After three or four weeks, the Magnesium Sulphate or "Epson Salts" can be applied, also in late afternoon to reduce evaporation and applied when fine weather is predicted. To aid the uptake of Magnesium as a foliar spray, Nitrogen needs to be added to tepid water, usually in the form of Urea, because apart from being fairly cheap, it contains a massive 43% water-soluble Nitrogen, so you don't need to use much.

It all sounds a bit complicated doesn't it, but your Oranges and Lemons will put on lush foliage and flower prolifically as a result and of course set better quality fruit, so it's worth the effort.

While I'd like you to be able to diagnose the specific deficiencies on your Citrus, I have to admit it's a lot easier to make the soil a little more acidic on alkaline soils, then foliar spray with the three minor trace elements even if you're not sure of the exact deficiencies on your particular trees.

If you insist though, try following this for a simple identification of Citrus deficiencies.

Zinc deficiency appears on the young leaves as a general mottling that causes the areas between the veins to turn pale yellow. In it's more chronic stage, the leaves turn totally yellow and die back leaving dead twigs, so that every attempt to sprout new leaves sees tiny rosettes of leaves form that lack vigour. This is most pronounced on the Northern and Western side of the tree, due to the fact that those are the warmer sides that metabolise more rapidly. Well that's my explanation anyhow!

Manganese deficiency looks a bit like chlorosis if you are familiar with that, but the green veins are less easily defined and the leaf stays basically green. It looks a bit like Zinc deficiency in its early stage, but does not result in the small leaf growth that is common with Zinc deficiency. When applying Manganese be sure to avoid spraying at fruiting time because the compound leaves a dark discolouring residue on the foliage and fruit.

Magnesium deficiency has very distinctive symptoms that turn the leaf pale yellow at the tips and usually leaves the base of the mature leaves with a green triangular shape. This is often described as an inverted green V. Magnesium can be deficient in both acidic soils and alkaline soils, where it may be available, but chemically locked up in such a way that it is not available to the Citrus plant through its root system, hence the need to add it as a foliar spray.

I'm surprised that there don't appear be any dual packs to remedy these minor trace element deficiencies on the market and that most manufacturers only market minor trace elements as soil-applied fertilizers, rather than as foliar sprays. On a recent trip to five nursery outlets in two states , I could not find a single nursery or garden centre that could supply me with the necessary compounds containing these vital trace elements to use as foliar sprays on Citrus. Some outlets had one only but they were only available in 500 gram packets, which would last one tree about 50 years! I hope you have better luck with your suppliers, but persevere in seeking them out and using them several times this Summer. You'll certainly notice the difference with more robust trees that can almost be heard to thank you for your consideration!

Further reading

Baxter, P. and Tankard, G. The Complete Guide to Growing Fruit in Australia, Fourth Edition, published by Macmillan Co of Australia Pty. Ltd. South Melbourne, 1990.

Bond, R. [Ed.] All about Citrus & Subtropical Fruits, published by Ortho Books, Berkeley Ca, USA), 1985.

Department of Agriculture and Rural Affairs' Gardening Guide (Vic), published by Schwartz & Wilkinson, Melbourne, 1990.

Department of Agriculture New South Wales, Home Fruit Growing, 1985.

Geoff Godden, Growing Citrus Trees, Lothian Publishing, Port Melbourne, 1988.

Judy McMaugh, What garden pest or disease is that? Companion Edition, Weldon Publishing, Willoughby NSW, 1986.