From the Mossman Branch, Archives of the Rare Fruit Council of Australia, inc.
by Jim Wait


Seasons in Australia are opposite to those in the US.  Summer is Dec. Jan. Feb. Autumn is Mar. Apr. May. Winter is June July Aug. Spring is Sept. Oct. Nov.

The Use of Trace Elements in Tropical Fruit Crops


All elements have their uses. All are equally important. They just vary in the quantities needed by the plant. Nitrogen, Phosphorus, and Potassium are the major or macro elements. Calcium, Sulphur and Magnesium are the minor elements.

Zinc (Zn), Iron (Fe), Copper (Cu), Boron (B), Molybdenum (Mo), Manganese (Mn) and Chlorine (Cl) are the trace elements.

DECIDING THE PLANTS' NEEDS

The best way to diagnose deficiencies is by soil and leaf analysis on a yearly basis. Then by comparing the year by year results with cropping records and nutrient inputs, it is possible, over time, to accurately decide what range of nutrients need to be applied.

To warrant a yearly soil and leaf analysis would need probably 2 ha of an individual crop species. Good diagnosis by soil and leaf analysis, however, requires large numbers of samples taken and analysed to establish good working background figures.

Smaller areas could be done every second year to bring the costs to an economic level.

But what of the mixed orchard of a couple of trees of many species. This situation relies on a combination of several rather inaccurate methods to come up with something workable.

Things such as visual symptoms, soil and leaf analysis on adjoining farms, or soil analysis on the same farm or just plain guessing from recorded needs of the crop or similar crops, can all be used to establish a workable nutrition program.

Visual analyses are poor. Plants can be deficient of 80% of their individual nutrient needs before obvious visual symptoms are seen. On top of this the deficiencies of various nutrients tend to merge or have points in common. Looking at colour plates of deficiencies on one crop to try to diagnose deficiencies in a different crop can be confusing and misleading. Also there are no deficiency colour plates for our new tropical fruit crops.

Working off visual symptoms is probably about the worst way to be growing your crop. It is shutting the gate after the horse has bolted - a year or season's crop lost or severely depleted, before some corrective action can be taken.

Soil analysis from surrounding farms is a good general guide if we have a standard soil type. But coastal soils vary considerably, with the potential for making some disastrous recommendations.

Cane soil analysis is a help, but at least until recently, these did not include trace elements analysis.

My diagnosis in the field here this afternoon will be largely on visible symptoms, but as I have explained - you must be aware that even with the best guesstimation this can be a highly inaccurate method.

Some deficiencies have very tell-tale symptoms which are confined to individual elements. With others the usual symptoms are indistinct and it is not possible to diagnose visually.

ZINC

Zinc is an element that is deficient in most of our North Queensland soils. In the plant, zinc is used to produce plant growth substances called auxins. Auxins give full cell expansion in new growth, producing large leaves on long stems.

Common deficiency symptoms are leaves of uneven size on the tree, with the largest leaves being at the start of the growth flush when the small amounts of zinc available would produce enough auxin to fully expand the first few leaves. Thereafter the leaves get progressively smaller as the growth flush continues.

Pale, uneven green leaves are another symptom, but this can be shared with deficiencies of other elements.

Mottle leaf in citrus is a severe long term deficiency. It is one of the most distinct of all deficiency symptoms. This symptom is often more common toward the end of the fruiting season.

Small leaves and dieback of terminal growth occur in Annonas, Rollinia and ornamental Inga sp. Autumn symptom in custard apples carrying a heavy crop of fruit is similar to mottle leaf in citrus. Leaf growth in this case is often of normal size, as the deficiency has occurred as the fruit fill and the seeds mature.

Macademias produce the small leaf symptom, and if pruned back they will produce clusters of leaves in a rosette form.

Lychee, Rambutan and Longan also show the small leaf symptom. In addition the Rambutan and Longan can show a weak, mottle leaf symptom.

Pulasan - as the zinc deficiency gets progressively more severe, the plants go through the range of symptoms from small leaves to the weak mottle leaf symptom to leaf shedding and finally terminal dieback. Adequate zinc levels are needed to get pulasan to flower and set fruit.

Zinc deficiency can usually be cured with a foliar application of zinc sulphate hepta hydrate at the rate of 1 gram per litre of water with a good coverage of the foliage. Use the hepta hydrate form of zinc sulphate - not the monohydrate form which is fairly insoluble. Spray to wet the foliage without runoff. Zinc chelate can be used in place of zinc sulphate.

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Rollinia, Sour Sop, Custard Apples - One spray on matured spring growth. Trees can be resprayed in Autumn if needed. Soil applications can also be made at the rate af 25 gms per square metre of the soil surface of the main root zone. This treatment should be sufficient for three years.

Citrus - A foliar application in spring time when the growth flush is about three quarters expanded should be sufficient. Alternatively, treat the soil with zinc sulphate at 10 grams per square metre. On newly planted trees treat 1m2, adding a m2 each year till year 10 when 10m2 is treated using 100 grams of zinc sulphate. The rate then remains constant at 100 gms over 10m2 per tree per annum. Spread the zinc sulphate evenly over the soil surface.

Macademia - Use foliar spray in spring as in citrus. Soil application can also be applied.

Papaws - Use 5 gms of zinc sulphate heptahydrate per litre of water as a foliar spray. Use two applications per year e.g. spring and autumn. Dithane (fungicide) sprays used alone do not provide zinc to the papaw tree. If mixed with an oil spray, the zinc does became available.

Avocado - Soil rates as for citrus.

Longan, Lychee, Pulasan, Rambutan - Foliar and soil rates as for citrus fruits, but with Pulasan give a second foliar spray in autumn as a matter of course.

Croton and Hibiscus - These ornamental plants also respond to foliar applications of zinc sulphate. Apply twice per year in each of spring and autumn at the rate of 1 gm/L.

WARNING: Foliar sprays at a concentration higher than recommended can cause leaf burn and defoliation.

COPPER

The symptoms of copper deficiency are best documented in citrus and macadamia, i.e. crops which have been grown in large areas for many years with the consequent build-up of knowledge.

In citrus, large dark green leaves on soft, angular stems are the first symptoms. This is followed by wavy branches due to uneven growth. As the deficiency increases, terminal dieback and multiple terminal branching with some stem cracking and gum ooze develop. Copper deficiency is also recorded as causing split fruit in citrus.

Copper deficiency depresses zinc uptake, so copper and zinc often go together even if zinc is not actually deficient in the soil.

In citrus, copper deficiency is overcome with the routine copper oxychloride fungicide sprays. In the tropical high summer rainfall areas of North Queensland, a routine copperoxy spray is recommended as a branch rot preventative measure to be applied in late December - early January before the onset of the wet season. This spray is recommended for all ages of citrus trees from planting onwards.

WARNING: Citrus soils in the Koah area have naturally high levels of copper in the soil and leaf drop occurs if copper sprays are used. On these soils, copper deficiency is not a problem, and other types of fungicides have to be used in place of copperoxy.

In macadamia, the wavy branches with some twisting of the leaves are the common signs. Copper oxychloride as a foliar spray will overcame the problem, or use a copper sulphate application to the soil. Use copper sulphate at the same rates as used for zinc sulphate in citrus.

Copper deficiency has not been diagnosed in the newer tropical (rare) fruits. This is a lack of knowledge rather than implying that we know copper is/is not deficient.

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IRON

Iron is used in the photosynthesis and respiration cycles of plants. It is needed to form chlorophyll, but is not actually in chlorophyll.

Iron deficiency may be caused by high levels of phosphorous or calcium in the soil holding the iron in forms unavailable to the plants. Expect to find iron deficiency on beach sand soils, especially those with coral visible in the soil. Iron deficiency can be extreme on some of the coral atoll islands of the Torres Strait. Iron deficiency shows as creamy white, bleached terminals.

On coral sands, all leaves may be bleached due to the very severe deficiency.

Deficiencies can be overcome with foliar application of iron sulphate or iron chelate at 1 gm/litre or by soil application of above.

Foliar application must be used when correcting iron deficiency on coral soils as the calcium levels in the soil would quickly immobilize any soil-applied iron.

BORON

Boron has its major part to play in the growth of young tissues and it is in young tissues where all the symptoms start. Severe deficiency may completely kill the growing points in seedlings of some vegetables.

Boron helps with the movement of calcium in the plant. Although it is the above-ground symptoms we usually see, there are, as well, often quite severe symptoms in the root system. Boron also helps in the sugar and/or starch accumulation in plants, i.e. increasing the sweetness of fruit.

Like zinc and magnesium, adequate boron levels play an important part in the development of completely sexually-functional flowers and consequent good fruit set. Boron is deficient in most of our North Queensland soils.

Visual symptoms:

(a) Bumpy fruit on papaws, also downward hooking leaf ends (last 10-15 mm).
(b) Gum packets in citrus and also small, deformed seeds and 'dry' fruit.
(c) Bending of fruit stem in avocado.
(d) Poor growth in many of our vegetable seedlings.
(e) Shrivelling of young passionfruit fruit (one cause).

Citrus - foliar application at 12-3 grams of borax/litre onto the spring growth flush. This should last the tree for about 3 years.

Papaw - Apply a foliar spray at 2 grams/litre twice yearly or apply 5 grams of borax evenly over 2m2 of soil at each planting site every 6 months for the first two years of the crop only.

Lychee - apply an annual application of borax at the following rates. Some of the yearly rates are shown.

Year 1 - 2: 5 gms/tree over 1m2 of soil.
Year 3 - 4: 10 gms/tree over 4m2 of soil.
Year 5 - 6: 20 gms/tree over 6-8m2 of soil.
Year 7 + : 30 gms/tree over 10-12m2 of soil.

Instead of using borax, you can use solubor, but because solubor contains double the strength of boron as does borax, you will need only half the quantities listed above.

WARNING: Borax and Solubor can be toxic to plants if used in excess. Do not exceed rates mentioned until such time as a proper soil and leaf analysis have been done and the plants' needs accurately determined. Borax at high rates was commonly used as a total-spectrum, long-term soil-sterilant type herbicide in industrial areas. (It will do the same job on your orchard trees if you insist on using excess amounts).

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MOLYBDENUM

Molybdenum is needed for the proper functioning of the nitrogen cycle in plants. Lack of molybdenum blocks the oxidisation of nitrites (toxic) to nitrates (non-toxic) in the leaf.

Molybdenum is one of the more 'trace' trace elements.

Not recorded as a problem in tree crops. This may be a lack of knowledge more than not having molybdenum deficiency.

(Commonly deficient in cabbage and cauliflower, cucumbers, melons, tomato, beetroot crops where the deficiency is controlled by foliar spraying with either sodium molybdate or ammonium molybdate at the rate of 1 gram per litre of water.

Two sprays, one in the seedbed stage and one 2-3 weeks after planting out).

MANGANESE

Manganese is used in several enzyme reactions in plants. When it is deficient, the symptoms look similar to zinc deficiency (mottle leaf) in citrus except that leaf remains normal size.

Manganese is a common element in most of our soils and is only rarely at deficiency levels. Manganese commonly becomes deficient when the pH reach 7.0 and it becomes tied up by the calcium in the soil.

At the other end of the scale, manganese becomes increasing soluble after the pH starts to drop below 5.0. By pH 4.5, manganese can be available in toxic quantities.

Manganese toxicity is overcome by liming the soil to increase the pH and bring the manganese back to an insoluble form. Deficiency symptoms are cured by foliar spraying with manganese sulphate at 1 gm/litre or manganese chelate at 2 gms/litre. If spraying citrus add 7 grams of urea/litre of the manganese sulphate spray to increase the manganese uptake.

GENERAL

Nutrients may be in the soil in sufficient quantity but chemical reaction in the soil with other elements can at times make them unavailable, or only weakly available to the plant. e.g. iron can be tied up by high soil rates of both phosphorus and calcium. Manganese may be toxic at pH levels below 4.5 and deficient when pH rises above 7.0.

Foliar application (followed by at least 1 day and preferably 2 days of fine weather) gets the chemical into the plant where it is needed. Foliar rates are usually sufficient if the deficiency is only weak, but strong deficiencies generally need the higher rates that can be achieved with a soil application.

Magnesium, zinc and boron all have important roles in producing fully sexually-functional flowers and subsequently good fruit set.

CAUTION

The rates of application of chemicals has been made for the wet tropical coastal areas of North Queensland. It is based on my local knowledge of the soil deficiencies and the leaching effects of the high rainfalls.

Foliar recommendation could well be applied to other soil and climate areas if the deficiencies occur. The soil application rates are for coastal North Queensland. In drier areas, the soil application may well last for a longer time and if you are in one of these drier zones you should look for a local source of information.

Powdered, mixed trace elements and single trace element chemicals are available from fertilizer companies. Fruit tree fertilizers with and without a full trace element mix are also available from the companies.



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Bibliography

Wait, Jim. "The Use of Trace Elements in Tropical Tree Fruit Crops." rfcarchives.org.au. Archives of the Rare Fruit Council of Australia. A talk given to the members of the Mossman Branch of the RFCA on 23 Sep. 1990. Jan. 1992. Web. 18 May 2015.

Published 18 May 2015 LR. Updated 24 July 2015 LR
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