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