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Fruit Tree Fertiliser Rates _DBIRD_NT_


                                                                       No. D10

                                                                       December 2005

                                                                       Agdex No: 210/19

                                                                       ISSN No: 0157-8243

Fruit Tree Fertiliser Rates in
the Top End
M. Poffley* and G. Owens, Crops, Forestry and Horticulture, Darwin
* Formerly Horticulture Division, Darwin

Climatically the Top End is well suited to growing a wide range of tropical fruit trees. The year
round warm temperatures are conducive to rapid growth. To get the most out of the good
growing conditions, fruit trees need to have a proper balance of nutrients available at all times,
together with regular irrigation.


Generally soils in the Top End are very poor in nutrients, have low water holding capacity and
are acidic. Young fruit trees which are not fertilised and watered regularly will grow very slowly,
if at all. For strong healthy plant development the following nutrients are essential in relatively
large amounts.

Nitrogen (N) promotes growth and is a major constituent in chlorophyll (the green pigment in
leaves). Too much results in lush vegetative growth, at the expense of flowering, or soft fruit
prone to breakdown and fruit rots. Deficiency symptoms are stunted growth and yellowing of the
leaves. Sources of N are ammonium nitrate, urea and sulphate of ammonia.

Phosphorus (P) is important for healthy root development, and is thus necessary early in plant
growth. Deficiency symptoms are stunting, sparse foliage and the leaves often are dark green or
purplish in colour. Sources of P are single, double and triple superphosphate. Superphosphate
is also available with added trace elements; this is the preferred form.

Potassium (K) promotes disease and insect resistance in plants; it also plays an important role
in fruit quality. Deficiency symptoms include scorching of tips and margins of older leaves and
poor fruit size and quality. Sources of K are muriate of potash and sulphate of potash.

These elements can be applied individually; however, the most convenient way is in the form of
one of the many fertiliser mixtures available on the market. The recommended mixed fertiliser
must be a 'complete' fertiliser i.e. must contain N, P and K. A high analysis fertiliser is
recommended, as it is cheaper per unit of fertiliser, and should be in the range of :

        N 10-15% P 10-15% K 10-15%

Complete fertilisers with added trace elements are preferable when available.

Complete fertilisers come in two forms. The most common is a fertiliser mixture, i.e. the
individual N, P and K. are physically mixed together and can be separated out as particles of N,
P and K. The other form is as a compound, where the N, P and K are manufactured by a
chemical reaction. In compounds each particle of fertiliser is uniform i.e. contains N, P and K in
the same ratio and cannot be separated out. This form usually has a slower nutrient release rate
than a mixture, and is less prone to rapid leaching from over watering or heavy rains.

The above elements are used in relatively large amounts by plants and need to be applied
regularly to meet requirements. The elements listed below are also major elements, but are
usually available in the soil, or are found in some fertilisers as by-products.

Calcium (Ca) is found in dolomite, agricultural lime, gypsum and single superphosphate.

Magnesium (Mg) is found in dolomite. It can also be applied as Mg sulphate (MgSO4) (epsom
salts) to the soil at 15-20 g per square metre or as a foliar spray at 10-15 g/L. High levels of Mg
are often found in bore water in the Top End. This could lead to excessive levels building up in
the soil, which can affect K uptake and cause an inbalance with Ca leading to complications.

Sulphur (S) is found in single superphosphate gypsum and all sulphate forms of fertiliser, e.g.
sulphate of ammonia as well as some fertiliser mixtures.

In addition to these major elements there are a number of 'trace elements' which are equally
important in plant growth, but only in very small amounts. In fact in large amounts they are often
toxic to plants, so it is important not to use them in excess. Trees, with their extensive root
systems, can usually find enough trace elements occurring naturally in the soil. However,
deficiencies of boron, iron and zinc are quite common in fruit trees in the Top End.


Trace elements can be applied in a number of ways. As already mentioned fertiliser mixtures
are available that contain trace elements. They can be applied direct to the soil, or sprayed on
the leaves by themselves or mixed with an insecticide.

Soil Application Rates
Boron as borax at the rate of 3 g/m2.
       Or Solubor® at 0.5–1.2 g/m2.

(Do not exceed the recommended rate as borax can be toxic to plants at high levels).

Iron as iron chelate at the rate of 2 g/m2.

Zinc as zinc sulphate at the rate of 7 g/m2or as zinc oxide at the rate of 5 g/m2.

Molybdenum as sodium molybdate at 0.05 - 0.1 g/m2

Foliar Application Rates
By using this method it is possible to mix a trace element with an insecticide, or to mix trace
elements together, but it is not advisable to mix more than three chemicals together for spraying
as they may precipitate. When mixing trace element sprays, each chemical should be dissolved
individually in a small amount of water and added separately to the tank to avoid precipitation.

Urea added to the mix at 5 g/L improves absorbtion by leaves. The best time to apply trace
element sprays is just after the trees have produced a new flush of leaves, as such young
leaves readily take up the nutrients.

Boron as borax at the rate of 1-3 g/L (not compatible with other sprays).
       Or as Solubor® at the rate of 1.5-2 g/L.

Iron as ferrous sulphate at the rate of 3-5 g/L (apply by itself).

Molybdenum as sodium molybdate at 0.5-1.0 g/L.

Zinc as zinc sulphate (heptahydrate form) at the rate of 2-8 g/L (compatible with most sprays).

Trace element deficiency symptoms only appear when the tree has used up all its reserves. In
this case it may be necessary to apply trace elements more frequently for a year or so to build
up plant reserves, then returning to the regular trace element program.


Manure is a good soil conditioner as well as a source of nutrients. Due to the variability in the
analysis of manures they should not be relied on as the sole source of plant nutrients, but rather
regarded as an additional supplement for the plants. Manure can also introduce weeds,
depending on the source. Avoid using fresh manure, especially chicken manure, as it can burn
plant roots. It should be allowed to weather for at least six months before being used. An
application rate for well weathered chicken manure would be about 2 kg. per square metre per
year. In young trees this should be split into two applications as a precaution against burning
the roots. This can be regarded as an optional extra for most fruit trees, except mango trees.


Almost any organic matter can be used for mulching under trees such as hay, grass cuttings,
tree prunings, old cardboard cartons, shredded paper etc. The mulch should not be heaped up
against the trunk, but kept at least 5-10 cm. away from it.

Young trees benefit from mulches in a number of ways because mulch:

•   keeps the ground cool, encouraging the growth of surface feeder roots;
•   smothers weeds eliminating their competition with young trees for water and nutrients;
•   reduces evaporation;
•   slows down the movement of fertiliser, spread on it, into the soil allowing the plant more
    time to utilise it before it leaches below the roots;
•   itself eventually breaks down into humus providing and storing additional nutrients for the

Mulching is most beneficial for young trees, one to two year old. As trees grow bigger they
shade out the area under their canopy and reduce weed competition. Older trees become self
mulching with natural leaf drop and prunings if these are left under the trees to rot.


Top End soils are usually acidic with a pH of 5-6. This is quite satisfactory for most tropical fruit
trees. Regular applications of chemical fertilisers tend to acidify the soil. To counteract this and
keep up a good supply of calcium to the plant it is beneficial to apply agricultural lime annually at
the rate of 100-150 g/m2 over the same area where the fertiliser is normally placed. The best
time is at the beginning of the Wet.


Many people get into trouble at the very start by putting too much fertiliser in the planting hole,
leading to the roots being burnt by direct contact with fertiliser. The result is that the young tree
either dies or is set back so severely that it would be quicker to replant with a new tree next to
the original site. Trees do not need fertiliser immediately on planting out, as it takes a week or
so for the roots to grow out into the surrounding soil.

The average planting hole, of about 10 L in size, could be given a maximum of 100-200 g of
dolomite or lime and 100 g of single superphosphate. This should be spread over the soil dug
out of the hole and mixed with it on back filling. A couple of slow release fertiliser tablets could
also be placed in the hole as an optional extra, but do not over do it as too many of these can
also damage plants. Filling the hole with "good" soil, mulch, manure or compost can often lead
to more problems than benefits. To avoid complications the best material to back fill a planting
hole with is the soil that came out of it.


As mentioned earlier, care should be taken if fertiliser is put in the planting hole to ensure that it
is well mixed with the soil. Fertiliser should never be dumped in a heap under the tree, always
scatter it evenly over the ground. With larger trees the fertiliser should be placed evenly from
about 0.5 metre away from the trunk out to the edge of the canopy, where most of the feeder
roots are found. Fertiliser can be scattered on top of the mulch, it is not necessary to dig it into

the soil, as it will leach down into the soil with irrigation. If fertiliser is applied during the dry it
needs to be placed in the wetting pattern of the sprinkers to be available to the plant.


Fertilising can start two to three weeks after the tree has been planted out, but initially at a
reduced rate. This allows time for the tree to settle down and become acclimatised to its new
environment, and the roots to start growing out into the surrounding soil.

Young trees require frequent light applications of fertiliser to ensure continuous vigorous growth.
It is better to fertilise monthly for the first year or two, than give them a large bi or triannual dose,
which is leached away before the young trees can utilise it. An average fertiliser rate for fruit
trees is in the region of 500 g of a complete (NPK) high analysis fertiliser, per year of age. This
would make the monthly application for a one year old tree about 40 g ( about a small, closed
hand full).

As trees get older the frequency of fertilising can be reduced to 4 - 6 times a year, or even less
as is the case with mature mangoes to only twice annually. As a general rule trees which fruit all
year around, e.g. bananas, pawpaws and carambolas should be fertilised all year round (i.e.


Below is an outline for general rate for the average fruit tree.
High analysis NPK fertiliser 500 g/tree/year of age*.
Trace element zinc applications three times per year.
Single superphosphate 50 g/ m2 (December to January).
Agricultural lime 100 - 150 g/ m2 (December to January).

* Where rates are given as kg or g per tree per year of age it should be remembered that
this increases up till year 10 and then the rate remains at that level. For example, at 500 g
per tree per year of age a 12 year old tree receives 5 kg of fertiliser.


From planting till first flowering:
Complete NPK fertiliser 250-300 g/site worked into the soil at planting.
Complete NPK fertiliser 100-200 g/plant site per month till first flowering.

Mature plants should be fertilised every four to six weeks alternating between:
Muriate of potash at 180 g/plant site and urea at 90 g/plant site.

Annual applications November to December of:
Single superphosphate (+ zinc) at 50 g/m2.
Agricultural lime at 100-150 g/m2.

Young newly planted banana plants should receive three sprays of zinc sulphate in the first
year. Older plants get enough zinc from regular sprays of Mancozebr applied to control leaf


These are heavy feeders and benefit from monthly applications of fertiliser.
Complete NPK fertiliser at 1 kg per year of age per tree.
Agricultural lime at 100-150 g/ m2 (November to December)
Zinc sulphate sprays 2 g/L three times per year.
Boron/Solubor®1-2g/L at flowering may improve pollination and fruit shape.


Complete NPK fertiliser at 500 g per tree per year of age.
Urea at 120 g per tree per year of age.
Or sulphate of ammonia at 250 g per tree per year of age.

The above rates should be split into two applications for mature trees and applied in March/April
and October/November.

Zinc sulphate sprays at 2 g/L three times per year.

Agricultural lime at 100-150 g/m2.


These rates are meant only as a guide and should not be blindly followed. Older trees may not
need any additional fertiliser as they may have built up reserves and are self mulching.

Complete NPK fertiliser applied after harvest to young trees (up to seven years old) at 20-50
g/m2. For larger older trees reduce or stop nitrogen altogether, unless nitrogen deficiency
symptoms are observed.

Gypsum 100-200 g/m2 at flowering.

Single superphosphate (+ Zn) 50 g/m2, at flowering and after harvest.

Muriate of potash or sulphate of potash 30 g/m2, at flowering and after harvest.

Agricultural lime 100-150 g per square metre, at the beginning of the Wet. This may not be
necessary for growers in the Katherine area where they have alkaline soil/water.

Zinc sulphate sprays at 2-8 g/L. Trees should receive three to four applications per year in the
form of a foliar spray. Alternatively, three to four soil applications of 7 g/m2 can be applied.

Sodium molybdate sprays at 0.5-1.0 g/L applied as a foliar spray one to two times per year.

Ferrous sulphate sprays at 3-5 g/L applied as a foliar spray three times per year.

Boron should also be applied annually. Trials have indicated it is most effective when applied to
the soil at the rate of 50 g/mature tree (this can be in the form of Solubor®). This should be
applied following lime or gypsum.


Pre planting:
Complete NPK fertiliser 250 g/m2/plant site.
Single superphosphate at 70 g/m2 plant site.
Dolomite at 200 g/m2/plant site.

Mature trees alternate monthly applications between:
Complete NPK fertiliser at 100 g per site and urea + muriate of potash 30 g of each per site.

Optional extras:
Boron - two foliar applications per year September to March.
Magnesium sulphate at 15-20 g/m2/year.


Young plants are prone to fertiliser root burn and can easily be killed with kindness. Although
they are heavy feeders care should be taken in the first year or two not to over fertilise i.e. small
frequent applications (monthly). Avoid fertilising during June/July when cooler weather can be
expected. They are very sensitive to chlorides. Typical symptoms are the scorching/browning
effect on the edges of leaves. Avoid fertilisers containing chlorides such as potassium chloride
(muriate of potash).

Amount from year 3.

Complete chloride-free NPK (10:5:9) fertiliser starting at 2 kg per tree and increasing by 0.5 kg
per tree per year up to year 10. This should be split into four and applied about – January, May,
August and October.

Zinc sulphate heptahydrate foliar sprays 2 g/L (three to four times per year).
Solubor® foliar sprays 2 g/L (three to four times per year).
Ferrous sulphate foliar sprays 3-5 g/L (three to four times per year).
Or iron chelate applied to the soil 2 g/m2 three times yearly.

Annual applications of:
Agricultural lime at 100-150 g/m2.

Optional extras:
Chicken manure at 1 kg/m2/year/bearing tree. This should be split and applied in December and

For more detailed rambutan fertiliser recommendations see Technical Bulletin No. 261.


An alternative method of applying fertiliser is by injecting it with the water. The fertiliser is
dissolved and then injected into the irrigation water at the time of irrigation. A pump is necessary
to inject the fertiliser into the line while irrigation is in progress. This method is a very convenient
way of applying fertiliser, especially in large orchards once the equipment has been set up and

The fertiliser mentioned in this section is only for replacing the complete NPK mix. All the other
trace elements, dolomite etc. should still be applied as recommended in the previous section.

General Fruit Tree Rate

Urea 2 g per tree per year of age per week.
Potassium nitrate 3 g per tree per year of age per week.

Exotics - Rambutan, Carambola, Sapodilla, Abiu

Urea 4 g per tree per year of age per week.
Potassium nitrate 6 g per tree per year of age per week.

Mangoes (from flowering on)

Urea 8 g per tree per year of age per week.
Potassium nitrate 12 g per tree per year of age per week.

It is not necessary to apply the fertiliser weekly. It can be applied every two to four weeks at two
to four times the rate. When applying fertiliser through the irrigation system the pump/injector
should be calibrated to inject the recommended amount of fertiliser continuously over the whole
irrigation cycle. If this is not possible then it should be injected towards the end of the cycle to
ensure that it is not leached below the feeder root zone. Fertiliser injection should take at least
half an hour to ensure even distribution of the fertiliser. This should be followed by a period of
irrigation to flush all the fertiliser out of the system.

The fertiliser rates given in this Agnote are only a general guide. Different situations may require
adjustments. It is also important to note that it is of little use to fertilise trees if they are starved
of water, or over watered, or if the area they are growing in is unsuitable.

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Published: Tuesday 13 December 2005.

While all care has been taken to ensure that information contained in this Agnote is true and correct at the time
of publication, the Northern Territory of Australia gives no warranty or assurance, and makes no representation
as to the accuracy of any information or advice contained in this publication, or that it is suitable for your
intended use. No serious, business or investment decisions should be made in reliance on this information
without obtaining independent/or professional advice in relation to your particular situation.

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