Agnote 340 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. TYPES OF NUTRIENTS 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. 2 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. 3 TRACE ELEMENT APPLICATION RATES 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. ANIMAL MANURE 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. 4 ORGANIC MULCHES 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 tree. 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. MAINTAINING pH LEVELS IN THE SOIL 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. PLANTING OUT 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. FERTILISER PLACEMENT 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 5 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. FREQUENCY OF FERTILISING 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. often). FERTILISER RATES 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. Bananas 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 spot. 6 Carambolas 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. Citrus 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. Mangoes 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. 7 Pawpaws 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. Rambutans 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 April. For more detailed rambutan fertiliser recommendations see Technical Bulletin No. 261. FERTIGATION 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 calibrated. 8 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. Please visit us on our website at www.horticulture.nt.gov.au 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.