Rambutan removes a lot of nitrogen (N) and potassium (K) from fruits and harvested twigs. As twigs are harvested with the fruit, fertilisation after harvest has to replenish the tree for the loss from twigs as well. They also require more N and K than P during fruit development. Trees should be irrigated after fertilisation to increase the uptake of nutrients.

In rambutan fruit, the sequence of nutrient removal is as follows:

nitrogen > potassium >calcium> magnesium > phosphorus (N>K>Ca>Mg>P).

For every 100 kg of fruit and 100 kg of twigs removed per hectare, the following amounts of macro-elements are removed:

Fruits = N - 199 g, P - 26.8 g, K - 152 g, Ca - 72 g, Mg - 36.78 g

Twigs = N - 256 g, P - 390 g, K - 240 g, Ca - 171 g, Mg - 32 g.

During harvest the weight of twigs harvested with the fruits is around 10% of the fruit weight harvested. We have to replenish this amount as well as the amount of nutrients consumed for root, leaf and twig/branch growth.

Mix 5-10 kg of organic matter, 0.5 kg of limestone and 0.2 kg of rock phosphate should be mixed with the top soil and incorporated into the planting holes.

The usual practice before planting is to lime the soil with lime or dolomite to make it less acid. Lime is more effective in reducing acidity but supplies only Ca whereas dolomite supplies both Ca and Mg as well as reducing the acidity. The actual amount of lime or dolomite used depends on the soil type, soil pH and fertilizer source. For example, to raise the pH from <5 to 6 may require up to 5 mt/ha of lime or dolomite.

NPK fertiliser used for rambutan should be chloride free as rambutan is sensitive to chloride toxicity at higher levels. Symptoms of chloride toxicity are yellowing and death of leaf margins and tips and the young leaves falling prematurely.

Trees should be fertilised more in a year when they are heavily pruned.

Macronutrients

Lack of nitrogen reduces the growth of branches and leaves, leaflets are small and yellowish with slightly curled margins.

In plants lacking in potassium, leaf margins become brown and have a scorched appearance. In severe cases, the necrosis spreads towards the midrib and eventually the leaf falls off. Deficiency symptoms first appear on older leaves but will eventually also affect younger leaves.

Ca deficiency is common on acid soils. It may also occur in soils rich in nitrogen and phosphorus . Soil Ca and Mg levels can be increased by using dolomite, which contains 15-22% Ca and 8-20% Mg, depending on the source. Gypsum and lime, which supply only Ca, can be used to replenish the soil Ca removed.

Micronutrients

Zinc, boron , iron, manganese, copper, molybdenum – are required in very small quantities. These deficiencies are usually detected as foliar symptoms and may be corrected with foliar or soil applications. Foliar applications should be timed with the appearance of new vegetative flushes as young leaves absorb the spray better.

Organic Fertiliser

Different types of inorganic fertilisers can be used for rambutan. The amount and frequency of application depends on, among others, the soil structure and composition and climatic factors. Normally, compound fertiliser NPK 15: 15: 15 is split into 4 applications for the first and second year, while compound fertiliser NPKMg 12: 12: 17: 2 + TE is given in 4 applications from the third year onwards.

The fertiliser is broadcasted under the tree at a distance of 60 cm from the main trunk or applied in a wide band below the perimeter of the tree canopy. Fertiliser application can also be carried out using a tractor mounted row fertiliser spreader. This machine applies the granular fertiliser on both sides of the tractor along the rows.

Critical periods for fertiliser application appear to be before flowering, during flowering and fruit set, several weeks after fruit set and after harvest.

The crop does not remove much nutrient: according to Malaysian findings 15 kg N, 2 kg P, 11.7 kg K, 5.9 kg Ca and 2.7 kg Mg per ha for a crop of 7300 kg/ha.