THE HUNGRY FRUIT TREE

(One of a series of articles about growing fruit trees in Samoa, written by Digby Gotts and published in the 'Samoa Observer' between December 2000 and April 2001)

Living in a land where the soil is as rich as that in Samoa, it’s difficult to remember that plants not only need to be fed, but also to be fed a balanced diet. Plants take most of their mineral nutrition from the soil, and as the plant ages and as the fruit is taken away, the levels of mineral nutrients in the soil may drop until the growth and health of the tree is being affected. The rule of the sewer applies here: That what you get out of it depends on what you put into it.

The main minerals needed by plants are nitrogen (N), phosphorous (P) and potassium (K). Most store bought fertilizers will contain these in varying amounts. The most common mixed fertilizer available in Apia is BOP with12% nitrogen, 5% phosphorous and 20% potassium. This is stated on the label as NPK12:5:20. This is a good general-purpose fertilizer with enough nitrogen to promote leaf growth, enough phosphorous to encourage strong wood and enough potassium to get the flowers moving. A useful rule of thumb is to apply one dose of fertilizer when the flowers show (NOT for avocado or mango!) and a second dose after harvest (double for avocado and mango). Each time, spread at least a kilo of BOP evenly on the ground under the tree right to the leaf edge.

Trees needing nitrogen are typically pale green to yellowish in colour, while heavy dark green leafy bunches are a sign of high nitrogen levels. When phosphorous is needed, young growth will be deformed and weak while poor flowering and fruit set is often due to inadequate potassium.

Spending good money on store fertilizers is convenient, but it isn’t actually necessary, if you have the time to collect and use local materials.

Nitrogen is contained in large amounts in fresh green leaves and grass clippings as well as animal urine and fresh manures. You can damage the tree by giving it too much fresh manure, so it is a good idea to leave some in a drum with water and allow it to break down for a few weeks, stirring sometimes. This “manure tea” should then be diluted about 20 to 1 and then watered into the trees roots. Phosphorous is one of the most common minerals on earth, but only a small amount is easily dissolved in water and therefore accessible to plants. Powdered rock, animal bones, soaps and detergents all contain good sources of phosphorous, but all need microbe activity in the soil to get them to break down and release their nutrients. Encourage microbes by adding a layer of grass and leaf litter, ensuring that a good source of rotting plant material (carbon) is available for them. Potassium easily dissolves in water and so can disappear very quickly in Samoa’s high rainfall. It is found in wood ash, banana trash, and some soaps. So again spread these materials around under your tree and leave it to nature to break them down.

Many other minerals are needed in small amounts and these may need to be added at some stage. A commercial fertilizer, (Thrive), is available in Apia, which contains these as well as a strong hit of nitrogen. This is mixed with water according to the directions on the packet and sprayed on the leaves. Again, you pay for the convenience. “Tea” made at home with animal manure, fresh seaweed or compost can serve the same purpose.