All You Need to Know About Plant Foods

Soluble fertilisers that can be watered on are just one of a few different forms of plant food. Image: © Odua Images

Most tropical soils are naturally infertile, so if you want to grow strong, healthy plants, you must add suitable plant foods. ln tropical climates chemical fertilisers don’t last long in the soil and therefore need to be applied at regular intervals of about 6-8 weeks.

The most important nutrients that are required by all plants are nitrogen, phosphorus and potassium. Fertilisers that contain all three elements are referred to as ‘complete’ or ‘N-P-K’ fertilisers.


The percentage of each element in the fertiliser is shown as the ‘N-P-K-ratio’. ln other words, a fertiliser with an N-P-K ratio of 1O-9-7 has in it 10% nitrogen, 9% phosphorus and 7% potassium (K is the elemental symbol for potassium).

Nitrogen is the element that promotes growth but should not be oversupplied otherwise the plant is liable to become leggy, the foliage soft and the plant susceptible to disease. A deficiency of nitrogen is usually shown in yellowing of the leaves and a reduction in their size.

Phosphorus builds strong roots and also assists in the formation of seeds and fruit. Most tropical soils are deficient in this element.

Potassium promotes disease resistant growth through the formation of fibrous matter and assists in the production of plant sugars which are particularly required for fruit and root crops. lt is also essential for good colouring in fruit and flowers.

Complete fertilisers are blended with different N-P-K’s for special purposes. For example, a good lawn fertiliser would have a higher N and P figures than K. This gives the lawn plenty of nitrogen to promote green leaf growth, phosphorus for strong root development, but the requirement for potassium is not as great as would be required for say fruits or vegetables.

Sulphate of ammonia contains about 20% nitrogen and can be applied on its own to promote new growth of leaves and shoots. Care should be taken not to let the fertiliser come into contact with the foliage otherwise burning may result. Use about a match boxful to the square metre once a month. When applied to lawns it must be watered in well immediately after application.

Urea is another nitrogenous fertiliser but contains more than twice the level of nitrogen as sulphate of ammonia. Extreme care should be taken not to overdo applications of this material. To minimise the risk of burning, urea may be dissolved in water and applied as a liquid.

Superphosphate is a source of phosphorus and contains about 9% of that element. lf applied on its own, it should be worked well into the soil as it is effective virtually only where it is placed and thus should be within easy reach of the plant’s root system.

Sulphate of potash contains about 48% potassium and can be used on its own at the same rate of application as sulphate of ammonia. Superphosphate and sulphate of potash are often used to follow up applications of blood and bone.

Blood and Bone

One of the most commonly used organic fertilisers in home gardens in developed countries is blood and bone but it should be realised that this fertiliser in its original form contains no potash. It usually contains less than 6% nitrogen, and only small proportions of phosphorus. However, some fertiliser manufacturers are now producing formulations that include added potash, but this is usually not more than 1-2%.

The main advantage of blood and bone is that it contains nitrogen in both fast and slowly available organic forms and rarely burns even the most sensitive plants. Blood and bone is an excellent fertiliser for mixing with the soil when planting young trees and shrubs as there is little danger of burning the plants through an over-supply of nitrogen.

It also acts as a soil improver in promoting the growth of soil bacteria and earthworms. The bacteria assist in the release of nutrients from the soil and the earthworms improve the soil structure and texture.

Most successful gardeners use organic and inorganic fertilisers to take advantage of the benefits of both types.

Another type of fertiliser that is extensively used in the tropics is controlled release plant food. This is a chemical fertiliser where each granule coated with an organic resin. When the granules come in contact with water, the fertiliser dissolves and is then slowly metered out through the resin coating over a period of about 3 months or 6 months depending on the thickness of the coating.

The advantage of controlled release plant foods over ordinary fertilisers is that they provide a constant supply of nutrients to the plant over an extended period and are not leached from the soil as quickly as conventional fertilisers. However they may be released more quickly in the Wet season.

Appropriately balanced blends are safer to use on native plants than ordinary fertilisers, which may burn sensitive species if applied too heavily. Controlled release fertilisers cost more than other types but their extensive use in nurseries gives an indication of their worth.

Soluble plant food

Concentrated soluble fertilisers are well known to home gardeners due to heavy promotion carried out by manufacturers. A teaspoonful or so of the product is dissolved in water to make up 5-10 litres of liquid fertiliser.

These types of fertilisers can be very time consuming to apply unless injected into a watering system. Their main advantage is that they provide a source of almost immediately available nutrients to the plants. They are best used in conjunction with other types of fertilisers to ensure adequate plant health.

For example, a gardener growing leaf vegetables will obtain the best results by using a complete fertiliser formulated for vegetables, dug deeply into the soil, and supplementing this with fortnightly feeds of a high nitrogen, soluble fertiliser.

Most of the brands of soluble fertilisers on the market are urea based and therefore have high nitrogen content. Care should be exercised when using these, as it is easy to burn plants with them. If applying with a watering can, ensure that the powder is thoroughly mixed before application.

Other brands are based on potassium nitrate, which is not as hygroscopic as urea and therefore stores better in the tropics. They have lower nitrogen content than the urea-based fertilisers but much higher potash content.

These fertilisers are particularly good for supplementary feeding of flowering and fruiting plants and palms which like a lot of potash. Don’t be deceived by the low nitrogen content when applying these though – potash can burn as badly as nitrogen if applied too heavily.

Minor Elements

Most of the fertilisers mentioned so far also contain some of the minor elements, calcium, sulphur and magnesium, which are required by the plant in smaller amounts than the major elements.

ln addition, some of them also contain trace elements such as iron, manganese, zinc, copper, boron and molybdenum, which are required in minute quantities to provide a balanced plant nutrition and to optimise the utilisation of the major nutrients.

The bag or container in which the fertiliser is packed will show the elements contained in the product so it is easy to determine whether something is missing.

lf no content of trace elements is shown, these can be provided by adding one of the many commercial trace element mixtures on the market or by supplementary feeding with liquid seaweed which contains a wide range of trace elements.



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