All you need to know about fertilisers

What is the role played by three vital chemicals in promoting good plant growth?

While compost and mulching are the best ways to improve the quality - or humus content - of the soil, compost alone will not provide sufficient nutrients for plants to thrive and stay healthy, and the major cause of poor plant growth is a lack of sufficient nutrients. This is because many of our natural soils tend to be light and sandy. They lack nutrient-holding elements such as clay and humus particles, so nutrients are leached out of the soil fairly quickly by water, be it natural rainfall or artificial watering. To keep our plants healthy and fruitful, gardeners need to give the soil supplementary amounts of fertiliser.

Major plant nutrients

Most gardeners tend to be somewhat confused by the range of chemical fertilisers available, and are not sure which fertiliser is the correct one to use on lawn, for fruit trees, in the vegetable garden and so on. Choosing a fertiliser is not a difficult matter, as long as you remember that the most important and vital chemicals for plant growth are nitrogen (N), phosphorus (P) and potassium (K). Any fertiliser that contains all three of these essential growth-producing elements is considered a balanced fertiliser.


Mixed Ornamental Plants

Nitrogen - the leaf-maker

Nitrogen (N) is the most important of the plant foods. Nitrogen in the soil stimulates growth of the leaves, improves the leaf colour of all plants, and increases the size of the plants. Nitrogen also gives bigger crops of better quality and increases the protein content of vegetables and fruit. It is therefore especially beneficial to leafy vegetables, such as lettuce, spinach and cabbage, and to lawns which are constantly being mown. South African soils generally tend to be low in nitrogen. Exceptionally heavy rains have a severe leaching effect on all the soluble nutrients in the soil, especially nitrogen. In the Western Cape, soluble plant nutrients are leached out of the soil all year round, by rainfall in winter and by the frequent watering that gardens require during the long, hot summers.

Lack of nitrogen in your soil will show clearly by yellowing of leaves and stunted growth of all plants including vegetables. However, if too much nitrogen is applied, it will lead to soft, lush growth, which renders the plant susceptible to damage by cold, fungal disease and insect attack.

Nitrates (the form of nitrogen used by plants) come from various sources:

  • Chemical fertilisers high in nitrogen.
  • Organic matter, such as farmyard manure, stable manures, or compost - when these decompose nitrates are slowly released.
  • Leguminous plants, such as peas, beans, clover and lupins, have nitrogen-converting root nodules which supply nitrates to the soil.
  • Nitrogen-fixing bacteria in the soil.

Phosphorus - the root-maker

Phosphorus (P) stimulates early root formation and germination of seeds. An adequate supply is essential in soil used for sowing seeds and for growing young plants, especially shrubs and trees. Phosphorus also influences the uptake of potassium in plants. Plants are better able to resist disease if there is plenty of phosphorous in the soil; it also encourages winter hardiness, and aids flower and seed formation. An adequate supply of phosphates in the soil also offsets the adverse effects of too much nitrogen in the soil.

There is normally a large amount of phosphorus in the soil, but only small amounts at a time are available to the plants. To compensate, soluble forms of phosphorus fertiliser should be added. Apply phosphorus fertiliser directly around the plant and avoid broadcasting (the usual method with general fertiliser).

A phosphorus deficiency is shown by poor germination of seeds, slow growth of plants, and poorly formed roots. Flowers and fruits are produced only in moderation and are often undersized. Where there is extreme shortage, the leaves of plants may turn purple. Too much phosphorus in the soil is very rare; it shows in the form of stocky growth with small buds and flowers. Bonemeal, superphosphate and rock phosphate are high in phosphorus.

Potassium - the flower and fruit maker

Potassium or potash (K) improves the quality of flowers and fruit, and the flavour of fruit and vegetables. It should be applied to all fruit and vegetable crops, except those in very acid soil, to increase size and yield. Adequate potassium also increases the amount and quality of seeds formed by plants.

Potassium is essential to plants for photosynthesis, the vital process by which plants make food for growth. It is needed to produce stiff stems, and is very necessary for fruit and flower production. Potassium is also necessary for root crops, where it is stored as starch.

Potassium encourages healthy plant growth, and makes plants resistant to temperature extremes and drought. It should be added to over-wintering plants in autumn, especially roses, and deciduous fruit trees, to make them more frost or cold resistant. Potassium deficiency in the soil will show by making plant leaves scorched at the edges. Fruit and vegetables are poorly coloured and lack flavour. Although there may be adequate potassium in a normal soil, it is only slowly available, and is easily washed out of the soil, so additional applications of potassium are necessary. It will give a boost to most plants when applied during their growing season. Potassium sources include manure, woodash and Condi's crystals.



Balanced and slow-release fertilisers

Both chemical and inorganic fertilisers will provide the 'big three' nutrients to the soil. Remember, however, that inorganic fertilisers do not build up the soil, so compost and mulches should be applied as well.

In order to provide these three important fertilisers conveniently and in the correct quantities, fertiliser manufacturers produce balanced fertilisers, which contain all three chemicals. The gardener can then give the soil a particular balance of 'the big three' in one application. Which combination you choose will depend on your soil type, the particular plant or plants, and the time of the growth cycle.

On a bag of fertiliser, you will always see three numbers separated by colons, for example, 2:3:2 (22). The first number represents the proportion of nitrogen in the fertiliser. The second number represents the proportion of phosphorus in the fertiliser. The third number represents the proportion of potassium in the fertiliser. The final number in brackets is the total amount of concentrated fertiliser or plant nutrients that is available to the plant. The rest is a carrying agent, which holds the concentrated fertiliser mixture.

If the three numbers are followed by the letters SR, this means that the fertiliser is a slow release fertiliser, which will supply the chemicals to the soil over a period of time, and not all at once. SR fertilisers have the following advantages:

  • The nitrogen in these fertilisers will not be leached from the soil quickly, so slow-release fertilisers need not be applied as often as the non-slow release ones.
  • If you apply too much fertiliser, plants are unlikely to be 'burned' by excess nitrogen.
  • Nitrogen is released by biochemical action, thereby providing nitrogen in a highly acceptable form as and when the plant needs it.

Foliar fertilisers

These are balanced fertilisers that are applied in solution to the foliage of plants. It is well known that soluble organic and inorganic fertilisers sprayed onto the leaves of most plants are more readily absorbed by the plants, resulting in quicker growth in contrast to fertilisers being absorbed through the roots. However, not all plants will respond the same, and it is usually soft-leafed plants that show the quickest response. Foliar fertilisers are recommended particularly for plants under trees, to make sure that the plants, and not the tree, absorb the fertiliser. Also use on the seedlings of annuals to speed up growth.

Tips for success

  • Always apply fertiliser over damp soil, and water in immediately, for a dry plant will soon die if fertilised when already dry, or if the fertiliser is left on the leaves or too close to the stems of the plant.
  • Sandy, light-textured soils need more frequent fertilising than clay soils because leaching of chemicals is faster in sandy soils.
  • Apply fertilisers high in nitrogen in smaller quantities than more balanced fertilisers.
  • More generous amounts of fertiliser can be given to fast-growing shrubs, such as hibiscus and hydrangeas. Slow-growing shrubs, such as azaleas, camellias, and gardenias, prefer small regular applications.
  • Apply extra fertiliser after an exceptionally rainy season in order to replace nutrients leached from the soil.

Overall size of the plant is relevant in determining how much fertiliser to apply and how often to apply it.

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