Plants and Fertilizers

To keep your house plants looking and feeling healthy, you must give them the nourishment they need, as well as the light, warmth and water.

A newly purchased plant which is perfectly healthy can soon decline as the nutrients in its compost become exhausted. They are used up by the plant and leached out of the compost during watering. Dull, small leaves, lack of growth, variegated leaves which turn green and poor flowering are signs that fertilizing is needed.

Top-dressing and repotting with fresh compost are good ways to replenish the food supply annually, but extra food is needed when plants are growing vigorously during spring and summer. Take care when feeding your plants as too much fertilizer can damage them.Plants and Fertilizers_picnik

What’s in a fertilizer?

A simple fertilizer contains one or two chemicals while a standard (or compound) fertilizer is a balanced mixture of several ingredients. The three main plant foods, often called ‘macronutrients’, are nitrogen, phosphorous and potassium. High nitrogen fertilizers are not suitable for house plants as they encourage soft, lush growth. Other macronutrients are calcium, magnesium and sulphur. These nutrients are taken up in varying amounts by the plant roots from the water in the compost. Trace elements are also added to fertilizers in minute amounts. Known as ‘micronutrients’, these elements include boron, copper, iron and zinc.

Standard fertilizers

Standard fertilizers are widely available, safe and easy to use providing you follow the printed instructions. They are suitable for the majority of house plants and come in several convenient forms. Liquids, powders and crystals are dissolved at the correct strength and watered in, or you may prefer fertilizer sticks, tablets or slow-release pellets. Foliar feeds are good as a quick pick-me-up for an ailing plant, but are less successful when they are used frequently.

Specialist fertilizers

There are some occasions when you will need to use a specialist fertilizer. For example, lime-hating plants such as Azalea and Gardenia may need extra iron to combat an alkaline soil caused by hard water. Use iron sequestrene if leaves and stems begin to lose their colour. Flowering plants and cacti benefit from applications of well-diluted, high-potassium tomato fertilizer. Special fertilizers are also available for plants such as African Violets, Orchids and Bonsai trees.

Know your nutrients

  • Nitrogen is essential for healthy growth and good leaf colour.
  • Phosphorous is important for healthy root growth and helps plant metabolism.
  • Potassium aids development of flowers and fruit.
  • Calcium is a general purpose nutrient needed for strong growth and development.
  • Magnesium helps with chlorophyll production. Chlorophyll is a vital ingredient as it helps the plant absorb sunlight as energy.
  • Sulphur helps convert carbon dioxide into sugars used by the plant (this process of conversion is known as photosynthesis).



  • Check that the fertilizer is right for the plant. Different plants require different amounts of nutrients: A high potassium brand may not suit a plant needing high phosphorous fertilizer. Ask a reliable supplier if you are in any doubt.
  • Look out for signs that plants need feeding: pale, undersized leaves, slow growth, variegated leaves which turn plain green and poor flowering and fruiting.
  • Check for other causes of ill-health before feeding. Has the plant been overwatered or the compost dried out? Are there signs of pests, virus or diseases present?
  • Follow the manufacturer’s instructions carefully and use the fertilizer at the correct strength. Feeding with a stronger ‘ solution will not make the plant grow more quickly, but may damage the roots and foliage.
  • Use the weakest solution indicated on the label when a manufacturer has not given a strength for house plants. Too strong a fertilizer can actually damage the plant, by possibly burning the roots, or even killing it.
  • Keep all containers out of reach of children and make sure they are clearly labelled.


  • Store fertilizers in discarded food containers or bottles, or keep them past their ‘use by’ date.
  • Feed plants at the wrong time. Feeding plants while they are resting will result in spindly, unhealthy growth.
  • Fertilize plants which have just been repotted. Peat-based compost contains sufficient food for between six and eight weeks while soil-based compost will be fine for up to twelve weeks. Vigorous plants will need feeding sooner than slow-growing species.
  • Let the fertilizer come into contact with the foliage unless you are using a foliar feed. Wipe off any splashes with a soft cloth or paper tissue as soon as possible. All other liquid fertilizers should be applied from below.
  • Feed young plants until they are well established. You may burn the roots and kill the plant.
  • Apply fertilizer to a plant with dry compost as this may scorch the foliage. Water first, then apply a weak solution of fertilizer.
  • Use foliar feeds all the time. They are excellent as a quick pick-me-up for a sickly plant, but less successful when used frequently.

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