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Healthy plants need healthy soil. Here's the basic information on soil fertilizers to get you started on sustainable gardening.

Fertilizers refresh and enrich your soil
Fertilizers refresh and enrich your soil, replenishing plant nutrients that have been used up or leached out.

Nearly all soil needs regular addition of soil fertilizers to remain healthy. Soil fertility constantly changes due to digging or tilling the soil, from plants using up nutrients, and from leaching due to rainfall.

There are two types of fertilizers: organic and inorganic.

Organic soil fertilizers

Organic fertilizers differ from inorganic fertilizers in that they feed plants while also building the soil. Soil with lots of organic material remains loose and airy, holds more moisture and nutrients, fosters growth of soil organisms, and promotes healthier plant root development.

Organic fertilizers can be made using plant litter/cuttings, animal droppings, cottonseed meal, blood meal, bone meal, rock phosphate, and greensand. Each of these is high in one of the major nutrients in the soil.

Inorganic soil fertilizers

These are also called chemical fertilizers. When inorganic fertilizer is used, the soil gradually loses its organic matter and microbiotic activity. When the fertilizer is used up, the soil structure deteriorates, becoming compact, lifeless and less able to hold water and nutrients.

Four basic nutrients

There are four basic nutrients that are found in fertilizers:

Nitrogen promotes leaf and stem growth, gives plant the green colour, and controls oxygen levels in plants. Plants need and use a large amount of nitrogen. The sources of nitrogen are blood meal, cottonseed meal, fish meal, cocoa shells and animal manures.

Lack of nitrogen in soil leads to leaves yellowing and falling off and a stunted growth. Too much nitrogen causes the plant to grow really fast, but results in a spindly plant that will not produce flowers.

Phosphorus promotes seed germination, fruit growth, flowering and healthy roots. Phosphorus also gives winter hardiness to fall plantings. Plants need this in reasonable amounts. The sources of phosphorus are bone meal, rock phosphate, animal manures and fish emulsion.

Lack of phosphorus results in dull green leaves with purplish tints and a stunted growth. Excess phosphorus generally does not affect the plant or soil.

Potassium (Potash)
Potassium promotes young leaves and buds, root growth and seed production. It also improves the flavour of fruit and vegetables. Its sources are potash rock, wood ash and greensand.

Lack of potassium causes loss of colour, particularly in the vein structure, and brown spots on the underside of leaves, and results in short and stocky plants.

Calcium is a soil conditioner usually applied in the form of lime. Calcium corrects an acidic soil and speeds up the decay of organic matter in the soil. Calcium also makes phosphates in the soil available to plants and improves soil structure (when added to clay soil, it will improve drainage). The source of calcium is lime.

Lack of calcium causes poor plant growth, and results in plants that are diseased and often dying. Excess calcium causes soil to become toxic and plants to die.

Need more help?

For more information on fertilizers and trees, and for help with planting, visit the Johannesburg Botanical Gardens Research Centre.