Farmer's Weekly (South Africa)

Organic phosphorus and plant nutrition

In organic farming systems, phosphorus is supplied mainly through recycling of materials such as compost and manure; it is not readily available to plants. Beware of depleting this element from your soil.


Phosphorus (P) plays an essential role in cell division and is therefore key to plant growth. Unlike nutrients such as nitrogen (N), carbon and sulphur, which are partly derived from the atmosphere, all organic P comes from available inorganic P in the soil.

However, the fraction of organic P available for plants is very small and not enough for the plant to grow properly, notes horticultu­

Organic forms of this element include dead plant and animal residues and soil micro-organisms.

These materials contain P mineralise­d by soil organisms, making this macronutri­ent easier for plants to use.

Organic material binds freely available inorganic P so that plants cannot use it. It is very important to plant nutrition as it becomes available through mineralisa­tion.

The rate at which unavailabl­e P becomes available to plants depends on various factors such as temperatur­e, wet/dry periods, bacterial activity in the soil, soil aeration and structure, and so on. The table alongside shows how variable P is in the soil.

Approximat­ely 50% of the N in fertiliser is used within the season, whereas only 20% of P in the fertiliser is used up, according to horticultu­re. org.Ȳza. The rest of the P is absorbed into the organic form, so is not readily available to the plant, or stays in soluble form which the plant can use at a later stage.

Phosphorus is more readily available in natural soils in warmer climates, as mineralisa­tion plays a significan­t role in breaking down the organic compounds. P is mineralise­d faster in added compost than in the organic material of the soil.

“Unfortunat­ely P soil analysis cannot provide a clear picture of how much P is available for plants, as these soil analysis methods also measure unavailabl­e organic P,” says horticultu­ “But at least a soil analysis will give a general indication of the P content.”

Always add P according to how much the plant uses or extracts from the soil. You don’t want P to become depleted in the soil. ȊȲSource: Lecuona A. 2022. ‘Organic phosphorus and plant nutrition’. horticultu­

 ?? FLICKR ?? The wheat plants on the left show a phosphorus deficiency.
FLICKR The wheat plants on the left show a phosphorus deficiency.
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