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Plants require 12 mineral nutrients that are essential for their growth and development. A shortage of any one element will result at best in stunted and poor growth. The plant however needs the various minerals in differing amounts. Phosphorus, together with Nitrogen and Potassium, being consumed in relatively large amounts, is considered one of the macro elements.

There are a number of vital functions that Phosphorus performs in the plant, such as the transference of energy within cells, and its role in building proteins. The problem for gardeners though, is how to ensure that adequate supplies are readily available to the plants, because the presence of phosphorus in the soil does not necessarily mean that the plants can take it up. It is necessary therefore to gain some insight as to how this nutrient behaves in the soil.

*Phosphorus is virtually immobile in the soil. Due to its low solubility, it tends to crystallize into a solid. This becomes more acute the more alkaline the soil conditions. As plant roots absorb nutrients as mineral salts dissolved in the soil water, it follows that in a solid state, the nutrient cannot be taken up. Furthermore, its molecules readily attach themselves to the mineral clay in soil particles.

*As a result of this immobility, phosphorus fertilizer spread on the ground, or applied in liquid form is liable to be out of the reach of the plants’ roots. On the other hand, the more acidic environment of the root zone causes the phosphorus to be more dissolvable. The conclusion to be drawn therefore is that the phosphorus source, whether chemical or organic, must be incorporated into the soil so that it can become available to the plant roots.

*As the solubility of phosphorus is connected to the soil pH (acid - alkaline scale) it follows that in alkaline conditions, typical of most dry and Mediterranean climates, steps must be taken to make those conditions become more acidic. This is most easily and safely affected by adding large amounts of compost into the soil.

*The presence of nitrogen in mineral form, aids in phosphorus take up. It is necessary therefore to ensure adequate supplies of nitrogen, preferably by way of compost and humus.

It follows then that phosphorus should be incorporated into the soil prior to planting, so that it is present at depths of about 30cm as well as on the top layer of earth. Very large amounts of compost, (say 30-50 liters per square meter) coupled with an organic or slow release fertilizer, may be adequate, while for general maintenance, realistic quantities of compost, say 5 liters per square meter, should do the job.

With regard to lawns however, it is far more difficult to ensure that the phosphorus gets down to the root zone. The application of slow release fertilizer together with occasional but deep watering, should allow for the passage of phosphorus at least to the upper layers of the soil. Make sure to read the label where the relative concentrations of Nitrogen, Phosphorus, and Potassium are expressed respectively as NPK. During the spring and summer the P for phosphorus should be relatively high, while for autumn and winter applications the K for Potassium should be high.

A final note for gardeners in an arid climate. Many fine garden ornamentals in use in dry climates are of Australian origin, such as species of Melaleuca, Callistemon, Grevillea and Eucalyptus. It should be noted that many Australian genera are particularly sensitive to high levels of phosphorus. It may be well worth your while conducting a soil test before amending your soil. If you are in doubt, it would be best to rely on organic matter only, excluding chemical fertilizer altogether.

About the author - Jonathan Ya’akobi.
I’ve been gardening in a professional capacity since 1984.
I am the former head gardener of the Jerusalem Botanical Garden, but now concentrate on building gardens for private home owners.
I also teach horticulture to students on training courses.
I’d love to share my knowledge and experience with you.
So you’re welcome to visit me on http://www.dryclimategardening.com

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