It is generally acknowledged that Australian and particularly Western Australian soils are some of the poorest in the world. This is especially so in regard to phosphorus (P).

Australian native plants have evolved over time in this low P environment with adaptations to enable them to survive on very limited P availability.

Introduced plants, including agricultural crop and pasture species, have no such adaptations and consequently would not survive without the addition of P in the form of fertilizer. Certainly they could not produce to their maximum potential or produce viable economic returns without P fertilizer being inputted into the system.

So, the addition of P fertilizer (along with the addition of other nutrients) has become essential for agriculture in Western Australia to remain viable and sustainable.

Role of P in plants

Phosphorus is critical in the plant in that it is involved in many of the biochemical processes that are essential for plant metabolism, such as, photosynthesis and energy production. It is also a major component of the genetic material DNA. It's critical role in early growth is demonstrated by the lack of root growth

Relatively small amounts of P are removed in grain (see Nutrient Removal). Significantly, phosphorus is not quickly recycled from straw and other plant matter.

Phosphorus in soils

Whilst P is very mobile once within the plant it is very immobile in the soil. Once P from water-soluble fertilizers is dissolved into the soil solution it quickly becomes ‘bound’ by iron, aluminium and calcium particles, rendering it very much less available to the plants for which it was intended. For this reason, crops are sown with P containing fertilizer placed as close to the seed as practically possible to enable seedling uptake before adsorption onto soil particles takes place.

Phosphorus Flow Chart

Phosphorus Deficiency

Given the major role P plays in plant metabolism, a deficiency will affect all aspects of plant growth. Plants lacking P are very stunted, often with a reddish colouration on the stems. The leaves of the plant become very much darker green due to the continued production of chloroplasts (the green cells) but with no leaf expansion possible the green colour intensifies in the small leaves. The leaf tip will then start to die back.

Overcoming P Deficiency

The easiest way to prevent P deficiency from limiting crop or pasture growth is to apply adequate rates of P containing fertilizer, either at seeding with crops or prior to germination with pastures. To establish what is an adequate rate it is suggested that soil analysis be used, in conjunction with tissue testing.

Correction of P deficiency in an established crop is very difficult given the immobility of applied P in soils and that such P would be bound by soil before getting into the root zone.

Supplying adequate phosphorus to the plant can be difficult in some soil types and some seasons.

Recent research has highlighted the problem in soils with high PBI (phosphorus fixing) particularly in seasons where the surface of the soil dries out for extended periods between rain events, early in the plants' life.