Phosphorus is one of the major plant nutrients in the soil. It is a constituent of plant cells, essential for cell division and development of the growing tip of the plant. For this reason it is vital for seedlings and young plants.
Without phosphorus, plant growth is retarded. Plants have stunted roots, and are stunted and spindly. Deficiency symptoms also include dull greyish-green leaves and red pigment in leaf bases and dying leaves. Phosphorus deficiency is difficult to diagnose, and by the time it is recognised it may be too late to do anything. If plants are starved of phosphorus as seedlings they may not recover when phosphorus is applied later.
Like most Australian soils, North Coast soils are naturally low in phosphorus due to extensive weathering. While native plants are adapted to these low levels, introduced crops and pasture grasses are not, which means you need to apply phosphorus fertilisers to soil to achieve productive yields. Australian farmers use much more phosphorus than nitrogen and potassium compared with farmers in Europe and USA.
Phosphorus fertilisers are available in several forms, all based on rock phosphate.
Superphosphate (9% P)
This is produced by treating rock phosphate with sulfuric acid, and is the most commonly used phosphorus fertiliser.
Double superphosphate (17.5% P); Triple superphosphate (20% P)
In both of these fertilisers, phosphoric acid is used instead of sulfuric acid in manufacture.
Monoammonium phosphate (MAP) (21% P); Diammonium phosphate (DAP) (20% P)
These two fertilisers are produced by treating phosphoric acid with ammonia.
Rock phosphate (15.5% P)
Crushed rock phosphate is sometimes used as a fertiliser but is very insoluble. Reactive phosphate rock (RPR) can be more soluble than crushed rock phosphate where there is acid soil and high rainfall, such as occurs on the North Coast.
All manures contain phosphorus, and manure from grain-fed animals is a particularly good source. For this reason it is advisable to leave manure from grazing animals where the animals have grazed.
Phosphorus is available in slag from steelmaking in Europe, but there are no commercially available forms.
Chemically, phosphorus is a very stable element. Fertiliser phosphorus does not move far from where it is applied because it reacts rapidly with soil.
It quickly binds with iron and aluminium in the soil and becomes unavailable to plants, especially when soil pH is below 5.0 (CaCl2).
The term CaCl2 after the pH figure signifies that the pH was measured in a solution of calcium chloride, a test preferred by most soil scientists. pH tested in CaCl2 is 0.5-0.8 pH lower than if tested in water.
Because phosphorus is so easily fixed in the soil, crops and pasture take up only 5–20% of phosphorus applied to the soil.
When broadcast in permanent pastures, phosphorus accumulates on the soil surface and is readily available to plants when moisture allows roots to grow to the surface. However, it becomes less available to plants if the surface soil dries out.
There are several possible reasons if your pastures or crops seem to get little benefit from phosphorus.
Because phosphorus is so easily fixed in the soil, plants can take up only a small amount of the phosphorus you apply. For this reason it is important that you apply phosphorus every year and test your soil regularly to monitor phosphorus levels.
From the Soil Sense leaflet, 10/92, Agdex 531 produced by Rebecca Lines-Kelly, formerly soils media officer, Wollongbar Agricultural Institute, for CaLM and NSWA, north coast region, under the National Soil Conservation Program, October 1992.