Why phosphorous is important

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.

Deficiency symptoms

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.

Phosphorus fertilisers

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.

Manure
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.

Steel slag
Phosphorus is available in slag from steelmaking in Europe, but there are no commercially available forms.

Phosphorus in the soil

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.

Uptake problems

There are several possible reasons if your pastures or crops seem to get little benefit from phosphorus.

  • Your soil is so low in phosphorus that even with some phosphorus applications, there is little available in the soil solution for the plant to take up. This is a common problem on the North Coast where superphosphate application is too low.
  • You have cultivated the phosphorus into the soil where it is not readily available to most crops because its effect is diluted and there is greater likelihood of the phosphorus binding with other minerals.
  • Your soil may have reached its optimum phosphorus level, and adding extra will only maintain the level, not boost it. This is the most usual reason for non-response in well-farmed land.
  • Other factors may be contributing to poor crop growth.

Improving uptake

  • Do a soil test to check your phosphorus levels and see whether your soil already has enough phosphorus for plants, in which case they won't respond to extra phosphorus.
  • When sowing seed, place phosphorus fertiliser close to seed. This is very effective in low-phosphorus soils, for you need only half as much phosphorus as you do when broadcasting the fertiliser.
  • Incorporate lime in your soil to raise your soil pH to 5.0 (CaCl2) and reduce the availability of aluminium in the soil. In this way, applied phosphorus will not be readily tied up in aluminium compounds and will be more available to plants.

Conclusion

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.