PHOSPHORUS and SOILS by Rob Cumming
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Phosphorus as a fertiliser and a nutrient source:

 Phosphorus is a key element in almost every process taking place in plants. The importance of phosphorus is related to the transfer of energy from sunlight to the plant. This energy is essential to the plant activities occurring during the life cycle of a plant to allow germination, growth, maturation and reproduction leading to the next generation of plants.

 What are the levels of phosphorus in soils ?

 Soils in Australia compared to the rest of the world are very low in phosphorus. Soils in Australia can have levels from 2 kg P/ha up to 800 kg P/ha. The higher levels tend to occur in a few isolated pockets such as volcanic or alluvial soils.

Thus in most cases phosphorus must be added to the soil to enable adequate plant growth. A typical level would for a pasture would be about 40 kg P/ha.(15 ppm as Bray) In a horticultural situation, phosphorus should be around 80 kg P/ha (30 ppm) for adequate growth. The only method for checking soil phosphorus is by soil testing. If phosphorus levels are adequate there is no need to add further phosphorus.

 How is phosphorus held in soil ?

 Much of the phosphorus is held as organic residues, iron phosphates or tightly bound with aluminium. Iron can be very important in soils where these are fixed. Examples of these are found on Kangaroo Island and the north Coast of NSW. In these soils phosphorus can be continually added and just "disappears" into the soil and is not available to the plant.

 A similar process occurs in soils with soil soluble aluminium. This is particularly important in soils which are becoming acidic as phosphorus applications are "tied up" with aluminium and again becomes unavailable to plant growth. Thus in using phosphorus it is important to understand what the current health of your soil is before using phosphorus.

 And what is used by plants ?

 Product Phosphorus removed (CSIRO Div. Soils)
Cereal grai n      3 - 6 kg/ha
Milk                  4 - 5   "
Animals            1 - 2   "
Hay                  8 - 15 "
Potatoes           5 - 15 "

 And what sources are there of phosphorus ?

The most common phosphatic fertiliser is superphosphate. There are now a range of phosphate fertilisers available which have different levels of phosphate. These are used for a range of purposes and often are mixed with other nutrients such as nitrogen.

Further details can be found by e-mailing us !

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