Phosphorus in the soil - Soil Essentials: Managing Your Farm's Primary Asset - page 29

6
Phosphorus in the soil
Background
Phosphorus is a chemical element and is written using the symbol P. Plants require
phosphorus for cell division and growth, especially in their early stages. As most
Australian soils are low in plant available phosphorus (in the orthophosphate
form), adding phosphatic fertilisers is necessary for good plant growth. Results of
soil tests will give a guide to how much fertiliser is needed.
Alternatively, if high levels of fixed phosphorus are present, it may be converted
to plant available forms by adding the appropriate VAM (vascular arbuscular
mycorrhizae).
If phosphorus is the limiting factor to crop growth, the yield will be
substantially lower than the maximum yield potential.
Phosphorus in the soil
When a phosphatic fertiliser is applied to the soil a number of complicated
chemical reactions start to occur. As the fertiliser dissolves it releases phosphatic
salts, some of which react with the soil ions, iron (Fe), aluminium (Al) and
calcium (Ca), forming insoluble compounds that makes the phosphorus
unavailable to plants. This problem is much worse in acidic soils (where iron and
aluminium are the problem), or highly alkaline soils (where calcium is the
problem), than in soils close to neutral pH. These reactions can occur in a short
space of time, maybe even in hours.