APPENDIX 2. Australian Soil Resources Information System
Total phosphorus (topsoil)
Phosphorus is an element that is present in small amounts in some rocks (average total phosphorus content of rocks is 1.2%). As these rocks break down the phosphorus is released and becomes available for plants to take up and incorporate it into organic compounds. Total phosphorus measures the amount of phosphorus tied up in soil mineral particles (which come from the break down of rocks) and in organic matter. Total phosphorus is generally much higher than the amount of phosphorus actually available for plant uptake (see also Figure A12).
Why is it important?
Phosphorus is essential for plant growth. The total phosphorus content of most Australian soils is low by world standards and many soils require phosphate fertilisers to maximise production.
Phosphorus can occur in many kinds of compounds in the soil-both organic and inorganic-and these are very sensitive to pH. If the pH is too low or too high, phosphorus is not available for plant uptake. Phosphorus is most available for plant uptake at pH ranging from 6 to 7.
How does phosphorus vary and what is it related to?
Most Australian soils contain less than 0.02% phosphorus (percent by weight or grams of phosphorus in 100 g of soil). Variations in total phosphorus are mainly due to different rock types. Soils with higher phosphorus levels are derived from basaltic rocks because these rocks are higher than average in phosphorus; phosphorus generally appears to be very low in areas where Calcarosols occur. Phosphorus content should also co-vary with organic carbon and nitrogen due to the phosphorus that is bound up in organic compounds.
* Australian Soil Resources Information System map estimates for total phosphorus appear anomalously high in portions of the Northern Territory.
How and why does it vary across Australia?
Mapping of total phosphorus in topsoil can be used to identify areas where natural soil fertility is low and fertiliser inputs would be required for crop production. It can be combined with pH maps to:
- predict where and how much phosphorus is likely to be available to plants under natural conditions;
- identify the areas where fertiliser needs to be added; and
- estimate the amount of lime required to bring the pH to a level where the phosphorus will be available for plant uptake.
Level of uncertainty
Very few measurements of total phosphorus were available for soils in New South Wales and Victoria with most measurements coming from Queensland and Western Australia. Modelled estimates are most reliable for Queensland and the Murray-Darling Basin, and least reliable for New South Wales, eastern Victoria, South Australia and Northern Territory.
Error diagnostics
| Error diagnostic | Topsoil |
| Number of points used | 7327 |
| R2 | 0.684 |
| Relative error | 0.49 |
Table A7 Total phosphorus (%).
| < 0.02 % | 0.02 - 0.05% | > 0.05 % | Total land use class area (ha) | |
|---|---|---|---|---|
| Conservation and natural environments | 63 | 28 | 10 | 54 814 200 |
| Production from native environments | 60 | 33 | 7 | 184 376 300 |
| Cropping | 71 | 24 | 5 | 22 240 900 |
| Grazing modified pasture | 65 | 28 | 7 | 18 482 500 |
| Horticulture | 60 | 24 | 16 | 351 500 |
| Irrigated cropping | 46 | 47 | 7 | 948 900 |
| Irrigated modified pasture | 84 | 12 | 4 | 1 079 900 |
| Total area* | 282 294 200 |
* Area of river basins containing intensive agriculture
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