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Soil phosphorus
Soil phosphorous is an important macronutrient in soil. Crop production is often enhanced by the application of this element as relatively large amounts are removed through harvest.
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Phosphorous is provided either from the soil or soil amendments, such as commercial fertilisers or animal manures. Large amounts are removed through harvesting. The availability of soil phosphorous to plants is highly dependent on soil pH (Figure 1).
(Source: http://www.extension.umn.edu/distribution/cropsystems/DC6795.html )
Figure 1: The availability of phosphorus is affected by soil pH.
Desired outcome
To maintain soil phosphorus at optimum levels for agricultural production.
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- This outcome corresponds to the National NRM M&E Framework (2004a) outcome to ‘maintain or enhance resource quality, maintain productive capacity’.
Indicator
Total soil phosphorus.
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Other possible indicators:
Available soil phosphorous would be a better indicator of phosphorous accessable to plants, as the major portion of total soil phosphorous (96% to 99%) is unavailable to plants. However spatial data are not available for the whole of Australia. Some data exists for NSW.The bulk of the soil phosphorous exists in three general groups of compounds - namely, organic phosphorous, calcium-bound inorganic phosphorous, and iron- or aluminium-bound inorganic phosphorous. Most of these groups have very low solubility and are not readily available for plant uptake.
Phosphorous is usually plant-available in soil as inorganic phosphate ions (HPO42- and H2PO42-) and sometimes as soluble organic phosphorous. The HPO42- anion dominates in strongly acidic soils while the H2PO42- anion dominates in alkaline soils. Both anions are important in near-neutral soils.
Summary measure
The proportion of land with total soil phosphorous above 0.02%.
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Values of total soil phosphorus above this level were classified as medium or high by the National Land and Water Resources Audit (NLWRA, 2001).
Results
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Figure 2. Summary measure for Soil phosphorous
The summary measure (figure 2) shows the extent to which the desired outcome is being achieved on a scale of 0 to 1. The summary measure for the year 2000 is between 0.3 and 0.4 based on the indicator values displayed in Map 1. Tables 1, 2, and 3 provide information by GRDC Agro Ecological Zones. The results vary considerably among GRDC Zones with particularly low values in Western Australia.
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Map 1: Total soil phosphorous (%) in land under cropping in the GRDC zones.
Table 1: Area (in 000’s Ha) of cropped land with total soil phosphorous > 0.02%.
| Region | GRDC zones | Total area of cropped land (000’s of hectares) | Area of cropped land with total P > 0.02% (000’s of hectares) | Proportion of cropped land with total P> 0.02% |
| Northern Region | Qld Central | 1623 | 1565 | 0.96 |
| NSW NorthWest-Qld SouthWest | 1577 | 1489 | 0.94 | |
| NSW NorthEast-Qld SouthEast | 3810 | 3644 | 0.96 | |
| Southern Region | NSW Central | 1656 | 655 | 0.40 |
| NSW Vic Slopes | 3139 | 2258 | 0.72 | |
| SA Vic Mallee | 3272 | 646 | 0.20 | |
| SA Midnorth-Lower Yorke Eyre | 2184 | 384 | 0.18 | |
| SA Vic Bordertown-Wimmera | 4116 | 695 | 0.17 | |
| Vic High Rainfall | 1254 | 947 | 0.76 | |
| Southern Region | Tas Grain | 306 | 253 | 0.83 |
| WA Northern | 2211 | <0.5 | 0.00 | |
| WA Eastern | 1555 | 0 | 0 | |
| WA Central | 5898 | 52 | 0.01 | |
| WA Mallee and Sandplain | 1188 | 0 | 0 | |
| All Regions | All GRDC Zones | 33789 | 12588 | 0.37 |
Source: NLWRA (2001) Australian Soil Resources Information System.
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Table 2: Area (in 000’s Ha) of cropped land with low, medium and high phosphorous levels.
Region GRDC zones Low <0.02% Medium 0.02-0.05% High >0.05% Total cropping area Northern Region Qld Central 58 637 928 1623 NSW NorthWest-Qld SouthWest 88 1080 409 1577 NSW NorthEast-Qld SouthEast 166 2175 1470 3810 Southern Region NSW Central 1001 601 53 1656 NSW Vic Slopes 881 2085 173 3139 SA Vic Mallee 2626 279 367 3272 SA Midnorth-Lower Yorke Eyre 1800 332 53 2184 SA Vic Bordertown-Wimmera 3421 633 63 4116 Vic High Rainfall 307 797 150 1254 Tas Grain 53 220 33 306 Western Region WA Northern 2211 <0.5 0 2211 WA Eastern 1555 0 0 1555 WA Central 5846 52 0 5898 WA Mallee and Sandplain 1188 0 0 1188 All Regions All GRDC Zones 21201 8891 3699 33789 Source: NLWRA (2001) Australian Soil Resources Information System
Table 3: Total soil phosphorus for cropped land compared to all land in a zone.
Region GRDC zones Average total phosphorous in whole area (%) Average total phosphorous in cropping area (%) Northern Region Qld Central 0.382 0.397 NSW NorthWest-Qld SouthWest 0.316 0.267 NSW NorthEast-Qld SouthEast 0.295 0.281 Southern Region NSW Central 0.132 0.137 NSW Vic Slopes 0.185 0.170 SA Vic Mallee 0.197 0.198 SA Midnorth-Lower Yorke Eyre 0.235 0.251 SA Vic Bordertown-Wimmera 0.257 0.258 Vic High Rainfall 0.213 0.209 Tas Grain 0.410 0.466 Western Region WA Northern 0.352 0.379 WA Eastern 0.401 0.457 WA Central 0.437 0.459 WA Mallee and Sandplain 0.387 0.422 Source: NLWRA (2001) Australian Soil Resources Information System.
- Data
Lineage: The dataset is a part of the NLWRA (2001) Australian Soil Resources Information System. This data for total soil phosphorous was presented as a map surface of predicted total phosphorous of top soil (A Horizon) for the intensive agricultural areas of Australia. Data were modelled from the spot observations taken by State and CSIRO soil agencies. The point information was extrapolated to produce the map. For details of modelling methodology, see Henderson et al (2001). Total phosphorous is presented as weight % phosphorous (for example g of P per 100g soil).The NLWRA Landuse map was used to identify areas that were cropped for grains (more information- Landuse Map metadata). Thereafter using the identified cropped area, the area of land under each nutrient category was extracted for each GRDC Zone (more information- GRDC Zones metadata). For further information on methodology see more information- Soil phosphorous metadata.
Data quality: This raster data set has a grid resolution of 0.001 degrees (approximately equivalent to 1.1 km). The data set is a product of the National Land and Water Resources Audit (NLWRA) as a base dataset. This map can be used to identify areas where the natural fertility of soils is low and fertiliser inputs would be required to enhance crop production. It can be combined with the pH map to predict where and how much phosphorous is likely to be available to plants under natural conditions.Very few measurements of total phosphorous were available for soils in NSW and Vic. Most measurements were from Qld and WA. The modelled estimates are most reliable for Qld, and least reliable for NSW, eastern Vic, SA and NT. The models were created using the regression tree software Cubist and a suite of 40 environmental predictors. Uncertainty surfaces corresponding to ASRIS point model prediction surfaces is also available from NLWRA for this soil property.
Other datasets
Responses
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The most common management actions to increase soil phosphorus include adding phosphorous fertilisers, and adopting conservation practices, such as minimum tillage and stubble retention. Minimum tillage and stubble retention promotes improvements to the physical, chemical and biological condition of agricultural soils (NLWRA, 2001). In cropping areas, phosphorous fertilisers are directed mainly to crops, with pastures relying on residual soil phosphorous.
Environmental concerns with phosphorous centre on eutrophication — an increase in the fertility status of natural waters that causes accelerated growth of algae or water-plants. Reducing phosphorous entry to surface water limits the rate of aquatic plant growth and decreases the chances of fish kills caused by oxygen depletion when plants die and decay.
Phosphorous, primarily in the form of phosphate, is not as soluble as nitrate and is primarily transported by sediment in runoff, often ending up in rivers and streams.
More information on management practices-Maintain cover along drainage lines
Interactions with other components
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Primarily interacts with Agricultural species- Varieties. Also may impact on farm income, soil nitrogen, water quality- nutrients and microbiology for surface water or groundwater.
Effective management of phosphorous in soil will have profound impacts on the productivity of cropping systems across Australia, on their environmental performance, on rural societies and economies and on the balance of trade. Management of phosphorous in soil requires procurement of chemical fertilisers that depends on farm income and for the uptake of phosphorous by crops the soil pH must be maintained towards optimum levels.
External drivers
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Balance of trade, rainfall, income, phosphorous.
References
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ANRDL (2001) Soil total phosphorous for Australian areas of intensive agriculture (derived from site measurements) Available from: http://asdd.ga.gov.au/asdd/tech/zap/basic-full.zap?&target=brs-4&syntax=html&cclfield1=all&cclfield2=phrase&cclfield3=any&cclterm1=total%20phosphorous&cclterm2=&cclterm3=&start=1&number=1, Accessed 23/9/2005.
NRM (1999) Resource Condition Indicators. NRM Australian Government. Website http://www.nrm.gov.au/monitoring/indicators/index.html#list, Updated, Wednesday, 22-Dec-2004, Accessed 23/9/2005.
Henderson, B; Bui, E; Moran, C.; Simon, D.; Carlilie, P. (2001) ASRIS: Continental-scale soil property predictions from point data. Technical Report 28/01, CSIRO Land and Water, Canberra. 107p.
National NRM M&E Framework (2004a) National Framework for Natural Resource Management (NRM) Standards and Targets, NRM Australian Government. Website- http://www.nrm.gov.au/publications/standards/index.html, Updated, 04-Aug-2004, Accessed 23/9/2005.
NLWRA (2001) Australian Agriculture assessment 2001. Available from: http://audit.ea.gov.au/ANRA/agriculture/docs/national/Agriculture_Contents.html, Accessed 23/9/2005.
Standing Committee on Agriculture and Resource Management (1998) Sustainable Agriculture: Assessing Australia's Recent Performance, Vol. Report 70, CSIRO Publishing, Collingwood.
