Purpose

To ensure adequate phosphorus supply to grain crops.

Most Australian soils, including those used for grain production, are naturally infertile and often contain phosphorus levels considered too low for optimal grain production under the prevailing climatic conditions. Phosphorus (P) is commonly the second most limiting plant nutrient after nitrogen.

• Losses from paddocks may enrich regional water bodies and coastal estuaries with dissolved phosphorus, increasing the risk of algal blooms and lowering water quality. This can affect the quality of domestic and stock water. These processes may pose future problems for rural communities.
• Soil acidification may cause reduced availability of a number of important micronutrients and increased potential of aluminium toxicity.

Components targeted:                      Soil phosphorus

Components that may be affected:   Soil acidity; Soil nitrogen and Surface water phosphorus

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Some characteristics of sustainable management practices were investigated by Cary et al. (2002). The results are presented in Table 1.

Table 1: Characteristics of sustainable management practices as assessed by Cary et al. (2002)

Sustainable practice	Geographic applicability	Relative advantage	Risk	Complexity	Compatibility	Trialability	Observability
(Ideal rating)	(high)	(high)	(low)	(low)	(high)	(high)	(high)
Nutrient balance accounting (soil and plant sampling)	low	low	low	high	medium	low	low
Soil and plant tissue tests to determine fertiliser needs-a	low	low	low	high	medium	low	low
Regular soil testing	medium	medium	low	low	medium	low	low
Fertilising of pastures	medium	high - medium (locality)	medium	low	high	high	high - medium

• Geographic applicability—refers to relative appropriateness of a practice, in terms of whether it is effective or adapted to only specific localities or, more universally, across many localities.
• Relative advantage—relates to the financial advantage or other convenience or personal advantage to the farm business or the adopter.
• Risk—refers to uncertainty about likely benefits or costs associated with a sustainable practice, the effectiveness of the practice, when the benefits might be realised and the social acceptability of the practice.
• Complexity—implies that a practice comprises more than one or two simple elements and that its elements interact with each other and, in sometimes complicated ways, with elements of the farming system into which it is to be incorporated.
• Compatibility—refers to the extent to which a practice fits in with existing farm practices, knowledge or social practice.
• Trialability—where practices can be implemented on a small or pilot scale, decisions can be more easily made about the value of a new practice without the risks associated with full implementation.
• Observability—practices where the impact or advantage is easily observable or the outcome quickly realised are more likely to be adopted.

Indicator

Total application of phosphorus (tonnes per GRDC agro-ecological zone per year)

More desirable indicators would be the level of achievement of the desired nutrient balance for each farm or paddock, aggregated to regional or national level, although this is rarely reported, or the application of phosphorus per unit area.

Results

Map 1 displays the total amount of phosphorus applied for crops per GRDC agro-ecological zone in 1996, while Table 2 provides results for multiple years.

Map 1: Total amount of phosphorus fertiliser (t/year) used for crops in GRDC agro-ecological zones during 1996.

Table 2: Total amount of phosphorus fertiliser (t/year) used for crops in GRDC agro-ecological zones during 1992-96.

^{Source- NLWRA (2001) Australian Agriculture Assessment: Farm gate nutrient balance.}

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The National Land & Water Resources Audit (NLWRA) Australian Agriculture Assessment 2001 report made the following assessment using the same data.  Consumption of phosphorus fertilisers in Australia have increased in recent years. Use of nitrogenous fertilisers now greatly exceeds phosphorus consumption.

Factors affecting fertiliser decisions

Land use and climate have major influence on fertiliser use decisions confirming that:

• trends in the levels of use of fertilisers were consistently lower in more arid, low yielding environments of the cropping zone than in more reliable, higher yielding regions—decisions on fertiliser use match anticipated returns.
• adverse seasonal conditions experienced in the drought year, 1994, markedly depressed fertiliser use generally during this year and the next year—especially noticeable across dryland cropping regions.
• Phosphorus use in dryland cropping areas is directed to the cropping phase.  

Status

Australia has a long history of phosphorus fertiliser application and the phosphorus status of most agricultural soils has been raised.

• 1.4 million hectares of agricultural land (estimate only) still had very low surface soil phosphorus levels (< 10 mg/kg Colwell extractable soil phosphorus). These areas tended to be mainly in the drier regions in each State, where lower input farming is practised and were especially evident in South Australia.
• 24.6 million hectares—across all States—and making up 28 per cent of the land assessed (estimate only), had soils of marginal phosphorus status (10 to 20 mg/kg extractable soil phosphorus).
• 3.2 million hectares had high values (> 80 mg extractable P/kg), located mainly in Queensland, New South Wales and Victoria. Intensive, irrigated agriculture, especially dairying and horticulture are often located in areas with naturally higher fertility status soils. Levels are augmented with fertiliser application.

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Metadata: The Farm gate map for Australia was downloaded from the NLWRA. The dataset contains nutrient inputs from various sources, nutrient exports in harvested agricultural products and the calculated differences between nutrient inputs and exports (termed farm gate nutrient balance). The data are reported by Statistical Local Areas within the intensive agricultural zone of Australia and cover the period from 1992/93 (1989/90 for WA) to 1996/97. The nutrients were nitrogen (N), phosphorus (P) and potassium (K) for that span of years and sulfur (S), calcium (Ca) and magnesium (Mg) for 1995/96 (More information- Farm gate nutrient balance metadata summary). Fertiliser sales data for each SLA were mapped to GRDC zones (More information- GRDC zone metadata summary). 

References

Cary, J. Webb, T. and Barr, N. (2002) Understanding landholders capacity to change to sustainable practices: Insights about practice adoption and social capacity for change. Bureau of Rural Sciences, Canberra, p. 17.

NLWRA (2001) Australian Natural Resources Atlas: Australian Agriculture Assessment 2001 - Appendix 2. Australian Soil Resources Information System: Total phosphorus. National Land & Water Resources Audit, Canberra. Website- http://www.anra.gov.au/topics/agriculture/pubs/national/agriculture_asris.html. Accessed 16/07/2008.