Nutrient Loss – Export (Phosphorus)
Phosphorus (P) is an important contributor to the eutrophication of waterways. Like nitrogen, P is required by plants in relatively large quantities for normal growth. Unlike nitrogen, P rapidly forms insoluble compounds when applied to soil and plants are unable to absorb it directly from the atmosphere. Nutrient management programs commonly target phosphorus as the limiting nutrient for algal growth.
P is transported in moving water. Thus, increased export is linked to excessive clearing of native vegetation, drainage of waterlogged soils, the application of highly soluble fertilisers to correct nutrient deficiencies (e.g. superphosphate) and the establishment of pasture with limited root systems and limited ability to take up applied nutrients.
Provided that sufficient contact time has occurred, P in soils tends to be firmly tied to soil particles. Hence, water erosion is generally the main mechanism of P export.
Sources of P from agriculture can be broadly classified as diffuse (coming from a wide area) or point (localised discharge, such as from a pipe) and include:
- broadacre grazing on fertilised pasture land (cattle and sheep)
- cropping on arable land
- intensive livestock industries (dairies, feedlots and stock holding yards, piggeries, poultry farms)
- orchards and intensive horticulture.
The coastal soils have a low capacity to retain nutrients. Any land clearing for productive agriculture increases nutrient run-off (Brearley 2005). Nitrogen and P are exported in solution or bound to soil particles in surface run-off from clay soils and both surface and sub-surface flow (leaching) in sandy soils, when saturated with water (Summers et al. 1999). Soluble P is more likely to be exported from sandy soils through leaching and run-off and from clay soils in run-off when the soil surface is saturated with P.
Increased rainfall increases nutrient export. On the other hand, reduced rainfall magnifies the impact of lost nutrients because it results in modified catchment hydrology. Winter river flows have generally decreased in recent years, reducing the amount of nutrients and organic matter flushed out to the ocean. Instead they accumulate in the sediments of waterways. In summer, saline water moves further upstream, often remobilising nutrients stored in the sediment.
Summer storms may become more frequent in the future. These stimulate nutrient export and re-suspend stored nutrients during periods of warm temperatures, further exacerbating the potential to create algal blooms (Giller and Malmqvist 1998; Chuwen et al. 2009)
Export of P from agriculture, urban and industrial sources has contributed to nutrient enrichment of streams, rivers and estuaries, resulting in water quality problems (eutrophication), such as algal blooms, occasional fish kills, odours from decaying algae, and potential accumulation of toxins in shellfish.
Phosphorus is an essential element for plant and animal growth and many of WA’s agricultural soils were originally amongst the most P deficient in the world. While P is critical for profitable agriculture, its export through surface water run-off and leaching increases the chance of algal blooms in receiving water bodies.
Susceptible soils and occurrence
Susceptibility to P export risk is primarily a concern along the coastal zones of the south-west and the south coast. Areas such as the coastal plain, which are characterised by sandy soils and intensive drainage, are particularly at risk.
The main mechanisms of control are stabilisation of catchment processes and reduction of nutrient outputs.
Stabilisation of catchment process focuses on controlling the mechanism by which P is transported into water bodies. The key strategies are riparian and drainage management. By managing riparian vegetation and streamlining drains with vegetated buffers nutrients are filtered out before they enter waterways. Riparian management includes stock exclusion, rehabilitation and revegetation, construction of stock crossings and off-stream watering points. For intensive developments, water-sensitive residential design guidelines (DPUD 1993) should be adhered to.
Reduction of nutrient outputs can be achieved through a number of methods., particularly effective fertiliser management. This involves matching fertiliser application to plant requirements through regular soil and tissue testing, and managing the timing and method of application to avoid leaching and runoff.
Nutrient outputs can also be reduced through use of specifically formulated fertilisers such as New Coastal Superphosphate or RedCoat and applying soil amendments to sandy soils such as high PRI loams (Gilkes et al. 1992) or ‘Red mud’ (Summers et al. 1993).
Replacing annual pastures with perennial pastures will minimise nutrient losses from leaching as they use nutrients longer and from greater depth. Adherence to good irrigation design and specifically tailored irrigation strategy will reduce phosphorus loses, as will restricting levels and intensity of developments (e.g. urban, horticulture) in waterlogged areas, and treatment of point source pollution (e.g. piggeries, stock-holding yards, dairies) effluent prior to discharge.
Water quality improvement plans (WQIPs) have assessed and modeled the magnitude of P from different sources and suggested ways to reduce nutrient export. Within the SW NRM Region, WQIPs now exist for the Vasse Wonnerup Wetlands and Geographe Bay, Leschenault Estuary and Hardy Inlet – Stage 1 Scott River catchment.
Key Messages across south-west Western Australia (DAFWA Report Card 2013)
Condition and trend
- While phosphorus (P) is a critical plant nutrient for profitable agriculture, its export in surface water run-off or leaching accelerates eutrophication in waterways.
- The export of nutrients used in agriculture was assessed as high to very high hazard for most of the coastal catchments.
- Algal blooms caused by excessive nutrients and organic matter in waterways are the major cause of fish kills and seagrass losses in rivers and estuaries across the south-west of WA
- A range of factors influence P export from agriculture, including fertiliser application method, storage, timing of application, rate applied and source, soil type and texture, slope, paddock fertility, connectivity to drainage, irrigation management, manure disposal and potential for erosion.
- Management practices that can reduce P export from paddocks include:
- amending soils to increase low P retention capacity
- using soil testing to inform decisions on fertiliser application rates
- using perennial pastures to increase water and nutrient uptake and decreased erosion
- accurately placing fertiliser
- having fertiliser-free buffer zones close to waterways
- using appropriate fertiliser for the soil type and the plant requirements
- improving irrigation efficiency – replace flood with central pivot irrigation, recycle nutrients within irrigation systems
- improving livestock management around feedlots and dairy sheds, and using effluent to offset nutrient inputs
- licensing of agricultural point sources of nutrient discharge into catchment/estuarine systems
- protecting and revegetating wetlands and waterways
- adopting agricultural best management practices.
Projects addressing Nutrient Loss – Export (Phosphorus)
SWCC supports a number of landholders and partners to undertake projects addressing agricultural practices including nutrient loss – export (P). One project addressing nutrient loss is:
- AccuSpread – Improving Nutrient Management project
In addition to working with agricultural managers, SWCC and its partners have developed the Home River Ocean campaign, Save the Crabs, Then Eat Them…which targets the suburban gardeners and their fertiliser practices.
Rain carries excess fertiliser from suburban lawns and gardens into stormwater drains, rivers and estuaries; all the way to the ocean. Fertilisers can threaten these sensitive environments and the health of fish and shellfish that live there, like the Blue Swimmer Crabs.
The up side is that small and simple changes in our homes, gardens and everyday lives can make a big difference. That’s what Home River Ocean is all about: People working together towards healthier waterways to protect the habitat of the Blue Swimmer Crab.
So, let’s ‘Save the Crabs, Then eat Them!’.
To find out more about the campaign check out the Home River Ocean website here.
One of the ads airing currently:–
- Ruprecht J, Vitale S and Weaver D (2013). ‘Nutrient export (phosphorus)’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.
- Van Gool, D., Vernon, L. and Runge, W. (2008). Land Resources in the South-West Agricultural Region. A shire-based summary of land degradation and land capability. Department of Agriculture and Food Western Australia.