Reducing phosphorus loss

What is phosphorus?

Phosphorus is a chemical element used by plants and animals for growth. It is found naturally as rock phosphate in sedimentary and igneous rock. Because it is bound tightly to mineral components in the soil, only a fraction of phosphorus is available for plant uptake.

Phosphorus typically enters a dairy farm system via purchased feed and phosphate fertiliser (phosphate is the form of phosphorus that can be taken up by plants).

Of the phosphate eaten by the cow in grass or supplement, approximately 30 percent will leave the farm in products such as milk and meat. The remainder will be excreted by the cow as dung.

Phosphorus behaves very differently to nitrogen. It binds with soil and only dissolves slowly in water over time. Although it doesn’t readily leach into groundwater, it can still pollute waterways via critical source areas, erosion, runoff, and direct deposition of dung.

The diagram below shows a simplified on-farm phosphorus cycle.

How phosphorus loss affects the environment

There are three ways that phosphorus is lost from dairy land to water:

• Surface runoff: During high rainfall events or when the soil is saturated, phosphorus in dung, fertiliser or farm dairy effluent can be lost to waterways, particularly if applied on land with a slope.
• Sub-surface flow: Lost via mole and pipe drains or soil macropores and leaching on freely draining soils with a low ability to bind phosphorus.
• Erosion and soil movement: Phosphorus is sticky and binds to soil particles. Any activities that disturb the soil, such as pugging, cultivation, or erosion, can cause soil particle losses and therefore phosphorus losses. Increased applications of phosphorus (fertiliser or effluent) may result in more being bound to each soil particle, which increases the amount at risk of being lost to water. This is why soil testing is essential.

The loss of phosphorus to water from dairy-grazed crops, pasture systems and catchments can vary widely, from 0.5 to 4.2 kg/ha/yr.

Excess phosphorus causes growth of plants in waterways, lakes and estuaries and leads to eutrophication. This can cause algal blooms, rapid weed growth and oxygen depletion, which are detrimental to freshwater ecosystems. Toxic blue-green cyanobacterial blooms can also cause human health issues.

Practical tips for reducing phosphorus loss

There are five areas of your farming operation to consider when taking action to reduce phosphorus loss:

Nutrient management

• Test soil phosphorus regularly (known as the ‘Olsen P’ test) and manage phosphate fertiliser application to ensure phosphorus levels are within the optimum range for your soils. This minimises the amount of phosphorus leaving your farm in loss events. See the Fertiliser Association booklets for further information.
• Manage the amount and timing of fertiliser inputs, taking account of all sources of nutrients, to match plant requirements and minimise risk of losses.
• Store and load fertiliser to minimise the risk of spillage, leaching and loss into water bodies.
• Ensure equipment for spreading fertilisers is well maintained and calibrated.
• Store, transport and distribute feed to minimise wastage, leachate and soil damage.

Waterways

• Use riparian planting as a buffer between paddocks, races, and the water. The plants act as a filter, slowing down runoff and catching sediment and phosphorus.
• Identify the risk of surface runoff of sediment and faecal bacteria on the property and implement measures to minimise transport to water bodies.
• Locate and manage critical source areas such as farm tracks, gateways, water troughs, self-feeding areas, stock camps, wallows and other run-off sources to minimise water quality risks.
• Exclude stock from water bodies to the extent compatible with landform, stock class and stock intensity. Where exclusion is not possible, mitigate impacts on waterways.

Land and soil

• Manage periods of exposed or fallow soil between crops and pasture rotation to reduce the risk of erosion, overland flow and leaching.
• Manage or retire erosion-prone land to minimise soil losses through appropriate measures and practices. (Implementing this principle may mean that Class 8 land is not actively farmed for arable, pastoral or commercial forestry land uses as this land is generally unsuitable for these uses as described in the Land Use Capability Handbook.)
• Select appropriate paddocks for intensive grazing, recognising and mitigating possible nutrient and sediment loss from critical source areas.
• Manage grazing to minimise losses from critical source areas.

Effluent

• Ensure the effluent system meets industry-specific Code of Practice or equivalent standards.
• Have sufficient, suitable storage available for farm effluent and wastewater to avoid applying at inappropriate times.
• Ensure equipment for spreading effluent and other organic manures is well maintained and calibrated.
• Apply effluent to pasture and crops at depths, rates, and times to match plant requirements and minimise risk to water bodies.
• Avoid ponding and runoff. Direct any effluent that collects on hard surfaces such as concreted races, bridges or culverts into your effluent system.

Water and irrigation

• Manage the amount and timing of irrigation inputs to meet plant demands and minimise the risk of leaching and runoff.
• Design, check and operate irrigation systems to minimise the amount of water needed to meet production objectives.