Dr Richard McDowell from AgResearch at Invermay is concerned about phosphorus being lost from the soil and causing water pollution via eutrophication.

Phosphorus – necessary in small doses

Phosphorus is an important macronutrient for both plants and animals. Naturally occurring phosphorus compounds found in soil can supply the needs of the plant. However, intensive agricultural practices often require larger amounts of this key macronutrient to increase plant production. To provide this important macronutrient, it is common practice to spread ‘superphosphate' fertiliser on the paddocks. Care is needed to ensure that phosphorus run-off to waterways does not occur

Agricultural pastures and crops need phosphorus in the soil to be healthy. A lack of phosphorus can result in poor plant growth and a decrease in crop yields. Soils deficient in phosphorus are replenished with phosphorus fertilisers or effluent. The problem arises when phosphorus is lost from the soil via surface run-off and subsurface flow (leaching) and deposited into waterways.


Once in a waterway (streams, lakes or estuaries), phosphorus stimulates growth of aquatic plants and weeds. These plants block sunlight from getting into the water, killing off other plants and organisms. When the plants die, they use up oxygen from the water, also killing off other organisms that need oxygen to live. This is eutrophication.

Monitoring phosphorus

Richard’s research enables farmers to work out how much available phosphorus they have in their soil and how much they need to add in the form of fertilisers or effluent for optimum plant growth, while minimising the potential for phosphorus to be lost to water.

Richard and his team are investigating ways in which farmers can decrease phosphorus loss. This involves examining the processes and pathways of phosphorus loss. For example, compacted soils contribute to phosphorus loss. Compacted soils have fewer pore spaces between the soil particles, making it difficult for water to enter into the soil. This increases the potential for erosion. Dissolved phosphorus adsorbs (sticks) to soil particles. Run-off water carries the soil particles (sediment) and the phosphorus away.

Management practices to decrease phosphorus loss from land to water

Avoiding soil compaction helps decrease surface run-off. Putting stock on stand-offs or herd shelters when the soil is likely to be wet for long periods (such as in winter) helps to keep soil from being compacted and lessens the likelihood that run-off will occur.

Fencing off riparian zones including wetlands helps prevent losses by preventing stock depositing dung (extra phosphorus) near waterways where it can be washed away and stream banks destabilised.

Farm dairy shed effluent can be collected and returned to the land through spreaders at low rates and when the soil has a sufficient moisture deficit that run-off or subsurface flow will not occur.

Careful attention needs to be given to the quantity of phosphorus fertiliser and to where and when it’s applied. Application during cooler, wetter parts of the year should be avoided when the risk of run-off is high. Some less soluble forms of phosphorus may pose a lesser risk of run-off than others.

Farmers should give careful thought to their landscape – especially areas more prone to sediment loss. Areas that account for the majority of contaminant losses but occupy a small part of a farm catchment are called critical source areas. For example, it may be that 80% of the loss comes from 20% of the land, so targeting these critical source areas with mitigation strategies is much more efficient than a blanket approach.

Richard and other scientists have devised the online Overseer® Nutrient Budgeting tool to help farmers determine phosphorus loss from land to surface water. The model determines potential risk of phosphorus loss from the soil and from application of fertiliser or effluent to waterways. The tool estimates the total amount of phosphorus loss annually. The Overseer® tool can be used by farmers and regional councils to set annual performance targets.

Nature of science

When developing explanations about their data, scientists often use models. Development of the model may allow it to be used commercially as a predictive tool. With Richard’s research into phosphorus loss from soils, he and others have developed an online budgeting tool that estimates the total amount of phosphorus loss annually.

    Published 30 July 2013