Leaching of fertilisers can cause serious environmental and economic damage
Most commonly used nitrogen and phosphorus fertilisers are very soluble and are designed for average annual rainfall of less than 600mm or soils that can retain these nutrients.
In the high rainfall areas (more than 600mm average annual rainfall), especially on sandy soils, these nutrients can be leached into the groundwater and washed into waterways, even after short periods (months and years, rather than decades). Nitrogen and phosphorus can then remain in sediments of watercourses and estuaries to be used as food for algae when conditions become favourable.
Effect on groundwater
Nitrogen in a nitrate form can cause health problems when it accumulates in groundwater used for drinking. It may be particularly harmful to young children and has been shown to combine with food to make cancer-forming compounds.
Effect on waterways
Excessive use of phosphorus fertilisers has already contributed to algal blooms in waterways and estuaries in Western Australia. These algae can create a range of problems, such as making life unpleasant near waterways (nauseating odours, aesthetics), causing skin irritations from contact with the water, and in extreme cases, poisoning. There have also been economic impacts through damage to fisheries, and impacts on the value of real estate and tourism in affected areas.
Effect on nutrient legislation
Farm businesses can improve environmental and financial outcomes by minimising leaching losses and maximising effective use of nitrogen and phosphorus fertilisers. In other parts of the world, pollution from these fertilisers has resulted in farming being forced to reduce or abandon fertilisers through strong regulation. For example, all farmers in the Netherlands must now account for the nitrogen and phosphorus that is entering and leaving the farm, paying a levy for any escaping nitrogen above 180 kilograms per hectare (kg/ha).
Nitrogen loss pathways
- Nitrate leaching occurs when soil nitrogen, in nitrate form, is dissolved and passes through the soil or when the soil becomes so saturated with water there is overland flow direct to waterways.
- Nitrate concentrations are highest in the watercourses with the first drainage after the autumn break. Therefore, care should be taken with nitrogen fertiliser applications during periods of high rainfall, especially before and during winter.
- Dung and urine patches are far too concentrated for the soil to retain the nutrients, resulting in leaching to groundwater and escape to the atmosphere through conversion to gas.
- Ammonia gas can be generated from urea applied as fertiliser (or as urine) after rainfall at the end of spring and in summer. Fertiliser applied to warm and already wet soil usually results in the highest loss of ammonia.
- Other sources of nitrogen, like diammonium phosphate, ammonia nitrate and calcium ammonium nitrate, are less likely to turn to gas under our acidic soil conditions.
- Nitrogen can also be lost via short circuit pathways, such as through cracks or macropores in the soil, similar to small tunnels. This is neither normal leaching nor run-off. Water travelling via this route can reach streams very quickly and because nutrients have very little contact with the soil column, large amounts can be transported quickly.
Minimise nitrate leaching from pasture
- Fertiliser nitrogen should not exceed 50kg/ha in any single application: smaller applications of nitrogen – 25kg/ha or less – should be applied during autumn. The sandier the soil, the less nitrogen that should be applied in one application.
- Nitrogen fertilisers should not be applied near streams or drains.
- Ammonia sources of nitrogen (diammonium phosphate and urea) should leach less nitrate in cold, wet soils, than nitrate (ammonium nitrate and calcium ammonium nitrate) sources of nitrogen.
- Apply nitrogen when the pasture is actively growing and can use the fertiliser.
- Avoid applying nitrogen fertiliser to well-drained soil if heavy rains are predicted.
- Avoid excessive pugging of fertilised paddocks in winter.
With increased use of nitrogen fertiliser on pasture over the past 15 years, there is concern about the potential environmental impact and human health concerns.
Phosphorus loss pathways
- Most of the phosphorus that enters watercourses occurs in autumn during the first flush of winter rains, before pasture plants have emerged and developed sufficient roots, or during extreme storm events in the growing season.
- Phosphorus is not converted to a gas and will persist in the soil or sediment and can become mobile in warm, wet conditions or during waterlogging/anaerobic conditions.
- Ordinary superphosphate is 80% water-soluble and most of this can be washed deep into the soil or across it before annual plants have successfully established roots.
- Phosphorus is either dissolved and leached down into sandy soils or it runs across the surface when the soil profile fills up with water. On clay soils the water flows across the surface to nearby drains and waterways with phosphorus dissolved or attached to clay particles.
- Phosphorus can also be lost via short circuit pathways, such as through cracks or macropores in the soil, similar to small tunnels. This is neither normal leaching nor run-off. Water travelling via this route can reach streams very quickly and because nutrients have very little contact with the soil column, large amounts can be transported quickly.
Minimise phosphorus leaching from pasture
- Only apply sufficient fertiliser required for the current year’s pasture: soil test analysis will provide this estimate.
- Phosphorus fertilisers should not be applied near streams or drains. The phosphorus will make its way to the lower parts of the landscape with the passage of water.
- Phosphorus is best applied in split applications – one early in winter soon after plants have germinated, and the other in spring.
- Avoid excessive livestock in areas near drains in winter. Livestock stir the soil surface and pug the soil causing particles to lift off during the rain and carry phosphorus to drains.
- Avoid applying phosphorus fertiliser if heavy rains are predicted and only apply after pooling from heavy rains has drained.
Making better nutrient management decisions
We encourage grazing (dairy, beef, sheep) managers in the lower south-west of Western Australia to base any nutrient decisions on good evidence.
Nitrogen and phosphorus are only effective if there are no other limitations or deficiencies. Soil, and possibly tissue, testing for nutrients will help identify nutrient or chemical imbalances.
We recommend regular soil testing and consultation with FertCare-accredited advisers to plan profitable and responsible fertiliser use.