Reducing Phosphorus Loss to Water

There has been at lot of focus on Nitrogen in media coverage in recent times, but it is another nutrient, Phosphorus, that in causing issues in many watercourses draining heavy soils. Ivan Kelly, Teagasc ASSAP Advisor provides some advice on reducing Phosphorous loss to water.

Nutrient application followed by significant rainfall on poorly draining and low permeability soils leads to overland flow transporting nutrients to waterbodies. Targeted fertiliser application at optimum times throughout the main growing season, particularly on low permeability soils, along with suitable land management can help mitigate against the risk of phosphorus & sediment making their way to our rivers and streams.

Farmers should have a Nutrient Management Plan for the farm prepared and implemented to ensure the nutrients in slurry, FYM and chemical fertiliser are directed to where most needed.

Use of slurry

It is prohibited to spread slurry from the 15^th October to the 15^th January in the West, but even outside these dates care must be taken to avoid losses. Slurry should only be applied when soil temperatures are above 6 degrees and ground conditions and weather forecast are suitable. It is important that the applied slurry gets down to the roots of plants such as grass in the growing season, preferably bare fields or fields with low grass cover. On very heavy land it may be necessary to delay spreading until after the first cut silage. Extra slurry storage allows more flexibility on spreading times, particularly in a very wet spring. Under the nitrates directive, Slurry must not be spread if heavy rain is forecast within 48 hours, but on poorly drained soil this period should be extended further. Spreading slurry with Low Emission Slurry Spreading (LESS) equipment such as a trailing shoe, dribble bar or the injector system can dramatically reduce losses and improve nutrient efficiency. LESS results in reduced sward contamination which allows more flexibility to spread on heavier covers in improved weather and ground conditions.

Riparian buffer zones

A riparian buffer zone is an area adjacent to a water body where no chemical and organic fertilisers, cultivation or spraying can be carried out. These zones vary in width and are required to protect waters from diffuse losses of nutrients, sediment and chemicals. The introduction of trees or rough dense vegetation in these areas can act as a barrier, shade streams and stabilise river banks while the roots can absorb soil nutrients. To be effective, riparian buffer zones must be located at the points on the farm most likely to allow nutrient, sediment or pesticides enter a waterbody. These are often low-lying parts of farms where surface runoff accumulates in high concentration.

Phosphorus does not bind to peat soil particles, so unlike mineral soils, peat soils do not have the capacity to build up a store of phosphorous. Only apply the phosphorus that the plant needs and can use for growth immediately – do not apply excess amounts of P (e.g. reduce slurry rates) as it risks being lost to waterbodies.

Sediment loss to water

Sediment loss to water has been identified as a major concern in recent years. If sediment finds its way to the stream, it can settle on the river-bed in slow flow areas, resulting in the loss of macroinvertebrate habitat and spawning ground. Phosphorus binds to sediment and when washed into the watercourse, can cause excess nutrient load and promotes algal blooms which reduce oxygen levels in the stream. Agricultural practices such as land drainage, cattle access drinking points to streams and poor management of farm roadways can lead to loss of sediment & phosphorus.

Mitigation options to reduce sediment loss include:

• Prevent access by livestock into drains and streams and providing alternative drinking water sources.
• Divert all surface runoff from farm roadways to a field or soak pit
• Establish targeted riparian buffer zones
• Employ proper drain maintenance practices including the following:
  • Only carry out drain maintenance during the months July to September
  • Only one side of a drain to be cleaned at a time
  • Drains should not be over-cleaned, retain as much vegetation as possible
  • Ensure the bank is sufficiently sloped afterwards to prevent collapse
  • Silt/ sediment traps should be in place prior to installing new open drains leading to streams.
  • Stone should not be filled to the surface of new field drains.

When farmers are applying fertilisers, cultivating, grazing or draining land close to watercourses, careful, site specific land management can minimise the risk of Phosphorus and sediment loss and help improve water quality.

The Agricultural Sustainability Support and Advisory Programme (ASSAP) is an advisory service available to all farmers situated in Priority Areas for Action to support with farming and water quality issues. 

For further information visit: www.teagasc.ie/assap