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Phosphorus Use on Peat Soils

29 May 2020
Type Media Article

Peat soils behave differently to mineral soils for certain nutrients such as phosphorous. What is the best approach to take to ensure fertiliser P applications get utilised by the grass or crop? Fiona Doolan, ASSAP Advisor Kildare/Laois has some advice

A recent analysis conducted by the Teagasc Rural Economy and Development programme indicates that approximately 6% of the country (or 420,000 Ha) is made up of cultivated peats across a wide range of farming intensities (though predominately low intensity farming). There are however some intrinsic differences between peat soils and mineral soils in relation to nutrients, particularly Phosphorous (P), and how the soil stores and uses them.

Phosphorous is a nutrient that can have negative effects on water quality if it is lost to streams and rivers. It is vital that we manage P applications carefully to improve efficiency, get the best financial return for the farmer and also reduce the risk of loss to our waters.

It is important to note that peat soils behave differently to mineral soils, when it comes to some nutrients such as phosphorous.  High organic matter soils (OM > 20%) do not adsorb P in the same way that mineral soils do. Therefore P does not bind to peat soil particles so do not have the capacity to build up or increase the store of phosphorous they hold. Applications of fertiliser P on peat soils may be lost (leached) to water if P is not utilised by the growing plant during periods of rainfall.

So what is the best approach to take to ensure fertiliser P applications get utilised by the grass or crop? Firstly, just apply the phosphorus that the plant  needs and can use  for  growth – do not apply excess amounts of P; peat soils cannot build up its phosphorous levels so it will be an expensive exercise to apply more than the crop needs with no long term advantage to the soil. This applies whether its phosphorous applied in chemical form or organic fertiliser.

Secondly is the timing of application. Match application to the growth period of the crop and crop requirement. Do not apply earlier, in advance of the crop requirement as this may be lost to waters and do not apply late in the season as growth rates will have slowed down. In grassland peat soils, a little and often approach is recommended to ensure applied P is taken up by the grass crop and not lost to waters.

So in summary:

P - Peat soils behave differently to mineral soils and are not capable of holding or building up phosphorous

E - Evenly match application to what the growing crop can use and do not target a build-up application

A - Apply this during the growing season so match the crop demand

T - Take account of your NMP plan and adhere to buffer margins when spreading.