Fertiliser potassium for intensively grazed dairy pastures

Page last updated: Wednesday, 17 January 2018 - 1:43pm

Please note: This content may be out of date and is currently under review.

The Greener Pastures project was set up to assist the Australian dairy industry meet the two major challenges in managing high performing pasture systems: maintaining profitability while meeting the expectations of a community that is increasingly sensitive to environmental issues. This page discusses the major findings from studies to better define the fertiliser potassium requirements of intensively grazed ryegrass pastures

This report is based on several studies:

  • Annual soil testing over 10 years of 48 dairy paddocks at Vasse Research Centre in the south-west of Western Australia.
  • Potassium experiments on commercial dairy farms in the south-west of Western Australia.

The main messages for farmers from this part of the study included:

For intensively grazed ryegrass-dominant pastures

  • Potassium (K) deficiency is most likely for legumes in pastures on sandy soils and in wet years.
  • K deficiency is likely to occur after silage and hay crops. Application of 50-100kg/ha K is recommended.
  • K can be redistributed around the farm from paddocks regularly cut for silage/hay to paddocks in which silage/hay are fed out.
  • Legumes have a higher K requirement than grasses.
  • Ryegrass responses to applied fertiliser K are rare in dairy pastures in WA. Routinely applying fertiliser K after each grazing is unlikely to be profitable.
  • Soil K testing is unreliable for indicating when fertiliser K is required.
  • Tissue testing is probably more useful than soil testing in deciding whether or not K fertiliser should be applied to pasture. Further research is required to assess this.

For traditional clover-ryegrass pastures which are not top-dressed with fertiliser nitrogen after each grazing but instead rely on clover as the main source of nitrogen for pasture production

  • Clover is very prone to K deficiency.
  • Soil testing is used to recommend when to apply fertiliser K to maintain clover production and persistence. However, only very crude recommendations can be made for when to apply fertiliser K for clover.
  • No clover production responses occur for soil test K values greater than 100mg/kg.
  • Clover production responses to applied fertiliser K always occur when soil test K is less than 30mg/kg.
  • Between 30 and 100mg/kg, clover production responses to applied K may or may not occur and the soil test is neither consistent nor reliable for indicating when a response is likely.
  • The crude recommendation is therefore to apply fertiliser K when soil test K is less than 100mg/kg, although this means that K is often applied when it is not required.