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.