Monitoring potassium in high rainfall pastures is important
Large amounts of potassium can move out of the root zone through leaching or plant uptake, especially in high rainfall (more than 600mm average annual rainfall) sandy areas. Potassium is removed in hay and silage and is redistributed around the farm by livestock, mosly in urine patches.
Pasture legumes – clovers in most Western Australian pastures – are very susceptible to potassium deficiency. Potassium deficiency greatly reduces seed production of legumes, and legumes will gradually disappear from potassium-deficient pasture. Re-seeding legumes after adding potassium fertiliser may be required.
In beef-grazed sandy areas where potassium deficiency is common, potash is applied annually, often in addition to phosphorus fertiliser.
Potassium deficiency is rare in ryegrass, even in intensively grazed dairy pasture. Ryegrass has deeper roots and can explore more soil and access potassium at greater depths, especially if there is clay in the subsoil.
Soil test and tissue test
If the tissue test shows less than 2% potassium (K), it is likely the pasture will respond to potassium application.
Soil test Colwell K values and clover growth
Potassium soil analysis can be very strongly influenced by potassium in urine patches. We recommend you follow our soil sampling guidelines and check the soil test with a tissue test. Maintaining records of annual soil tests and comparing them to tissue tests will improve the ability to accurately assess potassium requirements. See Table 1 for a guide on using Colwell K values to estimate the need for potassium fertiliser.
|Colwell K value (mg/kg)||High rainfall clover pasture requirements for potassium fertiliser and response|
|<50||Requires potassium fertiliser and responds with additional growth|
|50–100||Requires maintenance level of potassium fertiliser, and shows no additional growth|
|>100||Does not require potassium fertiliser, and would show no additional growth|
When to apply potassium fertiliser
Split applications of muriate of potash (potassium chloride) in autumn and spring provide a more even supply of potassium to leaching soils. This fertiliser is in addition to phosphorus and sulfur fertilisers or other fertiliser that is required.
Potassium deficiency on fertile clay or loam soils is less likely than on sands, and we recommend a single application of muriate of potash 4–8 weeks after the break of season, applied to emerging pasture. Potassium applied at this time is more likely to be absorbed by developed roots and less likely to be leached.
Use tissue testing to monitor the effectiveness of applied potassium.
Application rates of potassium
These rates are guidelines only and will vary depending on pasture production targets and economics:
- For a PBI* <300, 1kg of potassium is required for each unit increase in Colwell K required.
- For a PBI* >300, 2kg of potassium is required for each unit increase in Colwell K required.
- Deficient pastures with a PBI* <300 and a Colwell K of 40 would require 60kgK/ha.
* PBI is the phosphorus buffering index; see phosphorus for high rainfall clover pastures for more information.
A maintenance application of 30–50kgK/ha annually is likely to be required for high rainfall grazed pastures and 50–100kgK/ha for hay paddocks. We recommended that you soil test for an accurate assessment. The effectiveness of applications can be checked by tissue testing.
Dairy pastures and potassium
Potassium conentration in some pastures can be high enough to induce magnesium deficiency in lactating dairy cows and this reduces productivity and health. Refer to Bulletin 4812 'Managing potassium in dairy pastures'.
Making better nutrient management decisions
We encourage dairy, beef, sheep managers in the lower south-west of Western Australia to base nutrient management decisions on good evidence, and recommends regular soil testing and consultation with FertCare-accredited advisers to plan profitable and responsible fertiliser use.