Fact Sheet: Potassium

They key to plant vitality

Introduction to Potassium

Potassium (K) is an essential macronutrient for plants, involved in critical physiological processes such as photosynthesis, water regulation, enzyme activation, and overall plant health. Proper potassium levels are crucial for optimal plant growth, enhancing disease resistance, and improving drought tolerance.

Types of Potassium Tests.

Understanding the different potassium measurements provided in soil tests can help gardeners manage their soil's nutrient levels more effectively:

  1. Available Potassium

    • Method: Typically measured using an ammonium acetate extraction or a similar method.

    • Typical Values: Low: <150 mg/kg, Optimal: 150-400 mg/kg, High: >400 mg/kg

    • Interpretation: Indicates the amount of potassium immediately available for plant uptake, reflecting the soil's current fertility status regarding potassium.

  2. Potassium (Colwell)

    • Method: Involves a sodium bicarbonate extraction, adapted for potassium estimation.

    • Typical Values: Low: <150 mg/kg, Optimal: 150-400 mg/kg, High: >400 mg/kg

    • Interpretation: Provides a broader estimate of potassium, including both immediately available and potentially available forms over the growing season.

  3. Potassium (Amm-acet) (cmol(+)/kg)

    • Method: Measured using the ammonium acetate method and expressed in cmol(+)/kg, focusing on exchangeable potassium.

    • Typical Values: Low: <0.2 cmol(+)/kg, Optimal: 0.2-0.5 cmol(+)/kg, High: >0.5 cmol(+)/kg

    • Interpretation: Reflects the total exchangeable potassium in the soil, important for understanding how potassium is held within the soil complex and its availability for plant uptake.

Interpreting Results

  • Low Levels (Available Potassium & Colwell <150mg/kg - Amm-acet <0.2 cmol(+)/kg):

    Indicate a deficiency potentially leading to reduced growth and physiological issues in plants. Additional potassium may need to be added to the soil.

  • Optimal Levels (Available Potassium & Colwell 150-400mg/kg - Amm-acet <0.2-0.5 cmol(+)/kg):

    Sufficient for most plants, ensuring good growth and physiological functions. Regular monitoring is recommended to maintain these levels.

  • High Levels (Available Potassium & Colwell >400mg/kg - Amm-acet >0.5 cmol(+)/kg):

    While generally not harmful, excessively high levels can interfere with the uptake of other essential nutrients such as magnesium and calcium.

Factors Affecting Potassium Availability

  • Soil Texture and Structure: Sandy soils or heavily leached soils often have lower potassium due to its high mobility and leaching potential.

  • Soil Moisture: Adequate moisture is necessary for potassium to be available in the soil solution for uptake by plants.

  • Soil pH: Extreme pH levels can affect potassium availability by altering the ionic form that plants can absorb.

Actionable Recommendations

  • For Low Potassium Soils: Apply potassium-rich fertilizers such as potassium sulfate (SOP) to raise the potassium level to an optimal range.

  • For Optimal to High Potassium Soils: Continue regular monitoring to maintain potassium at a beneficial level without excess. Use balanced fertilisers that do not add unnecessary potassium if levels are already sufficient.

FAQs

  • What are signs of potassium deficiency in plants?

    • Symptoms include yellowing of leaf edges, scorched or burned look to leaves, and weak stems and roots.

  • How often should I test for potassium?

    • Testing every 2-3 years is typically sufficient, unless cropping practices or visible symptoms suggest a significant change in potassium levels.