Soil pH Impact on Potassium Retention

Soil pH impacts potassium differently than other nutrients. For phosphorus, soil pH affects the chemical form of the nutrient and the cations it bonds to. For potassium, however, the impact of soil pH is entirely about finding a place on the Cation Exchange Capacity (CEC).

Not all cations (positively charged ions) are created equal. The affinity, or lyotropic series, defines how strongly cations are held by the CEC. Assuming all cations are present in equal amounts, aluminum and hydrogen will bind to the CEC first, and sodium last:

Al³⁺ > H⁺ > Ca²⁺ > Mg²⁺ > K⁺ = NH₄⁺ > Na⁺

Aluminum and hydrogen have a high affinity for the soil CEC. At a low soil pH (below 5.5), aluminum becomes soluble and exchangeable, and the hydrogen concentration in the soil increases. At these low pH levels, these cations dominate, leaving little space on the CEC for the retention of potassium, ammonium, and sodium. (Sodium’s inability to bond strongly to the CEC is actually a good thing, as it leaches away).

High soil pH levels are created by the presence or application of calcium- and magnesium-based carbonates and bicarbonates. Calcium and magnesium have very similar affinities for the CEC, and both are much stronger than potassium, ammonium, and sodium. At high soil pH, calcium and magnesium dominate the sites bound to the CEC, again leaving little space for potassium retention.

Additionally, the typically low CEC of sandy soils reduces the chance of potassium finding a binding site. The structure of organic matter CEC is also not conducive to the retention of potassium at any soil pH.

In summary, the impact of soil pH on potassium is not about changing the form of the nutrient to limit plant uptake; rather, soil pH impacts the soil’s ability to retain potassium and prevent it from leaching down through the soil profile.