What it isClay minerals in the soil, such as Illite and Vermiculite, that have a high capacity to hold or "fix" potassium (K) ions within their layered structure. This fixed potassium is not immediately available to plants.CompositionClay minerals are typically hydrous aluminum silicates. Those that hold potassium are often 2:1 layer silicates (two silica sheets around one alumina sheet).Potassium RoleClay is essential for storing potassium. The potassium ions ($K^+$) fit tightly into the hexagonal holes between the mineral layers, making them non-exchangeable or only slowly available.SignificanceThis process of potassium fixation is crucial for long-term soil fertility as it prevents potassium from being easily washed away (leaching). However, in some cases, too much fixation can make the nutrient temporarily unavailable to crops.