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In search for a source for a slow-releasing K fertilizer, the plant availability of mineral K in selected crushed rocks and mine tailings was investigated by growing Italian ryegrass for six months in small volumes of peat, loamy sand or silt loam mixed with different K-sources. The K supplied as K-feldspar was nearly unavailable to the plants, whereas nearly 60% of the K supplied as biotite and nepheline in a carbonatite, was recovered in harvested plants parts. The carbonate content of the rocks and tailings seemed to be more important for the availability of the K than the specific surface of the mineral particles. It is concluded that a rock-based fertilizer containing biotite as its main K-bearing mineral and between 5 and 20% carbonate, will release K at a slower rate than soluble K fertilizers do and still supply considerably more K to the plants than is supplied from the fraction of non-exchangeable K in the soil.

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