The effectiveness of intensively milled gneiss and potassium (K-feldspar) as K fertilizers was evaluated through a glasshouse experiment with ryegrass. Plant were grown on two soil types for 12 months. Results show that the agronomic effectiveness of milled gneiss was nearly as great as potassium sulfate (K2SO4), but milled K-feldspar was much less effective because of the relatively small extent of dissolution of K-feldspar in the soil. The positive effects of K-silicate rock fertilizers (K-SRFs) included increases in plant biomass, uptake of K and silicon (Si), and soil pH with increasing application rate of the K-SRFs. The application of K-SRFs will be most advantageous for amending K-deficient soils, and high-energy milling provides a simple method for manufacturing effective multinutrient SRFs.