Amendments of a recirculating nutrient solution with potassium silicate were evaluated as a means to control Phytophthora capsici infections on pepper plant (Capsicum annuum L.). Supplying the solutions with 100 or 200 ppm of silicate significantly reduced motility, root decay, and yield losses attributed to infection of P. capsici. Treating inoculated plants with potassium silicate increased root dry weights and number of fruit, especially high-grade fruit. Results were slightly superior to non-inoculated controls. The two varieties, PBC 137 and PBC 602, responded similarly to the treatments. No significant differences were observed between the 100- and 200 ppm silicate treatments. Results were better when greenhouse conditions favored the spread of P. capsici. Silicon alone did not increase pepper yield, suggesting that it acts as a disease suppression agent rather than as a fertilizer. The phenomena by which silicon confers protection against P. capsici infection and disease development are not fully understood, but our results indicate that mechanisms other than a mechanical barrier to fungal penetration are involved.