Silicate slag was applied to soil of Captiva, Floratam, and Raleigh varieties of St. Augustinegrass to measure the effect of plant silicon on resistance of the varieties to southern chinch bugs, Blissus insularis Barber, and plant disease. In general, silicate slag addition increased Si and Cu content in leaves but tended to decrease P and Mg concentrations. Tissue nutrient contents were generally lowest for Floratam; this trend occurred both with and without silicate slag amendment, indicating a lower nutrient requirement than with other varieties. Chinch bug survival was lowest and development slowest on Captiva which was the only variety with resistance to the insects. There were no significant differences in survival or developmental rates of chinch bugs between silicon treatments within any of the 3 varieties. In disease assessments, Raleigh was more susceptible to gray leaf spot than Captiva or Floratam. However, the addition of Si resulted in significant disease reduction in Raleigh. The addition of Si resulted in disease reduction in Floratam as measured by disease incidence, but not severity. The addition of Si slag caused significant increases in stolon number, stolon length, and leaf blade width in 1 - 2 varieties. Varieties varied in response to Si fertilization among 6 growth characters measured.