Slag was found to increase yields by as much as 35 tons cane and 3.9 tons sugar per acre (2-year crop) on intensively weathered aluminous humic ferruginous latosols in Hawaii. Si uptake was increased sixfold by treatment. With other great soil groups, gains were either relatively small or nonexistant. Elimination, through field trials, pot tests, and soil analysis, of all other constituents of slag as possible causative factors, pointed to Si as being responsible for the improved yields. Confirmatory evidence resulted from pot tests with pure CaSiO3 in the presence of ample Ca. A highly significant inverse relationship was found in field experiments between extractable soil Si and response of sugarcane to slag. It is concluded that there is a level of available, or extractable, soil Si below which satisfactory growth of sugarcane will not obtain, regardless of supplies of macro-and micronutrients to which it has access. The need for additional Si can be met through the application of reactive silicates, and optimum yields of cane and sugar follow.