Two experiments were conducted to study reactivity for a blast furnace slag, both developed in a greenhouse, in pots planted with Brachiaria brizantha. A 3 x 4 + 3 factorial was used with four replications, with three slag doses corresponding to 0.6, 1.2 and 2.4 g dm -3 for the clay soil and 2.2, 4.4 and 8.8 g dm -3 for the medium texture soil, four particle sizes (ABNT 5-10, 10-20, 20-50 and <50) and three controls (without corrective application, slag and dolomitic limestone in a dose corresponding to V=70%). In the clayey soil, there was a low efficiency of Ca and Mg release to the soil with the use of slag. It was concluded the slag, in its different particle size fractions influenced differently the pH, Ca and Mg of medium texture soil, where the fraction that passed through the ABNT 50 sieve had a greater effect on the chemical attributes evaluated. The corrective agents, slag and limestone, acted similarly on pH, Ca, Mg and V%. For greater liberation of silicon to the soil and absorption by plants it is necessary to use smaller particle sizes in the medium texture soil and intermediate particle sizes in the soil with clayey texture.