We study the problem of convective movement of a reacting solute in a viscous incompressible occupying a plane layer in a saturated porous medium and subjected to a vertical magnetic field. The thresholds for linear instability are found and compared to those derived by a global nonlinear energy stability analysis. Then, the accuracy of the linear instability threshold is tested using a three-dimensional simulation. It is shown that the strong stabilizing effect of gravity field and magnetic field. Moreover, the results support the assertion that the linear theory is very accurate in predicting the onset of convective motion, and thus, regions of stability.