Highly weathered upland soils in the tropics are mostly acid, having high Al, but often deficient in Ca and/or Mg which limit crop production. Some studies were conducted in Malaysia to investigate the efficacy of ground magnesium limestone (GML) and gypsum as soil ameliorants, and to determine the response of the soils and maize to the application of the GML and gypsum. Results of the studies showed that Ca remained in the zone of incorporation in the GML treated soils. When the GML was applied together with gypsum, Ca moved deeper into the soil profile. SO₄^2- adsorption onto the surfaces of the oxides in the soils resulted in an increase in pH and negative charge. The increase in pH was due to replacement of OH^- by SO₄^2-. Soil solution Ca/Al concentration ratio was found to be a useful index of soil acidity. Beneficial effects of GML application at the rate of t ha^-1 lasted about 8 years; the effect was comparable to annual application of 1 t GML ha^-1.