Se estudiaron las relaciones Si/Mg y Ca/Mg, en tres transectos y 12 perfiles de suelos, con lluvias inferiores a 1400 mm anuales. MAis de 15% de minerales ferromagnesianos en la fracciA³n arena fueron garantA­a para la apariciA³n de contenidos de Mg+2 mA­nimos de 5 Cmol (+)/kg en todos los paisajes. Variaciones anuales de la precipitaciA³n entre 800 y 1400 mm no influyeron en la variabilidad del magnesio de 5 a 41 Cmol (+)/kg. El Mg+2 aumentA³ en el sentido en que decreciA³ la tasa de infiltraciA³n promedia. Los suelos con arcillas oxA­dicas acumularon SO4=, yeso en los poros y adolecieron de horizontes Bk; con influencia de piroclastos aumentaron Na+. Cuando presentaron Si / Mg > 5 tuvieron menores concentraciones relativas de Mg+2 y Ca+2, excepto en familias esmectA­tica y caolinA­tica. Las aguas de riego presentaron C.E. 10 Cmol (+)/kg y pH ­ 7.5 inhibieron la acumulaciA³n de Na+. ABSTRACT A sigh over the origin and properties of magnesic soils in Valle del Cauca-Colombia. They were studied as a function of Si/Mg and Ca/Mg relations in three transect and 12 soil profiles where rain precipitation was lower than 1400 mm. More than 15% of ferromagnesian minerals in sand fraction allows at least a minimum Mg+2 content of 5 Cmol (+)/kg. Annual rain variations between 800 and 1400 mm had not influence in Mg+2 variability between 5 to 41 Cmol(+)/kg. Mg+2 contents had a tendency to increase as well as medium infiltration rate decrease. Magnesic soils with oxidic clays in general allowed accumulation of S04= ion, gypsum in pore space, and lacking of Bk horizons; but with piroclastic materials Na+ tended to increase. When showed Si / Mg > 5, had relatively less Mg+2 y Ca+2 concentrations except in smectitic and kaolinitic families. All irrigation waters had C.E < 1 dS/m, most of them residual sodium carbonate (RSC), and most of times they foresee high apports of magnesium to soils. There were specific effect in all landscapes; for Ca/Mg < 2 was lessen when soil organic matter content was close to 5%, or if kaolinitic mineralogy; with micaceous mineralogy it occurred for Ca/Mg of 2-3. Ca+2 values greater than 10 Cmol (+)/kg and pH ­ 7.5 suppressed soil Na+ accumulation. Key words: magnesic soils, Mg specific effect, irrigation water quality.