Magnesium affected soils in Colombia

Abtract The Mg +2 accumulation on the exchange complex of soils to a very high saturation levels affect their chemical, physical and biological properties. Magnesium has some capacity to develop higher exchangeable sodium levels in clays and soil materials. Colombia has a large area of these soils, located mainly in the main rivers valleys and in the Caribbean Region. In the Cauca River Valley there are about 117.000 hectares affected.There is a lack of information about the soil forming processes, the Mg +2 effects on soils, the type and source of compounds responsible for the magnesium enrichment, their relationship with the landscape and the way this accumulation occurs. To identify and quantify Mg +2 saturated areas over 2500 soil profiles from different landscape positions of the Cauca River Valley were studied. The information was processed to generate Mg-saturation maps, to identify the different soil profile types and to estimate the affected area. A topographic sequence from the alluvial inundation plain to the hills, was used to explore the presence of diagnostic horizons and to determine the main soil characteristics and genetic, mineralogical or chemical evidences of soil forming processes. Two transects parallel to the river were used to study the type and source of Mg-compounds responsible for the Mg-enrichment and the way this accumulation occurs. By grouping together the soil profiles, five main type of Mg-affected soils were identified as being predominant in the different landscape units. Their distribution in relation to the landscape units in the valley showed two different origins: soils developed under hydromorphic conditions and soils related to igneous Mg-materials. The toposequence studied showed the presence of a natric or gypsic horizon on the basin soil profiles, and evidence of calcium supersaturation and precipitation as calcite and aragonite; as the drainage conditions change the vertic characteristics disappear but calcium accumulation and precipitation processes occur near the hills. The lack of structure or massive conditions on the soil of the three soil profiles closest to the river are notorious. Light alkalinity is ordinary in all the strata but in the upper horizons. Sulphate and bicarbonate ions were found in all profiles but the later was not found in the soil parent materials of the lower plains. As the soil becomes deeper sulphate ions become predominant and gypsum accumulation appears not only due to hydromorphic conditions but related to the presence of Na + and/or Mg +2. The mineralogical …