GRAZING AS A CAUSE OF LIME PRECIPITATION IN A NATRAQUALF

The movement of water from the watertable to the atmosphere is one of the most common causes for CaCO3 accumulation on top of the soil. In t he Flooding Pampa, the increase in water evaporation due mainly to the reduction of soil cover as a result of grazing has given place to pul ses of topsoil salinization. We investigated the influence of this pro cess on lime precipitation in the upper horizon. Data from continuous grazed and ungrazed areas were considered. The relative ion concentrat ions in the upper horizon of the ungrazed area and in the phreatic wat er were very similar. Under grazing, absolute ion concentration was la rger than in the water table. The anionic constituents did not increas e directly and linearly. Chlorides and sulfates showed increments. The carbonate proportion did not change, but bicarbonates showed a relati ve decrease. This latter reduction could be related to the precipitati on of CaCOs in the upper horizon of the soil. In agreement, the Ion Ac tivity Product (IAP) of the calcite of the soil solution indicated con ditions for saturation of CaCO3 in the grazed area. Conversely, the LA P showed CaCO3 undersaturation in the ungrazed area. In spite of the e vidence for the ocurrence of lime precipitation, the top horizon did n ot show measurable lime under grazing. This fact could be explained by considering both the usual time taken by CaCO3 precipitation, and the recent and episodic process of top soil salinization in this region a fter the introduction of livestock.