Quantification of Gypsum in Soils: Methodological Proposal

Gypsum is widely found in soils under arid and semi-arid climates due to its semi-soluble nature. In spite of that, they are less known than other soils, and this has generated some misunderstandings in some initial pedological concepts and in soil classification systems. In addition, the quantification of gypsum, and in particular of its secondary accumulations is affected by the sampling procedures and sample handling in the lab; besides by the methods used for the determination of gypsum themselves, since they differ on the accuracy, cost, and expertise needed. The objective of our research is to improve some laboratory procedures in order to determine and quantify gypsum in the soil, especially secondary accumulations. We applied several methods of sample handling and gypsum analysis to a loess profile in the Ebro Valley (NE Iberia), consisting of 10 horizons containing gypsum in varying amounts (0 to about 50%); of different sizes and morphologies. We propose a protocol considering procedures (sieving or not), qualitative determinations and two methods (turbidimetry and dehydration of crystallization water) for an optimal determination of gypsum depending on the characteristics of the sample and compared them with the acetone method (US Salinity Laboratory Staff, Agric. Handb., 1954, 60, 175; Nelson, 1978, 181), as it is the reference method in the main Classification Systems. The results obtained after applying the different methods for the analysis of gypsum in bulk samples have allowed us to propose a decision tree procedure for the determination of gypsum in soil materials. This procedure includes, determination of gypsum in all fractions, coarse and fine, the estimated amount of gypsum in the field (as a major or minor component) and the presence of other components that may interfere with the results. The most accurate results are obtained with those methods based on the loss of gypsum water upon heating when gypsum content is >4%, and with the turbidimetric method in case of lower amounts of gypsum. Finally, we discuss the implications of these analyses when a soil is classified according to the main soil classification systems (WRB 2014; Soil Survey Staff, SSS- NRCS, 2014).