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The familiarity of mankind with gypsum and its simple composition are in contrast with the frequent mistakes reported for its behavior and role in nature. Gypsum has been studied as a raw material, as a rock constituent, as an indicator of geological and archaeological conditions, and from other points of view. However, its role in Earth surface processes, its relationship to life through calcium in the equilibrium of carbonates and its structural water molecules, seems overlooked. Moreover, errors of gypsum formulation, analysis, and behavior obscure some of its roles within nature. The semi-solubility of gypsum explains its actions in many soils. The softness, fragility, and crystal water of gypsum are often not considered. Routinely drying at 105[degrees]C and pulverizing samples for lab analyses casts suspicion on all analytical results because gypsum becomes anhydrite and/or bassanite at this temperature. Specific physicochemical models are needed to predict the behavior of soils mainly composed of gypsum. doi: 10.2136/sssaj2008.0224

The presence of gypsum in soil, even in small amounts, is relevant from the genetic, taxonomic, and applied points of view. Moreover, those soils having gypsum as the main component, for example gypseous soils, host a number of rare and endangered organisms. Determining gypsum content in soils is crucial to understanding their behavior; however, current methods of determination are cumbersome or imprecise. This study was conducted to develop an accurate new method to determine soil gypsum content by reducing the inaccuracies or poor assumptions in previous methods. We determined gypsum content in samples using quartz sand and clay minerals mixed with gypsum reagent powder to 0 and approximately 2, 5, and 50% gypsum content; we also studied seven gypseous soil samples. Gypsum (CaS[O.sub.4] x 2[H.sub.2]O), after heating at 105[degrees]C, dehydrates, losing the two water molecules of the gypsum crystal cell and becoming mostly anhydrite (CaS[O.sub.4]). Anhydritc is very hygroscopic, absorbing atmospheric water very quickly; this process has been found to occur even in desiccators where the relative humidity (RH) is between 12 and 20% depending on the desiccant. Anhydrite rehydrates under laboratory conditions to bassanite (CaS[O.sub.4] x 0.5[H.sub.2]O), which is stable at RH values Abbreviations: E[C.sub.e], electrical conductivity of a saturation extract; RH, relative humidity.

Many land use decisions are made without the aid of soil maps in countries without National soil surveys. The purpose of this research was to create a soil survey delineating salinity phases at a regional scale that could serve as the basis for improved decision making, or highlight areas for further investigation. Soil salinity survey at a regional scale remains a challenge. We demonstrate a survey methodology suited to small fields, often found in Europe and developing countries, and present a streamlined survey methodology making this typically labor-intensive process more efficient. A regional soil survey was conducted at a scale of 1:25 000, mapping consociations and complexes of soil series. The soil survey information was then used to guide an electromagnetic induction (EMI) survey of the soils with observed salinity, to delineate phases. A calibration method for estimating the electrical conductivity of the extract of the saturated paste (ECe) from the EMI measurements was devised. After testing several statistical methods, a single calibration of the EMI data was found to be suitable using a nonparametric regression. The use of the EMI method reduced operation costs, where time, personnel, analysis time, and budget are limitations.

Determining the gypsum content in soil is lengthy and cumbersome using methods based on SO4 determination. Moreover, as these methods do not strictly titrate gypsum, inaccuracies can be produced by the presence of sulfate minerals other than gypsum. The thermogravimetric properties of gypsum, however, allow determination of its content in a rapid and easy way sufficiently accurate for many pedologic purposes. Our objective was to expand the lower limit of gypsum detection to make the test useful for soil classification and management. We have refined the differential water loss method by estimating the gypsum percentage from the loss of water in the soil sample between 70 and 90[degrees]C. Our results, compared with gravimetric determinations of precipitated BaS[O.sub.4], found coefficients of determination of 0.98 for gypsum contents ranging from 2 to 50%, and 0.99 for contents >50%. The method is valid for gypsum contents >2% and thus improves previous estimation procedures.

The weathering of lutites in aridic or near-aridic environments leading to soil horizons is not well known. Lutites are a common soil parent rock composed of silt and clay, that may be massive or may have their sedimentary origin marked by layers of alternating color or granulometry, or by fissility along planes. Lutites underlying soils are considered Cr horizons and show different alteration stages recognizable in the field by the degree of the layering disturbance or by the loss of coherence. In the study area, neither the dry climate nor the kind of clay minerals, mainly illite, can explain the weathering of lutites. This study was conducted to investigate the initial weathering reactions as encountered in Cr horizons, and to depict the processes responsible for pedogenesis from lutites in gypsiferous soils of the Ebro Valley (Spain). We described 12 soils in the field under two different soil moisture regimes, with further characterization through chemical analyses and micromorphology by thin section and scanning electron microscope (SEM). The microscopic study revealed two distinctive pedofeatures in the Cr horizons: (i) lenticular gypsum crystals; and (ii) 'queras,' sub-millimetric biotic features produced by calcification-decalcification processes. Growth of gypsum crystals in these horizons resulted in an 'isles fabric,' that is, isles of fine materials in a mass of gypsum crystals, with much greater porosity than the parent lutite. The growth of gypsum crystals and the development of queras result in a particle-size distribution change and an increased porosity of the Cr horizons.

Electromagnetic induction surveys are often used in practice to estimate field-scale soil salinity patterns, and to infer changing salinity conditions with time. We developed a statistical monitoring strategy that uses electromagnetic induction data and repetitive soil sampling to measure changing soil salinity conditions. This monitoring approach requires (i) the estimation of a conditional regression model that is capable of predicting soil salinity from electromagnetic (EM) survey data, and (ii) the acquisition of new soil samples at two or more previously established survey sites, so that formal tests can be made on the differences between the predicted and observed salinity levels. We examined two test statistics in detail: a test for detecting dynamic spatial variation in the new salinity pattern and a test for detecting a change in the field median salinity level with time. We applied this monitoring and testing strategy to two EM survey-soil salinity data sets collected at multiple points in time from the saline irrigation district of Flumen, Spain. Our results demonstrate that this monitoring approach was successfully able to quantify the temporal changes in the soil salinity patterns occurring within these two fields.

The relationship of hypergypsic soil morphology to its parent landscape in a northeastern region of Spain is described. The parent materials are gyprock in upper and eroded parts of the area and transported materials in lower slopes and valley bottoms. Transport of gypsum is in solution, by runoff and by mud flow. Mud flows allow the formation of microcrystalline gypsum. Tongues of microcrystalline gypsum flow downwards and are incorporated into the valley floor. A modification to the classification of gypsic horizons is also discussed.