Resumen La curva de retención de agua del suelo (SWRC) relaciona la humedad con la energía de retención de agua (potencial de matriz). En suelos retráctiles, este volumen varía de acuerdo con el secado o la humectación del suelo, lo que puede provocar errores en la humedad calculada. El objetivo de este estudio es cuantificar la variación de volumen en los suelos retráctiles y elaborar el SWRC a través del método tradicional, que no considera la retracción del suelo, y un segundo método, que considera la retracción. Se han recolectado muestras de suelo en los horizontes A y B de seis perfiles de suelo. Por lo tanto, para suelos retráctiles, se recomienda que el ajuste del SWRC se lleve a cabo considerando el volumen real del suelo (retraído), que varía para cada potencial de matriz aplicado. Este ajuste reduce los errores, principalmente al determinar el punto de marchitez permanente y el agua disponible.

Abstract The soil water retention curve (SWRC) relates moisture to soil water retention energy (matrix potential). The calculation of the volumetric humidity considers the sample volume. In retractable soils, this volume varies according to the drying or wetting of the soil, which can result in errors in the calculated moisture. The objective of this study is to quantify the volume variation in retractable soils and to elaborate the SWRC via the traditional method, which does not consider soil retraction, and a second method, called adjusted, that considers the phenomenon of soil retraction. Soil samples have been collected in horizons A and B from six soil profiles Thus, for retractable soils, it is recommended that the adjustment of the SWRC be carried out considering the actual volume of the soil (retracted), which varies for each matrix potential applied. This adjustment reduces errors, mainly in determining the permanent wilt point and available water.

ABSTRACT Shrinkage capacity (caráter retrátil in Portuguese) is a new diagnostic characteristic recently introduced in the Brazilian System of Soil Classification (SiBCS) to indicate shrink and swell properties observed in subtropical soils from highland plateaus in southern Brazil, specifically in Oxisols with brown colors. In soils located in road cuts exposed to drying for some weeks, strong shrinkage of soil volume is observed in these soils, resulting in the formation of pronounced vertical cracks and large and very large prismatic structures, which crumble in blocks when handled. We hypothesize that such properties are related to their clay mineralogy, although there are no conclusive studies about this, the motive for the present study. Samples of the A and B horizons from six Oxisols with expansive capacity from the states of Santa Catarina and Rio Grande do Sul were analyzed. One Rhodic Hapludox, from the state of Paraná, without expansive capacity, was used for comparison. All the soils are very clayey, originated from basalt, and have similar iron oxide content. For identification of clay mineralogy, X-ray diffraction techniques were employed, together with the use of NEWMOD® software to investigate and describe the interstratified minerals. The results showed that most expansive soils have a similar mineralogical composition, with kaolinite, interstratified kaolinite-smectite (K-S), and hydroxy-Al interlayered smectites (HIS), unlike the non-expansive Rhodic Hapludox, which exhibited kaolinite with significant amounts of gibbsite and low amount of interstratified K-S. According to the mineralogical assemblage identified in the expansive soils, we can affirm that the mechanism of smectite expansion and contraction is related to the shrinkage capacity of the soil, considering that the level of hydroxy-Al intercalation is low. In addition, these mechanisms also are related to the presence of quasicrystals and domains that control the water dynamic in the smectites, contributing to shrinkage capacity.

ABSTRACT Brown Nitossolos (Nitisols) and Latossolos (Ferralsols), according to the Brazilian System of Soil Classification (SiBCS), have a “caráter retrátil” as their distinctive property. Because this is a new topic, it is necessary to propose methods for evaluation. The objectives of this study were to evaluate methodologies for quantifying the shrinkage of soil using the Syringe Method and the Metallic Mercury Method, and to propose a new one, the “Ring with Sand Method”. Soil samples from eight pedons were used, with six Nitossolos and Latossolos with shrinkage, a Latossolo without shrinkage, and a Vertissolo (Vertissol) with admittedly high shrinkage and expansion. The methods were effective in identifying the greater degree of shrinkage of the Vertissolo . However, the Ring with Sand Method was the only one to indicate significant differences between the Vertissolo and the Latossolo without shrinkage, and this method differentiated the shrinkable soils as to the intensity of the characteristic. The proposed method was effective and can serve as a standard to quantify shrinkage.