Structure and structure‐related characteristics of dryland and long‐term irrigated xerochrept soils

Typical soils from the Ebro Valley (NE Spain) under different land use regimes (flood irrigation for more than 100 years and dryland) were studied from various analytical data, including classical physicochemical and mineralogical parameters, and submi‐croscopic and porosimetric properties, in order to assess potential changes in soil quality (particularly soil structure). The submicroscopic study revealed marked differences in structural characteristics between the dry (uniform, complex granular structure) and the irrigated (weakly developed subangular blocky structure) soils, which may be explained in several ways. When dry soil is flooded, soil aggregates are stressed by swelling of the clay present and entrapped air causing immediate break‐up and slaking of aggregates. Another source of soil structure changes is the ploughing and traffic of agricultural machinery under moist conditions in the irrigated soil; these practices create mechanical stress in the top soil (Ap horizons) and lead to the disappearance of packing voids and the formation of a very dense fabric. The B horizons in the irrigated soil show clear signs of degradation typically associated with saline‐sodic soils irrigated with low‐salinity water (mainly structure collapse and reorganization of fine materials). The use of submicroscopic and porosimetric techniques proved very powerful for assessing soil structure degradation: It allowed detection of some processes that cannot be properly evaluated with conventional analyses. Based on the results, improved water management practices are required to avoid land degradation in semiarid irrigated areas similar to the ones studied.