SOIL DEVELOPMENT IN A CHRONOSEQUENCE OF FLUVIAL TERRACES OF THE ALMAR RIVER (SALAMANCA, CENTRAL WESTERN SPAIN)

C. DORRONSORO and P. ALONSO

Dpto. de Edafología. Facultad de Farmacia. Univeridad de Salamanca.

ABSTRACT
The aim of this work is to analyze how soils formed fundamentally from the erosion of granites evolve under a subhumid Mediterranean climate as a function of age. Their evolution will be evaluated by means of the changes in components and properties, as well as the use of developmental indices. The soils are Xerorthents (Holocene), Haploxeralfs (Upper Pleistocene) and Palexeralfs (Middle Pleistocene). The properties have been subject to several evolutionary trends: i) some properties increase with regularity throughout the chronosequence: available water and COLE of the Bt horizons. ii) Other properties increase strongly only during the first phases; some continue to increase with age but only moderately-- retentions of water and the cation exchange capacity of the Ap horizon, Fed of the Bt horizon, solum thickness, clay accumulations, the quartz content and the quartz/feldspar ratio-- whereas other properties of the soils of Middle Pleistocene age cease to increase: the silt and available water of the Ap horizon, and retentions of water and cation exchange capacity of the Bt horizons. iii) Others decrease with age, with pronounced decreases during the first phases (feldspar contents andpercentages of sands of the Ap and Bt horizons and bulk density of Bt horizons). v) Some properties are not age-related (the base saturation and nitrogen content of the Ap horizon and the silt of the Bt horizon). The horizon development indices and the soil development indices indicate good relationships with age. In most cases the rate of increase declines strongly for the oldest soils. In the great majority of cases, the properties and development indices continue to evolve through the chronosequence without reaching a steady state