A COMPARISON BETWEEN SEASONAL-CHANGES IN SOIL-WATER STORAGE AND PENETRATION RESISTANCE UNDER CONVENTIONAL AND CONSERVATION TILLAGE SYSTEMS IN ARAGON

Low and extremely variable precipitations limit dryland crop productio n in the semi-arid areas of Aragon (NE Spain). These areas are also af fected by high annual rates of topsoil losses by both wind and water e rosion. A long-term experiment to determine the feasibility of conserv ation tillage in the main winter barley production areas of Aragon was initiated in 1989 at four locations, three on loam to silt loam soils (Xerollic Calciorthid) and one on a silty clay loam (Fluventic Ustoch rept), receiving between 300 and 600 mm of average annual rainfall. In this study, we compared, under both continuous cropping and cereal-fa llow rotation, the effects of conventional tillage (mouldboard plough) and two conservation tillage systems, reduced tillage (chisel plough) and no-tillage, on soil water content and penetration resistance duri ng the first two growing seasons. Whereas reduced and conventionally t illed treatments generally had similar soil water content during the e xperimental period, the effects of no-tillage were inconsistent. No-ti lled plots had from 26% less to 17% more stored soil water (0-80 cm) t han conventional tilled plots at the beginning of the growing season. In contrast to the conventional and reduced tillage treatments, penetr ation resistances were between 2 and 4 MPa after sowing in most of the plough layer (0-40 cm) under no-tillage at all sites. Fallow efficien cies in moisture storage in the cereal-fallow rotation, when compared with the continuous cropping system, ranged from -8.7 to 12%. The high est efficiencies were recorded when the rainfall in the months close t o primary tillage exceeded 100 mm. Since this event is very unlikely, long fallowing (9-10 months) appears to be an inefficient practice for water conservation under both conventional and conservation managemen t. Our results suggest that, up to now, only reduced tillage could rep lace conventional tillage without adverse effects on soil water conten t and penetration resistance in the dryland cereal-growing areas of Ar agon.