SOIL PHYSICAL-PROPERTIES, WATER DEPLETION AND CROP DEVELOPMENT UNDER TRADITIONAL AND CONSERVATION TILLAGE IN SOUTHERN SPAIN

Tillage methods affect soil physical properties and, thus, have a dire ct influence on the replenishment and depletion of soil water storage and crop performance. This study was conducted to determine the effect s of traditional and conservation tillage on soil physical properties, soil water replenishment and depletion, and crop development and yiel d under southern Spanish conditions, The experiments were carried out from 1992 to 1995 in a sandy clay loam soil (Xerofluvent). The traditi onal tillage (TT) method consisted mainly of the use of mouldboard plo ughing, and the conservation tillage (CT) was characterized by not usi ng mouldboard ploughing, by reduction of the number of tillage operati ons and leaving the crop residues on the surface as mulch, In both til lage treatments a wheat (Triticum aestivum, L.)-sunflower (Helianthus annuus, L.) crop rotation was established. In each treatment, systemat ic measurements of bulk density, resistance to penetration, infiltrati on rate and hydraulic conductivity (using tension disc infiltrometers) in the soil top layer were carried out. Changes in water profiles thr ough the experimental period were also followed using a neutron probe, Crop development and yield were determined. The soil bulk density in the 0 to 20 cm layer was significantly higher in the CT than in the Tr treatment, mainly after tillage operations (:between 10% and 24% high er in CT than in TT), After 3 years of continuous tillage treatments, the soil bulk density did not increase, The resistance to penetration at any time was higher in the CT than in the TT treatment, but not alw ays significantly different. Infiltration rates were significantly hig her in the TT than in the CT treatment (about 35% higher in TT than in CT). After 3 years of the tillage treatments the hydraulic conductivi ty of the soil surface layer, at a pressure head of 0 mm, was signific antly higher in the CT (124 mm h(-1)) than in the TT (66 mm h(-1)). Th is is related to the existence of preferential paths created by an inc rease of the earthworm population in the CT treatment. Soil water prof iles showed higher replenishment of soil water storage in the CT than in the TT treatment, particularly from October 1994 to June 1995 when the lowest precipitation of the experimental period was recorded. Plan t height, leaf area index (LAI) and root length density (RLD) of the f irst sunflower crop were significantly higher in the TT than in the CT treatment. Nevertheless, the seed yield was slightly higher in the CT than in the TT treatment. In the second sunflower crop, plant height, LAI, RLD were significantly higher in the CT than in the TT treatment except early in the season, and yield was significantly (and extremel y) higher in the CT (1521 kg ha(-1)) than in the TT (473 kg ha(-1)) tr eatment. During the wheat crop season, plant height and RLD were highe r in the TT than in the CT treatment, but grain yield was again slight ly higher in the CT than in the TT treatment. The conservation tillage applied seems to he highly effective in enhancing soil water recharge and water conservation, particularly in years with much lower than av erage precipitation.