INFLUENCE OF MULCHING ON THE PATTERN OF GROWTH AND WATER-USE BY SPRING WHEAT AND MOISTURE STORAGE ON A FINE TEXTURED SOIL

Eight tonnes ha-1 of stubble were used to mulch spring wheat (Triticum aestivum) on a fine textured soil with the aim of controlling both tr anspiration and soil evaporation during the wet pre-anthesis phase to increase moisture supply during grain filling in the eastern wheatbelt of Western Australia. Mulching reduced leaf area per plant by reducin g the culm number; consequently the green area index was reduced. Redu ced culm number was associated with low soil temperature which at 50 m m depth averaged 7-degrees-C lower under the mulched crop relative to the control crop in mid-season. The smaller canopies of the mulched cr op used 15 mm less water than those of the control before anthesis; th is difference in water-use was due equally to reduced transpiration an d soil evaporation. However, the mulched crop was unable to increase E T during grain filling, a response associated with the persistence of low soil temperature for most of the growth period. Hence, total ET fo r the season was significantly lower (18 mm) under the mulched crop th an the control crop. At harvest, mulching did not have significant eff ects on total above-ground dry matter and grain yields, but it increas ed water use efficiency for grain yield by 18%, grain weight by almost 17% and available moisture in both uncropped and cropped plots by an average of 43 mm. To determine whether there was any residual effects of soil treatment on moisture storage during the summer fallow period, soil moisture was monitored both in cropped plots and uncropped plots , that were either mulched or unmulched during the growing season, fro m harvest in October 1988 until next planting in June 1989. Available moisture at next planting was correlated with moisture storage at harv est despite the differences in run-off, soil evaporation and fallowing efficiency (increase in moisture storage as a percentage of rainfall) between treatments during fallowing. Therefore, the mulched treatment s had more moisture available (30 mm), mostly as a result of less wate r use during cropping in the previous growing season, than the unmulch ed treatment. The study shows that mulching may be used to restrain bo th transpiration and soil evaporation early in the season to increase availability of soil moisture during grain filling. Secondly, mulching during the previous growing season had little effect on soil moisture during the summer fallow period, however, the moisture saved by mulch ing during cropping was conserved for the following season. These resu lts indicate the importance of evaluating mulching of winter crops in terms of crop yield in the subsequent growing season as well as in the current season in which the soil was treated.