Sustainable crop production in the Pacific Northwest dry-farmed areas relies heavily on tillage and residue management systems to conserve water. Stable, sustainable yields cannot be achieved without adequate water conservation techniques. Frozen soil can reduce infiltration markedly, which decreases overwinter profile water storage and can cause severe soil erosion. Uncurbed evaporation losses throughout the year can greatly limit yields, particularly with summer fallow.In both summer-fallowed and annually cropped regions where soil freezes frequently, fall tillage is used to increase surface macroporosity and to provide open channels to below the frost depth. This enhances infiltration throughout the winter and insures better water intake during rapid snowmelt and rainfall when the soil is frozen. Fall tillage enhances overwinter water recharge under these conditions, whereas in areas where soil freezes infrequently, it does not improve water storage efficiency.In the dry-farmed regions receiving less than 330 mm annual precipitation, a winter wheat-fallow system is used to reduce the risk of uneconomical yields. Successful establishment of winter wheat following summer fallow is feasible only when proper management has suppressed evaporative loss. During the dry summer fallow, tillage is used to develop and maintain a soil mulch that restricts the flow of water, as both liquid and vapor. The tillage mulch effectively conserves stored soil water and maintains adequate seedzone moisture for fall establishment of winter wheat. However, the soil mulch can lead to high wind and water erosion.In the Pacific Northwest dry-farmed region, tillage by itself is not considered a substitute for proper residue management. Crop residues following harvest are important for conserving water and controlling erosion. Under conservation programs implemented since 1985, shallow subsurface tillage systems that maintain residues on the surface have substantially reduced wind and water erosion in the region. Surface residues are effective in decreasing evaporative water loss and trapping snow during the winter, and therefore increase overwinter recharge. While surface residues are much less effective in suppressing evaporative losses in dry-farmed areas during extended dry periods, residues provide substantial control of wind and water erosion during the fallow.Before conservation tillage systems came into use in the Pacific Northwest, water conservation frequently was achieved only through tillage. This helped to stabilize yields, but at a high cost to the soil resource. Poor use of surface residues and intensive tillage contributed to extensive wind and water erosion. Continued use of these practices would have caused yields to decline over time and required greater agrichemical inputs. To meet soil and water conservation needs, site-specific tillage and residue management systems were developed to account for the diversity and variability of soils and climate across the Pacific Northwest. Common to all these production systems is that both water conservation and effective residue management to protect the soil are required for long-term sustainable production.
Soil phosphorus fractions vary with harvest intensity and vegetation control at two contrasting Douglas-fir sites in the Pacific northwest
