Proceedings: Nitrogen cycling and partitioning under alternative organic orchard floor management strategies

Organic orchard systems are a significant and growing component of Washington State agriculture, yet sustainable methods of nitrogen (N) fertility and weed management remain a challenge. Nutrient supply is dependent on decomposition and mineralization of organic matter, yet intensive cultivation commonly used to control weeds can disrupt biological processes and cause loss of organic matter. To address the often-competing goals of organic fertility and weed control, a number of alternative orchard floor management strategies were implemented and evaluated for their impact on N cycling, soil quality, and tree health in a newly established orchard. The standard practice of weed control using extensive tillage resulted in trees with good growth and acceptable levels of leaf N and most other essential nutrients, conversely, soil quality was in decline and may negatively impact long-term nutrient dynamics. Maintenance of a living cover understory resulted in greater N retention and availability, and rapid soil quality improvement, yet it severely competed with young trees, resulting in reduced tree growth. Covering the understory with wood chip mulch enhanced soil moisture and resulted in adequate tree growth, but it also facilitated N loss and correspondingly resulted in low tree leaf N. Application of a clove oil herbicide resulted in lower leaf N and tree growth in comparison to cultivated treatments and did not positively impact soil quality parameters. In contrast, Brassicaceae seedmeal (BSM) applications enhanced N availability and soil faunal biomass, yet leaf N did not reach acceptable levels and many of the other essential nutrients were lowest in this treatment. Tree leaf chlorosis was observed following an early season BSM application and may have been the result of reduced soil iron availability. None of the treatments applied produced an ideal combination of weed control, maximum tree growth, adequate leaf nutrients, and improved soil quality. Rather, soil quality improvements tended to compete with treeHoagland, L., Carpenter-Boggs, L. Granatstein, D., Mazzola, M., Peryea, F., Smith, J., and Reganold, J. 2007. Nitrogen cycling and partitioning under alternative organic orchard floor management strategies. Proceedings Western Nutrient Management Symposium, Salt Lake City, UT. 7:117-123