Soil quality characteristics during conversion to organic orchard management

Abstract

Soil quality measurements were made as part of a larger study of conversion to certified organic management in a California apple orchard, and results are reported here. The general objectives of the soil research were: (1) to compare soil properties between the two production systems (conventional and transitional organic); (2) relate observed differences to soil processes and management practices; and (3) determine the relative usefulness of each of the measured parameters as soil quality indicators. Bulk density and water holding capacity were useful indicators of changes in soil physical characteristics. Because treatment differences in bulk density were detected, biological and chemical data were expressed volumetrically to more closely approximate in-field values. EC, Ca, and Mg readings were used as an indication of the absence of significant salinity problems in both production systems. Soil pH in surface soils was mildly acidic, and values were slightly higher in the organic production system plots. Potentially mineralizable nitrogen and biomass carbon (C) were more sensitive indicators of system change than total N or organic C. Because microbial respiration was higher from organic plots on some sample dates, C loss via CO 2 was evaluated against C inputs in the two systems. It was concluded that in both production systems, C inputs appear to outweigh C loss via microbial respiration, and therefore the potential for net organic matter loss is minimal. No significant treatment differences were seen in the respiratory ratio, or in the ratio of biomass C to total organic C. Increased colonization by mycorrhizal fungi seemed to provide an explanation for higher tissue P levels in the organic trees, and thus was an indicator of increased soil biological function in the organically managed plots. Soil invertebrates did not seem to be sensitive indicators of short-term responses to management changes in the current study, as no significant differences were seen during 1990–1991, the first 2 years of transition to organic management in the organic treatment plots. However, earthworm abundance and biomass increased significantly during 1992 in the organic plots, and seemed to provide an indication of long-term response to changes in weed, pest, and soil fertility management. Pitfall trap arthropods were too variable in abundance and diversity to discern any significant treatment effects with the sampling scheme used, and therefore these parameters were not useful indicators of soil quality changes.