Matthew R. Werner

Soil quality characteristics during conversion to organic orchard management

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 CO2 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.

Effects of conversion to organic agricultural practices on soil biota.

In the fifth year of an agricultural conversion experiment in Pennsylvania, we studied the soil biological community under three treatment regimes planted with corn: organic-manure, organic-legume, and a conventional system. The organic treatments consisted of complex crop rotations, cultivations, and organic matter inputs to control pests and maintain soil fertility. The conventional system consisted of a simple corn/soybean rotation with synthetic fertilizer and pesticide inputs. High rates of CO 2 evolution (a measure of potential microbial activity) in the organic plots corresponded with high levels of organic matter input. Soil nematodes were most abundant in organic plots, although seasonal patterns differed between the two organic treatments. Soil microarthropods were dominated by fungivorous Prostigmata mites, which reached peak abundance in organic plots two to five months after organic matter incorporation. Oribatid mites, which were rare throughout the study, followed the same pattern of abundance in each treatment and were probably most influenced by tillage disturbances. Predatory Mesostigmata were generally more abundant in organic plots. Surface-dwelling Collembola were abundant briefly in the spring, but soil-dwelling species dominated numerically throughout the cropping season. Spring tillage appeared to have a strong negative effect on earthworm populations in all plots. Small earthworm species became abundant in organic-manure plots during the summer. Larger earthworm species were abundant in organic-legume and conventional plots after the autumn harvest, when crop residues covered the undisturbed soil The systems-level nature of the Conversion Project experiment makes it difficult to identify cause-effect relationships. The data do suggest that organic amendments tend to enhance soil biological activity, while tillage disturbances tend to disrupt the biotic community.

Comparison of conventional and organic apple production systems during three years of conversion to organic management in coastal California.

Conventional and organic semidwarf Granny Smith apple production systems were compared during three years of conversion to certified organic management. Because of differences in fruit load with hand thinning compared with chemical thinning, apple tonnage was higher in the organic production system (OPS) in 1989 and 1991. The organic system was higher than the conventional system in number and weight of fruit per tree, but smaller in average fruit size. Using grower-receivedfarmgate premiums of 38% (1990) and 33% (1991) for unsorted, certified organic apples, comparative cost accounting showed greater net return per hectare for the OPS. The OPS required higher material and labor inputs in all years . Greater terminal growth in the conventional production system (CPS) in 1991 was the only significant difference in growth indicators between systems. N was generally higher in leaf and new wood bark tissues in the CPS. P was generally higher in the leaf and new wood bark tissues in the OPS. No decline in yield was associated with increased weed biomass in the OPS. There was no difference in fruit damage caused by codling moth between production system treatments (codling moth granulosis virus and pheromone-based mating disruption vs. synthetic insecticide). In 1991, secondary lepidopterous pests (apple leafroller and orange tortrix) caused greater fruit scarring in the CPS. In all years, tentiform leafminers caused greater leaf damage in the CPS. Apple leafhopper density and leaf damage were greater in the OPS in 1990 and 1991. Soil nutrient levels showed few significant changes during conversion to organic management. Soil bulk density and water holding capacity were useful indicators of changes in soil physical characteristics. Potentially mineralizable nitrogen andmicrobial biomass-C were more sensitive indicators of system change than total N or organic C. Two soil biological ratios, the respiratory ratio and biomass-C/total organic-C, were similar in the two production systems. Earthworm biomass and abundance increased in the OPS in the third year. The introduction of Lumbricus terrestris into the OPS greatly increased litter incorporation rates .

Colonization of Strawberry Roots by VA Mycorrhizal Fungi in Agroecosystems under Conventional and Transitional Organic Management

ABSTRACT Mycorrhizal colonization of strawberry roots was studied for two agricultural management systems on the California central coast, one conventional and one in transition to organic. Low levels of VAM colonization early in the first year of the experiment may be the result of past management practices, which included conventional production of a non-mycorrhizal crop with high levels of phosphorus inputs. In the second year colonization levels were higher in the transitional organic plots. Annual use of a fumigant and a fungicide in the conventional plots was implicated with depleting the VAM inoculum reservoir and maintaining low colonization levels. Inoculation of organically managed plots with Glomus intraradices at the second planting did not change colonization levels. Soil analyses showed that available soil phosphorus levels remained high (range 62 to 101 ppm) in both treatments throughout the study. In both years colonization increased during the spring and summer harvest season, and it is h...

A Comparison of Strawberry Plant Development and Yield Under Organic and Conventional Management on the Central California Coast

ABSTRACT Conventional strawberry (Fragaria ananassa Duch. cv. Chandler) production was compared with production undergoing conversion to certified organic management in a three-year, replicated, on- farm study. Plant vegetative development, measured as leaf number, leaf area, and vegetative biomass, decreased in the organic production system compared to the conventional system. Developmental differences were less significant in the third year of the study. Reproductive development, measured as number of flower buds, open flowers, and immature fruit, as well as total reproductive biomass, was also lower in the organic system. The timing and degree of difference in responses studied varied over the course of the study. The percent of total berries produced which were not marketable was larger in the conventional system in the second and third years of the study. Marketable fruit yield was consistently lower in the organic plots, but the margin of difference decreased over the course of the study from 39% to...