Saturday, 15 July 2006

Factors Influencing the Soil Nutrient Status of Organic Vegetable Farms in Northeastern US.

Thomas Morris1, Jianli Ping1, Robert Durgy1, George Hamilton2, Stefan Seiter3, Eric Sideman4, and Karen Anderson5. (1) Univ of Connecticut, Dept of Plant Science, 1376 Storrs Rd, Storrs, CT 06269-4067, (2) Univ of New Hampshire Cooperative Extension, 329 Mast Road, Goffstown, NH 03045, (3) Linn-Benton Community College, 6500 Pacific Blvd., SW, Albany, OR 97321, (4) Maine Organic Farmers and Gardeners Association, P.O. Box 170, Unity, ME 04988, (5) Northeast Organic Farmers Organization-New Jersey, 60 South Main Street, Pennington, NJ 08534

Soil nutrient status is of great concern in organic farming because maintaining optimum availability of nutrients is important for efficient production and environmental safety. The objective of this study was to document the soil nutrient status of organic vegetable fields in Northeastern US and to explore the effects of farmers' nutrient management on soil nutrient status. We collected soil samples in the fall of 2002 and 2003 from 214 fields at 38 farms in CT, MA, ME, NH, and NJ. The growers were interviewed to obtain field history information about nutrient applications, length of time in organic production, and crop rotation. Summary statistics indicated that the extractable nutrients by the modified-Morgan test (ammonium acetate at pH 4.8) were in the above-optimum category for Ca, Mg, and P in greater than 50% of the fields. The extractable K values were similarly distributed with slightly less than the 50% of the values in the above-optimum category. Extractable P values are of most interest due to concerns about accumulation of P in agricultural soils. Twenty-eight percent of the extractable P values were in the optimum range, 20% below optimum, and 52% above optimum. Cluster analysis was used to classify the extractable P values into four groups, and the average P values for the groups were 2.7, 14.3, 95.1 and 282.1 mg P L-1. The group with the greatest soil P value had a significantly greater length of time in organic production, an average of 16.3 years, compared with the length of time in organic production for the other three groups. Cross validation of the four groups showed that the average accuracy was 70%, with the group having the greatest P value also having the greatest accuracy. The Most Significant Linear Combination (MSLC) from a multivariate analysis of variance was used to determine the influence of field history and soil properties on the classification of the four groups. The largest contributor to the MSLC was soil organic matter, with application of composts made from manures and application of chicken manure as the next largest contributors. Factors that had no effect or inconsistent effects on the MSLC were: applications of mineral fertilizer, other organic material, and blended fertilizer; field use before organic production; crop rotation; and cover crops. These data suggest that long-term application of composts made from manure and application of chicken manure can add excess P to soil. Careful management of composts, especially compost made from manures, and chicken manure is necessary to avoid the accumulation of P on organic vegetable farms. Our data also suggest that many organic vegetable growers should use soil testing to more carefully manage the macronutrients Ca, Mg and K.

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