Wednesday, November 15, 2006
287-24

The Need to Monitor Soil Phosphorus and Soil Nitrate Values on Organic Vegetable Fields.

Thomas Morris1, Jianli Ping1, Robert Durgy1, George Hamilton2, Eric Sideman3, Karen Anderson4, and Stefan Seiter5. (1) University of Connecticut, 1376 Storrs Rd; Unit 4067, Storrs, CT 06269, (2) University of New Hampshire, 329 Mast Road, Goffstown, NH 03045-2422, (3) MOFGA, Anson Rd, Greene, ME 04236, (4) NOFA-NJ, 60 South Main St, Pennington, NJ 08634, (5) Lin-Benton Community College, 6500 Pacific Blvd. SW, Albany, OR 97321

The objective of this study was to explore the effect of farmers’ nutrient management on the nutrient status of organic vegetable fields in Northeastern US. We collected soil samples in the fall of 2002 and 2003 from 34 farms in CT, MA, ME, NH, and NJ. Samples also were collected from selected fields for the Pre-sidedress Soil Nitrate Test (PSNT) in June 2002 and June 2004. The growers were interviewed to obtain field history information about nutrient applications, length of time in organic production, and crop rotation. The extractable nutrients by the modified-Morgan test varied substantially with 15% of the values in the optimum range, 32% below optimum, and 53% above optimum.  Cluster analysis was used to classify the extractable P values into four groups, and the average P values for the groups were 2.6, 12.6, 89.4 and 270.9 mg P L-1. There were significant differences in the nutrients typically measured by soil testing laboratories (pH, extractable P, K, Ca, Mg) among the four groups, which suggests that extractable P values provide an index for the soil nutrient status of these fields. The group with the greatest soil P value had a significantly greater length of time in organic production, (16.3 years) compared with the length of time in organic production for the other three groups. Data in the literature indicates that accumulation of P in soils from applications of manure is linear while accumulation of soil organic matter (SOM) tends to be curvilinear. PSNT values on fields with long-term additions of organic amendments were optimum or below optimum with normal spring rainfall in 2002, but much above optimum with below-normal rainfall in spring 2004. Organic growers who apply organic amendments to maintain or increase SOM need to carefully monitor soil P and soil nitrate values rather than SOM content.