Monitoring Crop Nitrogen Status in Organic Vegetable Cropping Systems.

The applicability of traditional N monitoring approaches has not been fully demonstrated within organic vegetable cropping systems.  Over  4-yr, we monitored crop N status within on-farm and experiment station trials in western Oregon, USA.  Indicators included: preplant, in-season, and postharvest soil nitrate-N; potentially mineralizable soil N (Nmin, aerobic incubation at 22 ^oC), crop N uptake without current season N addition, and petiole nitrate-N.  Early-season soil nitrate tests were the simplest and most useful of the indices.  Early season soil nitrate-N values above the PSNT threshold (25 to 30 mg kg^-1; 0-30 cm depth) indicated N sufficiency.  Aboveground crop N uptake on unfertilized plots on established organic farms was typically 150 to 200 kg N ha^-1.   Potential Nmin (aerobic incubation) was 2 to 3x greater in soils with 5+ yr. organic history vs. soils from nearby fields under transitional or conventional management.  We recommend in-season monitoring of soil nitrate-N to adjust organic N fertility management practices to meet site-specific need.