Monitoring Nitrogen Availability in Organic Potato Production.

The largest gap in the N budget for most organically-grown vegetables is the estimate of N mineralized from soil organic matter.  We worked with western Oregon organic farms to measure on-farm N mineralization, and to assess traditional N monitoring methods (soil nitrate testing and petiole nitrate) for assessing N sufficiency.  Soils typically had 3 to 5% organic matter in 0-30 cm depth.  Median N uptake by an unfertilized (zero N applied) potato crop was approximately 170 kg N ha-1 near harvest (100 d after planting).  The typical rate of crop N uptake for unfertilized potato crops during tuber growth was 2 kg N ha-1 d-1.  Median net mineralized N from soil in laboratory incubations (0.7 mg N kg-1 d-1 at 22oC) was approximately equivalent to unfertilized crop N uptake in the field, taking into account degree days. We found that soil nitrate values were easier to interpret than petiole values, because soil nitrate was not as strongly related to cultivar as were petiole nitrate values.  We conclude that much (50 to 100%) of potato crop N needs on our organic farms can be supplied by soil N mineralization, and that a combination of traditional N monitoring tools can be used successfully to manage N to meet crop needs.