Nitrogen Credits from Cover Crops in Organic Vegetable Production.

Using cover crop to provide N for subsequent vegetable crops can reduce the cost of fertilizer for organic farmers, but better tools are needed to predict N credits or debits.  We are testing a Cover Crop N Availability Calculator for on-farm use that was adapted from the N subroutine of the CERES family of models.  It uses cover crop and soil characteristics along with local weather station data to predict N mineralization from cover crops. Cover crop N, carbohydrates, cellulose, lignin are measured by NIRS and used with soils data, cover crop biomass, and date of termination to predict available N and rate of release. 

The calculator has been used with sunnhemp and cowpea cover crops for fall broccoli production.   Organic broccoli yields following cowpeas were measured in cover-cropped (C) and no-cover crop (NC) plots with four rates of N fertilizer to develop yield curves.  The NC treatments received the full N fertilizer recommendation.  The C treatments received the recommended N fertilizer minus the predicted N credit.  At two farm sites, yields from C and NC treatments were also compared.  The certified organic site used sunn hemp cover crop followed by broccoli “Gypsy”.  Feathermeal was used to provide N.  The sustainable farm used cowpeas followed by broccoli ‘ Pakman’ and urea as the N source. 

 At the research farm, the average cover crop credit was 62 lbs N /ac.  Based on the equivalent yield from the NC yield curve, the cover crop supplied 79 lbs N/ac.  There was no statistical difference between the yields in the C and NC treatments. At both farm sites, there were no statistical differences between treatments receiving full N rate vs those that received the N credit.  This indicates that the Cover Crop Nitrogen Availability Calculator should be effective in helping farmers manage N fertilizer.