Predicting Nitrogen Credits from Cover Crops Using Minimob.

Better tools to predict the amount of nitrogen mineralized by cover crops are needed for organic as well as conventional farmers.  MinImob is a simulation model that uses cover crop and soil characteristics along with local weather station data to predict nitrogen mineralization from cover crops and soil organic matter. Soil (organic C, hot KCl N) and cover characteristics (N, carbohydrates, cellulose, lignin) are measured by NIRS and used with inorganic N, cover crop biomass and date of incorporation to predict the total N release and rate of release over user-selected  intervals.  The model was adapted from the N subroutine of the CERES family of models. 

MinImob was used to evaluate N mineralization or immobilization from sunnhemp, sudan-sorghum/cowpeas, and cowpea cover crops in organic vegetable production in two projects.  In the first project, the model results were compared to book values and yields of onions and carrots were compared to conventional county averages.  The model predicted an average total mineralized N of 112 kg/ha from the sunnhemp cover crop and soil organic matter of which 50% was attributed to the cover crop.  The crop was assigned a 100 kg N/ha credit and only 34 kg N/ha of additional N was applied.  Onion yields were comparable to average conventional county yields. The model predicted N immobilization for the sudan-sorghum/cowpeas. Carrots yields were low.  In the second project, broccoli yields were measured in cover-cropped and no-cover crop plots with four rates of N fertilizer to develop yield curves.  The cowpea cover crop was predicted to provide an average of 67 kg /ha and cover crop plots had an average increase of 735 kg/ha compared to no cover crop.  The data indicate the model has potential for use in organic vegetable production.