Nitrogen Credit from Legume Cover Crops in High Tunnel Production

WCROC High Tunnel Research Project

Liz Perkus, Julie Grossman, Mary Rogers - U of MN Department of Horticultural Science, and Steve Poppe, WCROC Sr. Horticulturist

Season-extending high tunnel production has been expanding rapidly across the U.S.  High tunnels are an important tool that small scale vegetable producers in the upper Midwest use to grow good quality, high value crops.  High tunnels extend the growing season earlier in the spring and later in the fall, in some cases allowing year round crop production with little to no additional heat.  High tunnels also create hotter summer conditions needed to produce high quality tomatoes and peppers in Minnesota.  Another benefit of high tunnel production is protection; the plastic cover protects fruit from rain-drop propelled soil splattering, minimizing the spread of soil borne plant pathogens which can reduce yield and market value of the crop. 

High-tunnel production is characterized by increased productivity, but due to intense cultivation strategies and fertilize intensively, these practices lead to some unusual soil health problems such as high salinity and low organic matter content.  The WCROC high tunnel project evaluates inclusion of short-lived, fall planted cover crop legumes to improve soil fertility and quality in high tunnel environments.  The long term goal for this integrated project (Research, Education), based on extensive feedback from growers, is to develop a comprehensive and economically viable model to address soil health issues in high tunnels across a wide geographic area, resulting in increased adoption of practices such as legume cover crop incorporation that promotes sustainable management of organic high tunnels.   Our project is designed to have far-reaching implications for how farmers manage their high tunnels for optimal soil health and economic returns.  


  1. Evaluate fall-planted legume cover crop mixes for nitrogen credit potential
  2. Assess weed suppression ability of cover crop mixes
  3. Determine the effect of cover crop mixtures on pepper production

Cover crop treatments in the high tunnel were 1) bare control, 2) red clover, 3) pea/rye mix, and 4) vetch/rye/tillage radish mix.  All 3 cover crop mixes have a legume, and we're trying to use the legume's ability to fix atmospheric nitrogen to eliminate need for nitrogen fertilizer.  Cover crops were grown from late August to early May.  At time of termination we estimated that the bare control will contribute no nitrogen fertilizer, red clover will contribute 78 pounds of nitrogen fertilizer/acre, pea/rye mix will contribute 139 pounds of nitrogen fertilizer/acre, and vetch/rye/radish will contribute 125 pounds of nitrogen fertilizer/acre over the growing season.  The recommendation for peppers in this soil type is 100 pounds of nitrogen fertilizer/acre, so in 2/3 treatments we exceeded the recommendation.  We did not add any fertilizer besides the cover crops to the tunnel this year and have already harvested more than 1,300 pounds of marketable peppers from a 30 by 48 ft tunnel, which are delivered and used by University of Minnesota Morris Food Service.