Lighting up the black box: Improving legume performance on organic farms by optimizing microbially-mediated plant and soil nitrogen cycling processes.

Project Overview

Project Type: Research and Education
Funds awarded in 2010: $192,000.00
Projected End Date: 12/31/2014
Region: Southern
State: North Carolina
Principal Investigator:
Dr. Julie Grossman
University of Minnesota

Annual Reports


  • Agronomic: corn


  • Crop Production: biological inoculants, cover crops, no-till, nutrient cycling
  • Education and Training: farmer to farmer, on-farm/ranch research
  • Production Systems: organic agriculture
  • Soil Management: green manures, organic matter, nutrient mineralization, soil microbiology, soil quality/health


    We evaluated four cover crop termination treatments (organic herbicide, flail mowing, rolling, incorporation) with three cover crop species (hairy vetch, crimson clover, Austrian winter pea) to determine decomposition rate and nitrogen delivery. Hairy vetch had the greatest overall average nitrogen contribution. Disking was found to release hairy vetch N most rapidly, contributing the most N at 6-10 weeks after termination. An evaluation of nodule rhizobia occupants found native rhizobia to be out-competing the inoculant for nodule occupancy, as most nodules evaluated were found to contain native soil rhizobia rather than inoculant rhizobia.

    Project objectives:

    Objective 1: Randomly survey 200 organic growers from North and South Carolina, in order to determine current rhizobia inoculant handling procedures, and production challenges and perceived benefits of legume cover crop use.


    Objective 2: Establish three demonstration plots on working organic farms to learn about the impact of inoculation on nodulation and legume production.


    Objective 3: Determine how various methods of legume cover crop termination common in organic farming systems (rolling, disking, and flail mowing) impact indicators of microbial activity and N supply to crop plants.


    Objective 4: Disseminate results to agronomic organic growers and jointly educate organic growers and students nationwide about soil microbial N-cycling processes in sustainable agriculture.


    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.