Perennial legumes as a sustainable source of soil organic matter in Southeastern organic farming systems

2006 Annual Report for LS06-190

Project Type: Research and Education
Funds awarded in 2006: $190,000.00
Projected End Date: 12/31/2008
Region: Southern
State: Georgia
Principal Investigator:
Carl Jordan
University of Georgia

Perennial legumes as a sustainable source of soil organic matter in Southeastern organic farming systems


  • In 2006, production of chili peppers in plots with alley cropping plus winter cover crops was not different from production in plots with winter cover crops alone, possibly because nutrients in the alleys were in excess of demand.

    In a preliminary trial, a leguminous shrub (Amorpha fruticosa) was successfully leaf-labeled with 13C and 15N. Labels translocated to roots within 2 weeks.

    Alley cropping trials were established in 3 organic farms in the Piedmont region of Georgia.

    Four members of the Agroecology lab gave a session dedicated to soil organic matter management at the 2007 Georgia Organics Conference.

Objectives/Performance Targets

  • To compare crop yield, soil organic matter, and soil properties of an alley cropping system plus winter cover crops with a more conventional organic farming system that uses composts plus winter cover crops but no perennial leguminous shrubs.

    To measure the time and effort needed to manage the two systems.

    To determine whether pruning of above ground biomass of a perennial legume causes an increase in root sloughing, and if so, to quantify the contribution of root sloughing to soil organic matter and nitrogen.

    To develop an outreach component that will disseminate research results, establish on-farm trials, conduct workshops and internships, and provide feedback to researchers;

    To continue, expand, and integrate the Agroecology Lab’s current educational program for undergraduates and graduate students into the outreach program.


  • Yield trials
    Yield trials were carried out to test the effectiveness of perennial legumes in an alley cropping system in supplying nutrients to crops compared to other organic and conventional systems of cultivation.
    In 2006, three varieties of chili peppers were grown in replicated plots under the following conditions: Alley cropping with perennial legumes plus winter cover crops, and plus or minus mulch for weed control (strip tilled); Winter cover crops without perennial legumes, and plus or minus mulch for weed control (strip tilled): No cover crops nor perennial legumes, fertilization with inorganic NPK (rototilled).
    Yields of peppers in the alley cropping system plus winter cover cropping system was not significantly greater than yields in the winter cover cropping system alone, possibly because nutrients in the alley cropping system were in excess of demand by the peppers.
    For the 2007 season, crops (corn and squash) that demand more nutrients will be used to put greater stress on both systems. Both of the above systems produced higher yields than plots fertilized with recommended levels of inorganic NPK.
    Plots that were mulched had fewer weeds than those that were not mulched, and thus required less time and effort for weeding, but mulched plots had lower yields, apparently due to increased soil moisture content resulting in greater fungal infestation (Fusarium wilt)

    Effects of Pruning
    The hypothesis is that pruning of the perennial leguminous shrubs during the growing season in the alley cropping system results in root sloughing due to a physiological imbalance between leaves and roots. The subsequent decay of the roots contributes to the fertility of the soil. The procedure to test this hypothesis is to leaf-label a series of potted plants of a leguminous shrub (Amorpha fruticosa) with 13C and 15N, then prune. In a preliminary trial in 2006, 20 plants were labeled with the stable isotopes, then half were pruned, and over the next six weeks, soil samples were taken from both pruned and unpruned plants. Leaves, roots, soil and leachates were examined for presence of the labels. Leaves were successfully labeled, and labels translocated to roots within two weeks. However, although the labels were found in the soil and leachates, it was difficult to distinguish effects of treatments due to high variability. The conclusion is that the root death and subsequent decomposition that enables nutrients to move from the root to the soil and to the soil microbial community may take longer than six weeks. In 2007, the experiment will be repeated with 40 plants in each treatment (experimental and control) to increase statistical rigor. Sampling will take place monthly instead of weekly, and will continue throughout the winter of 2007-08 and into the spring of 08 if necessary.

    Outreach and Education
    In early 2007, we participated in two conferences that will benefit the outreach and education component of this project.
    In Jan, a member of the Agroecology Lab was an invited participant in the SAWG (Southern Agricultural Working Group) sponsored “On Farm Exchange Group” to exchange ideas of experienced farmers using organic practices, as part of the SAWG Annual Conference “Practical Tools and Solutions for Sustaining Family Farms”. The contacts that were made during this workshop constituted a step in organizing the “Soil Organic Matter Management Working Group” that will advise and participate in our effort to improve methods for building soil organic matter on organic farms in the Southeast.
    In March, four members of the Agroecology Lab presented an invited session entitled “The Agroecology Tool Box: Agroecological Systems, Stories, and Tools for the Southeastern Farmer” at the annual Georgia Organics Conference at Douglas, Georgia. The session consisted of talks on why soil organic matter management is especially critical in Southeastern farms, the mechanisms through which microbial communities in the soil recycle and conserve nutrients for the system, an overview of approaches to building and conserving soil organic matter in organic farming systems, and the story of how one organic farmer became involved in using alley cropping to supplement the fertility of his soil.
    There was a tremendous interest in the audience in learning more about the approaches to organic matter management, especially the alley cropping system. As a result, Georgia Organics has agreed to sponsor a special field day in the Spring of 2008 at Spring Valley Farm, the site of research sponsored by the SARE program. In addition, the Agroecology Lab will organize a formal meeting of the Soil Organic Matter Management Working Group at the 2008 meeting of Georgia Organics at Dalton, GA.

Impacts and Contributions/Outcomes

Spring Valley Farm continues to host over 1500 undergraduate students per year from the Athens and Gwinnett campuses of the Univ. of Georgia, as well as the Watkinsville campus of Gainesville State College. These students carry out lab exercises in which they learn more about organic agriculture.
Spring Valley Farm is the site of the Univ. of Georgia accredited course in Organic Agriculture, given each May to about 20 students. Several of the students in the course have become interns at “Full Moon Farm”, the commercial organic farm that comprises part of the Spring Valley research farm. The course is also part of the new UGA program in “Certificate in Organic Agriculture”.
Examples of other groups that also visited the farm include those from the State Botanical Garden, the Univ. of Georgia’s Environmental Ethics Certificate Program, and the “Outstanding in the Field” Program to honor chefs specializing in organic preparations.
The Soil Organic Matter Management Working Group will be the principal conduit through which research will be evaluated and guided. Georgia Organics will be the primary institution for disseminating research information, through a website and mailing list addressed to their membership, and at their annual conferences.


Alice Rolls

[email protected]
Executive Director
Georgia Organics
P.O. box 8924
Atlanta, GA 31106
Office Phone: 6787020400