Soil Microbial Response to Seven Different Organic Transition Strategies

Project Overview

Project Type: Graduate Student
Funds awarded in 2012: $9,920.00
Projected End Date: 12/31/2015
Grant Recipient: GNC12-154
Region: North Central
State: Missouri
Graduate Student:
Faculty Advisor:
Dr. Randall Miles
University of Missouri

Annual Reports


  • Agronomic: corn, sorghum (milo), soybeans, wheat


  • Crop Production: conservation tillage
  • Production Systems: transitioning to organic
  • Soil Management: green manures, organic matter, soil analysis, soil microbiology, soil quality/health


    In the three years that is required in the U.S. to transition from conventional to organic practices, producers often experience decreased yields. We compared the effects of seven crop and tillage rotations on soil quality, weeds, and yields. We are reporting here only on soil microbial response to the transition strategies. This research was primarily funded by the Ceres Organic Trust and the full report, including weed response to the treatments, can be found on their website. We found that a three year rotation of winter and summer cover crops was the most effective in decreasing weed pressure but there were no treatment differences in soil quality, including microorganism biomass and community diversity. This was most likely because the study site had 4.3% SOM, indicating that soil quality was already high. We found that using organic no-till during the transition years severely reduced yields and that new methods of mechanical weed control need to be researched to make organic no-till feasible. We recommend that a combination of winter cover crops or wheat for early season weed control, SOC additions, and erosion control; summer cover crops for reducing the soil weed seedbank; and low N-use soybean are important tools for the transition from conventional to organic production methods.



    To gain organic certification in the U.S., a three year transitional period is required during which no prohibited materials, such as synthetic chemicals, fertilizers or genetically modified (GM) seeds, may be applied to the land. This transitional period from conventional to organic row cropping can be the most important and the most challenging time for an organic producer because of the need to control weeds, decrease the soil weed seedbank (Riemens et al., 2006) and improve soil quality and fertility (Delate and Cambardella, 2004). Soil quality is a composite view of the soil’s physical, chemical and biological properties and processes that sustain productivity, environmental quality and support healthy organisms (Doran and Zeiss, 2000). Soil organic matter (SOM) and its primary constituent, soil organic carbon (SOC), are closely linked to soil quality and soil fertility, thus maintaining or increasing SOM is an important goal in organic transition (Wander et al., 1994; Clark et al., 1998).

    To determine the effect of crop rotation and tillage practices on soil building during the transition into organic row cropping, we examined seven transitional cropping systems listed in table 2.1 These cropping systems utilized a winter cover crop mix (Table 2.2), the cash crops corn, soybean, wheat, and grain sorghum and the summer cover crops sorghum-sudangrass and sunn hemp. All systems utilized fall planted winter cover crops or wheat prior to spring planting. Treatment CCO, cover crop only, utilized three years of winter and summer cover crops with no intervening cash crop. Sorghum-sudangrass was the summer cover crop in year 1-2 and sunn hemp, a tropical legume, was used in year 3. Cover crops provide weed control through resource competition and allelopathic effects (Teasdale, 1996; Creamer at el., 1996). They can also improve soil nutrient cycling efficiency, reduce soil erosion, increase SOC, increase water infiltration and improve soil physical properties (Snapp et al., 2005; Dabney et al., 2001; Lundquist et al., 1999a). We hypothesized that the CCO system would provide the greatest weed control and soil quality building during the transition to organic.

    Treatment MCC, modified cover crop, utilized one year of a sorghum-sudangrass summer cover crop followed by grain sorghum in year 2 and corn in year 3. Sorghum species contain sorgoleone, an allelopathic chemical that has been found to reduce growth of weed seedlings (Einhellig and Souza, 1992), so we hypothesized that two consecutive years of sorghum species would lead to lower weed seed growth and seed production. Having grain sorghum in year 2 was expected to be more economically viable than two years of summer cover crop. In treatment MCT, modified conventional tillage, the transition began with one year of sorghum-sudangrass in the summer followed by winter wheat and a soybean double-crop in year 2 and corn in year 3, with tillage as needed to control weeds

    CONVCS, conventional tilled corn-soybean-wheat-double crop soybean rotation utilized pre-plant tillage for early weed control and between row cultivation for in-season weed control. NTCS, no-till corn-soybean followed the same rotation as CONVCS but utilized a crimped winter cover crop for weed control and did not use any soil tillage until year 3 when a soybean double-crop was planted following winter wheat. No-till can reduce the carbon footprint of organic farmers by using cover crops flattened and killed by a roller/crimper as a weed blocking mulch instead of relying on multiple tillage for weed control (Carr et al., 2013; Mirsky et al., 2013).

    In treatments NTSS, no-till sorghum-soybean, and CONVSS, conventional sorghum-soybean both the no-till and conventional rotations were replicated using grain sorghum as the 2nd year cash crop rather than corn. Grain sorghum has a history of success in MO, can get a majority of its fertility needs met through use of a winter legume cover crop (Reinbott et al., 2004), and does not contain any genetically modified (GM) varieties nor can it cross pollinate with GM varieties, which are prohibited by NOP regulations.

    Project objectives:


    The overall goal of this research project was to improve the competitiveness of transitional organic grain crop producers by researching cropping systems that can help maintain or increase productivity, suppress weeds and build soil health. The specific goal of the SARE funded project was:

    • Compare seven different rotational and tillage systems for transitioning into organic production and determine which strategies best contribute to microorganism biomass and community diversity as measured by phospholipid fatty acid analysis (PLFA) and response to a β–glucosidase enzyme assay.
    • Create outreach events to reach producers and government personnel interested in organic and sustainable agricultural practices.

    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.