Over the 4 years of the study, soil quality at the Iowa State University Neely-Kinyon Long-Term Agroecological Research (LTAR) site was consistently higher in the organic rotations (containing corn, soybean, oats, and alfalfa in a three- or four-year rotation) relative to the conventionally managed corn-soybean rotation. Soils under organic rotations had greater soil organic carbon, total N, labile organic N; higher P, K, Mg and Ca concentrations; and lower soil acidity than the conventional system. Organic farms in the study showed similar high soil quality, despite differences in soil types, crop history, and amendment use.
Organic agriculture continues to expand in the U.S. at a 7% rate in 2009, but strategies to optimize biological turnover to enhance soil quality in transitional organic farming are not well understood. In this project, we examined soil quality changes at a long-term organic research site established to examine the short- and long-term physical, biological, and economic outcomes of certified organic and conventional cropping systems. The ISU Neely-Kinyon Long-Term Agroecological Research (LTAR) farm is a systems experiment where treatments consist of a suite of farmer-developed practices (soil amendments, tillage, crop selection/rotation) established as complete management strategies. In addition, three on-farm organic sites were selected to monitor changes during the transition and beyond certification.
In this research, we are testing the hypothesis that organic systems relying on locally derived soil fertility inputs are capable of providing stable yields, while maintaining soil quality and plant protection, compared to conventional systems with less diverse crop rotations and greater levels of external, fossil-fuel based inputs. This project was started in 2006 to building on farmer-based experiences and our long-term research program experience to address the following research objectives:
Objective 1: Examine the effects of required organic farming practices, including crop rotations, cover cropping, compost application, and non-chemical weed control, on soil quality, crop yield and grain quality.
Objective 2: Examine how soil organic matter (SOM) quantity and quality influence the interrelationships among soil fertility, crop resistance to pests and diseases, and environmental conservation of nutrients and carbon.
Objective 3: Determine which crop rotations and nutrient management practices will increase the crop’s competitiveness with weeds, build soil fertility, and maximize biological control of insect pests and diseases.
In 2009, five randomly-located soil cores (0-15 cm) were removed from each plot in the fall after harvest but before plowing. The cores were mixed together to produce one composite sample from each plot. Soil samples were analyzed for a suite of soil quality indicator variables including soil organic (SOC), total N (TN), particulate organic matter C (POMC) and N (POMN), microbial biomass C (MBC), potentially mineralizable N (PMinN), inorganic N, Bray P, extractable K, Mg, Ca, electrical conductivity (EC), pH, macroaggregate stability (Aggs%), and bulk density (BD) at USDA-NLAE (National Lab for Agriculture and the Environment), Ames, IA.
Soil quality was higher in the organic rotations relative to the conventionally managed corn-soybean rotation. The organic soils had more soil organic carbon, total N, biologically active organic C and N, higher P, K, Mg and Ca concentrations and lower soil acidity than conventional soils (Table 1). Macroaggregation was especially high in the soybean-winter wheat system, possibly as a result of the dense rooting system of winter wheat and relatively fewer passes with the moldboard plow. The 3-yr organic rotation had higher inorganic P and K concentrations than the 4-yr organic rotation reflecting the greater manure application intensity (2 of 3 yrs) in the 3-yr rotation. Soil quality enhancement was particularly evident for labile soil N pools, which are critical for maintenance of N fertility in organic systems, and for basic cation concentrations, which control nutrient availability through the relationship with cation exchange capacity (CEC).
Results in 2009 at the LTAR site mirrored results obtained in 2006, 2007 and 2008, where higher soil quality was determined for the organic rotations, particularly for the rotation with two years of alfalfa (C-S-O/A-A).
On-farm organic sites also reflected the importance of alfalfa, clover and manure amendments in the rotation. Crop rotations at all the farms included corn, soybean, small grains and forage legumes. Soil biological, chemical and physical properties on the mid-length organic farm indicated high amounts of soil organic C, total N, particulate organic matter C and N, microbial biomass C, N mineralization potential, and macroaggregation in fields where alfalfa had preceded the study period.
On the organic farm that was closest to the organic transition period, soil quality was greater in fields where manure had been applied versus the presence of forage legumes, which had been present in every field’s history. The difference may have been related to the planting of clover versus alfalfa, however. The forage legume and manure impacts on soil quality may account for the differences among the 3 fields.
On the organic farm that had the longest history of organic production, soil quality was high, but differences between fields were more difficult to ascertain, probably due to a longer history of extensive crop rotations which included small grains, alfalfa and manure applications on every field. Thus, patterns in soil properties related to soil quality at this farm were less definitive than the other two farms.
One of the most significant outcomes is the result that the organic soils had more soil organic carbon, total N, biologically active organic C and N, higher P, K, Mg and Ca concentrations and lower soil acidity than conventional soils. With organic farming often criticized for excessive tillage, and the potential for degradation of soil carbon pools, this research shows the opposite: organic farming can lead to increases in soil quality, including carbon pools. This increase in soil quality in the organic system matched the increased profits compared to the conventional system. Farmers participating in this research also showed more interest in lowering tillage operations and improving soil quality on their organic fields.
Economic analysis in 2009 demonstrated higher returns to management from the organic rotation (C-S-O/A-A) compared to the conventionally managed corn-soybean rotation (Figure 1). Equivalent or greater corn and soybean yields in the organic rotations plus organic premium prices led to returns averaging twice that of conventional returns over the course of the study.
Farmer adoption of practices examined in this study has been high, with the majority of organic farmers in Iowa utilizing long-term crop rotations that include a perennial legume. The change in behavior from this study included the routine use of soil testing by participating farmers to improve farm sustainability.
Educational & Outreach Activities
Accomplishments and milestones included establishing a collaborative environment with farmer-cooperators with significant participation from farmers. Two of the farmer participants, Ron Rosmann and Earl Hafner, presented information on project results and concepts of soil quality in organic farming at the Iowa Organic Conference on November 19, 2007, at Iowa State University, Ames, Iowa, and again at the same conference on December 5, 2009, Ron Rosmann presented to a total of 143 farmers and ag professionals at both conferences. Mr. Rosmann also presented soil quality information at a Practical Farmers of Iowa Field Day at his farm in 2008. Kathleen Delate presented information on this project at the Iowa Organic Conferences in 2007, 2008 and 2009, to a total of 650 people. Results of this project are posted on the Iowa State University Organic Agriculture website, which has an average of main page viewing of 9,430 hits/yr and viewing of full site pages (with research reports) averaging 60,459 total hits/yr:
Areas needing additional study
Additional studies to further monitor the soil quality changes in the long-term (greater than 10 years) would be very beneficial.