Improving Soil Quality During and After Organic Transition
The organic agriculture industry continues to expand in the U.S.A. at 17% annually despite the economic downturn. With additional economic and environmental benefits associated with transitioning to organic agriculture, greater interest in organic transition has occurred. Strategies to optimize biological turnover to enhance soil quality in transitional organic farming are not well understood. At the long-term organic research site established in Iowa (the ISU Neely-Kinyon Long-Term Agroecological Research (LTAR) site), we are examining the short- and long-term physical, biological, and economic outcomes of certified organic and conventional cropping systems. The LTAR site 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. Over 4 years, soil quality has been consistently higher in the organic rotations relative to the conventionally managed corn-soybean rotation. The organic soils had more soil organic carbon, total N, labile organic N, higher P, K, Mg and Ca concentrations and lower soil acidity than conventional soils. In addition, three organic farm sites showed excellent soil quality, with differences attributed to inherent differences in soils in different Iowa counties. Differences in P and K are most likely related to differences in manure/compost type, rates of application, and application timing. Overall, the three farms exhibit remarkably similar values for soil quality indicator variables despite differences in crops, soil types, and organic amendments.
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, 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; and
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.
LTAR 2005-2008 Soil Data Summary
Five randomly-located soil cores (0-15 cm) were removed from each plot every fall after harvest but before plowing from 2005 to 2008. The cores were mixed together to produce one composite sample from each plot. Soil quality was consistently higher in the organic rotations relative to the conventionally managed corn-soybean rotation during this four-year period. The organic soils had more soil organic carbon, total N, labile organic N, higher P, K, Mg and Ca concentrations and lower soil acidity than conventional soils. Soil organic C was lower for the organic soybean-winter wheat rotation that the other organic rotations, reflecting the lack of carbon-rich inputs from corn. However, macroaggregate stability was higher in the soybean-winter wheat system in 3 out of 5 years, likely because of the dense, fibrous rooting system of the small grain. The 3-yr organic rotation had more inorganic P and K 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).
2006-2008 Organic Farm Soil Data Summary
The organic farms are located in Shelby County, Iowa (Errett and Rosmann) within the Marshall soil association and Guthrie County (Hafner), Iowa, within the Shelby-Sharpsburg-Macksburg soil association. Crop rotations at all three farms included corn, soybean, small grains and forage legumes. Three fields, each in a different phase of the rotation, were sampled at each farm. Sampling locations for the 3 fields at each farm were chosen to fall within the same soil type to reduce variability. Soil sampling was conducted in May 2006 and 2007, and in the fall of 2008. Three sampling transects were delineated at each field. Four sampling sites were located at 20-m intervals along each transect. Three soil cores (depth 0-15 cm) were removed from each sampling site and all cores were composited from each transect for a total of 3 composite soil samples from each field. 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). For the samples collected in fall 2008, a sub-set of the suite of indicator variables was analyzed.
Soil biological, chemical and physical properties for the Errett Farm (mid-length organic operation) indicated overall soil quality was highest for Field 8 compared with Field 7 and 9 in 2006 and 2007. Soil organic C, total N, particulate organic matter C and N, microbial biomass C, N mineralization potential, and macroaggregation were all significantly higher for Field 8. Soil quality for this field may be greater because alfalfa hay was cropped during 2003 and 2004. In 2006, we observed consistently higher values for soil properties related to soil quality for Field 3 compared to Field 2 and 4 at the Hafner Farm (shortest time in organic production). Soil organic C, total N, particulate organic matter C and N, microbial biomass C, and N mineralization potential were significantly greater for Field 3. All three fields at the Hafner Farm were planted to crop rotations that had 2-3 years of clover in the last 5 years so soil quality differences were likely not due to forage legume impacts. Soil quality at Field 3 may be related to manure applications since this field was planted to corn in 2002 and 2004 whereas Field 2 and 4 were cropped to corn only once since 2002. We also observed higher values for some of the soil properties related to soil quality for Field 3 compared to Field 2 and 4 at the Hafner Farm in 2007 and 2008, but the pattern was not as consistent as what we observed in 2006. Soil organic C, total N, and N mineralization potential were significantly greater for Field 3 but particulate organic matter C and N, and microbial biomass C were not. Field 2 was planted to barley/clover in 2006 and field 4 was cropped to corn, which received manure. The forage legume and manure impacts on soil quality may account for the 2007 differences among the 3 fields relative to 2006 data. Patterns in soil properties related to soil quality at the Rosmann Farm (the longest time under organic management) were less definitive than the other two farms. Nearly all of the soil properties we measured trended lower in Field 9, relative to Field 6 and 16, but the difference was significant only for soil organic C. No consistent pattern emerged for differences among the fields at the Rosmann Farm.
Estimates of whole-farm soil quality were made by averaging data for all fields within a farm across all three years. The Hafner Farm had more SOC, labile organic N, inorganic P and extractable K, and greater aggregate stability than the Errett and Rosmann Farms. Since SOC and aggregate stability are higher at every field for all years at the Hafner Farm, these differences can be attributed to inherent differences in soils in the two Iowa counties. However, differences in P and K are most likely related to differences in manure/compost type, rates of application, and application timing. Overall, the three farms exhibit remarkably similar values for soil quality indicator variables despite differences in crops, soil types, and organic amendments.
In the first year of the sampling, distinct differences were noted in soil quality, with longer years in organic production leading to higher soil quality, but after three years, all organic farms exhibited similar high levels of soil quality, suggesting that soil response to organic transitioning rapidly leads to greater soil quality. 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, with a total of 292 farmers and ag professionals participating in these sessions. In 2008, Ron Rosmann held a Field Day at his farm and discussed project results and soil quality in organic operations. Project results were also discussed by Kathleen Delate at the Iowa Organic Conference on November 24, 2008, for 236 people.
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Impacts and Contributions/Outcomes
One of the most significant outcomes to date 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. Farmers participating in this research also show more interest in lowering tillage operations and improving soil quality. In the final year of the project, we will quantify the economic benefits of transitioning to organic and the benefits beyond certification on participating organic farms.
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National Soil Tilth Laboratory
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