Farmers wishing to transition from conventional to organic management practices face numerous technical and logistical problems. Following conversion to organic management, crop yields frequently decline while weed pressures increase. Farmers, encouraged to build soil organic matter to overcome these constraints, are rightfully frustrated as they seek information about soil management. Even though soil organic matter is reputed to be a defining factor in organically managed systems, soil tests that document organic matter targets or provide useful management handles have not been developed. Even though organic standards require a soil building program, regulated components of the plan list things not to do instead of detailing how to manage soil effectively. We investigated two assays, particulate organic matter (POM) and amino sugar N plus hydrolyzable NH4 quantified by the Illinois N test (IL-N). Particulate organic matter is a sensitive and readily measurable index of soil C metabolism while IL-N can be used to predict soil N supply. The objective of this project was to develop soil test strategies and informational resources for soil fertility management. Post-transition soils were obtained from numerous long-term studies that included organic and conventional production systems. Organic systems that rely on animal inputs including manure or compost were considered separately from those that relied exclusively on mineral amendments, and plant based inputs including legume N. Use of organic practices increased organic carbon and total N contents and the concentrations of POM-C, POM-N and IL-N. Organic systems that relied on animal-based production systems did not do a better job of building total or labile SOM than organic systems that were purely plant based. Organic management increased the proportion of organic matter in POM but not in the IL-N fraction and the quantity of N in POM and the IL-N fraction were similar. POM appears to be a more sensitive indicator of organic management. It may be a more effective soil testing parameter than the IL-N fraction provided its contents can be linked to soil N supply, soil physical fertility, and/or diseases suppression.
The International Federation of Organic Agricultural Movements (IFOAM) standards, which were the first formalized organic standards, state “Soil and soil management is the foundation of organic production. Organic growing systems are soil based, care for the soil and surrounding ecosystems and provide support for a diversity of species while encouraging nutrient cycling and mitigating soil and nutrient losses.” IFOAM Norms (4.3.) require that certification follow a 36-month minimum period since prohibited substances have been applied and note a longer transition period could be required depending upon past land use, ecological context, and operator experience. The purpose of the conversion period is to establish an organic management system that builds soil fertility and re-establishes the balance of the ecosystem. Implementation of organic standards in a manner that ensures they achieve these lofty goals may be a challenge, given how general these standards are and how limited the research on this topic has been. The efficacy of organic practices is bound to be questioned by many, including producers, advocates, and enemies of the industry. This challenge may be greatest in the United States because the USDA National Organic Program (NOP) rules are even less proscriptive than those that preceded it (ATTRA, 2003).
In practical terms, what ‘enhanced’ organic matter characteristics means is a critical question. Enhancement of biologically-based fertility may have as much or more to do with changes in SOM quality as they do with quantity. Even though numerous studies have documented differences between organic and conventional soils in SOM and microbial properties, few measures of SOM or microbial biomass have found their way into practical use. Two fractions that hold particular promise as fertility indices are particulate organic matter (POM) and amino sugars. Measures of POM have been tied to microbial growth and nutrient supply and suggest it is closely related to biologically mediated C, N, and in some soils P availability. Amino sugars are derived from chitin and from bacterial and fungal residues. Newly immobilized N is disproportionately incorporated into the acid-soluble fraction that contains microbially-derived amino compounds. Related fractions are sensitive to organic amendments and have been related to plant N acquisition.
The ability of organic management to enhance soil fertility varies with context and crop. For example, cereal yields are frequently reduced in organic systems due to lower availability of nutrients and delayed timing of N release. In systems of organic farming without livestock, with no nutrients imported in feed or as bedding, or no manure to transfer nutrients within the farm, supplying adequate nutrients for crop growth, particularly nitrogen (N), is a major challenge. In cooler climates, cereals must complete their growth rapidly and have a high requirement for nutrients early in the growing season. Diagnosis of oversupply of nutrients might be as or more important to organic producers as is diagnosis of limitation. It is critical for organic producers to get a better handle on how to efficiently use manures and composts, which are the principle fertility tools in many systems. Inefficient use of manures is a key concern for the organic industry. Over application of amendments is a greater problem in intensive- high value systems or where livestock is concentrated.
The objective of this ongoing effort is to develop soil test strategies and informational resources for soil fertility management. The primary goal of the SARE funded component was to obtain post-transition soils from important long-term organic matter studies that included organic production to determine whether biologically-based soil fertility characteristics could be generalized according to organic farming system type (manure or legume based N fertility). Additionally, while not funded with SARE monies because of budget reductions, we’ve engaged in a number of participatory and collaborative efforts to document producers’ experiences with soil fertility and organic transition strategies and ascertain their information needs.
Samples were obtained from the long-term trials listed below. Thanks are due to Deborah Stinner (FCTE), Anne Conklin and Michel Cavigelli (FSP), Rita Seidel and Jeff Meyers (FST), Jeff Smeenk, Dick Harwood and Ann-Marie Fortuna (LFL), Andrew Corbin and Phil Robertson (LTER), Dennis Bryant and R. Ford Denison (LTRAS), Kathleen Delate and Cindy Cambardella (NK), Josh Posner and Janet Hedtcke (WICST) for providing assistance with and access to soil samples.
Long term trials: Numbers denote farming system type: 1. manure-based organic, 2. legume-based organic, 3. conventional.
Field Crops Organic Transition Experiment (FCTE)
Wooster, OH with 1: corn-soybean-oats-hay and 3: corn-soybean
Farming Systems Project (FSP)
Beltsville, MD with 1: corn-soybean-wheat and 3: corn-soybean-wheat/soybean
Farming Systems Trial (FST)
Kutztown, PA with 1: wheat/red clover/alfalfa-red clover/alfalfa-corn/rye-rye/soybean-corn silage/wheat, 2: hairy vetch/corn-corn/rye-soybean/wheat-wheat/hairy vetch, and 3: corn-corn-soybean
Living Field Lab (LFL)
Hickory Corners, MI with 1: corn-corn-soybean-wheat and 3: corn-corn-soybean-wheat
Long-Term Ecological Research in Row Crop Agriculture (LTER)
Hickory Corners, MI with 2: corn-soybean-wheat
and 3: corn-soybean-wheat
Long Term Research on Agricultural Systems (LTRAS)
Davis, CA with 1: corn-tomato, 2: winter legume cover crop/corn-tomato, and3: corn-tomato
Muscatine Island Research and Demonstration Farm (MIRDF)Fruitland, IA with 1: rye/hairy vetch-pepper and 3: rye-pepper
Neely-Kinyon Long-Term Agroecological Research (N-K)Greenfield, IA with 1: corn-soybean-oats/alfalfa and corn-soybean oats/alfalfa-alfalfa
and 3: corn-soybean
Wisconsin Integrated Cropping Systems Trial (WICST) Arlington, WI with 1: corn-oats/alfalfa-alfalfa, 2: corn-soybean-winter wheat/red clover
and 3: corn-soybean
Soil samples were obtained from the plow layer (approximately 0 to 25cm depth) of nine established farming systems trials with both organic and conventional farming systems in the spring of 2002 and/or 2003. The organic systems in eight of the nine sites had been established at least five years prior to sample collection and thus were well through the transition period thought to last three years. One trial was established three years prior to sample collection and so the organic system had just finished the transition period at the time of sampling. Farming systems were categorized as manure-based organic, legume-based organic, or conventional. The manure based farming systems received various types of raw or composted manure as their primary fertility source. Some crop rotations in manure based organic systems also included legumes as primary crops, cover crops, or green manures. Legume based farming systems used legumes as cover crops or green manures for their primary fertility source. Conventional farming systems relied on synthetic fertilizers.
Soil samples were air-dried and ground to pass a 2mm sieve. Recognizable organic residues greater than 2 mm were removed by hand. POM was fractionated as SOM >53 microns. Where present, carbonates were destroyed. TOC, TN in whole soil and POM samples were analyzed by combustion analysis (Costech Analytical C-H-N Analyzer). Amino sugar plus exchangeable NH4+-N (IL-N) was determined in whole soils using the Illinois Soil N Test. Exchangeable NH4+ also was analyzed to confirm that exchangeable NH4+ was not the primary component of the IL-N fraction.
On average, use of organic management increased total SOM concentrations in surface soils. Both TOC and TN were significantly higher in the organic than the conventional soils. The POM-C and -N fractions accumulated disproportionately in the organic systems while the IL-N fraction was not preferentially accumulated in organic farming systems. The POM fraction was more sensitive to differences in management than the IL-N fraction. IL-N was highly correlated with TOC and both climate and texture appear to have had a greater impact on soil IL-N contents than had management practices. Correlations between POM-C and TOC were comparatively weak, as were relationships with climatic and textural variables.
- The project allowed us to conclude that POM is a more sensitive indicator of organic management than IL-N, moreover, the quantity of N contained in the POM fraction equals that contained in the IL-N fraction. POM shows more promise as a soil testing parameter than the IL-N test.
We also found that a livestock component or animal based products are not necessarily needed to build soil organic matter. This challenges a widely held paradigm and should be investigated on farm to see how well replicated trials emulate reality.
The results from this study have helped us refine our research and management questions. In addition to providing information about soil N supply, POM can provide needed information about soils’ physical fertility and their potential to suppress disease.
This project supported the master’s program and research of Emily Marriott. Her skills and knowledge will likely have added future benefits to agriculture.
While our results have economic implications, economic analyses were not included in our work.
These tests are not yet commercially available. Our efforts move us toward endorsement of strategies that could be adopted.
Educational & Outreach Activities
Marriott, E.E. and M.M. Wander. November 2003. Soil organic matter characteristics of organic and conventional farming systems at long-term trials. Soil Sci. Soc. Am. Annual Meeting, Denver, CO. Poster presentation.
Marriott, E.E. February 2004. Soil organic matter in organic and conventional farming systems at long-term trials. Program in Ecology and Evolutionary Biology Graduate Student Symposium, Univ. of Illinois, Urbana, IL. Oral presentation. *Her paper won the GEEB award for the best masters presentation.
Marriott, E.E. April 2004. Soil organic matter in organic and conventional farming systems at long-term trials. Dept. of Natural Resources and Environmental Science Graduate Student Symposium, Univ. of Illinois, Urbana, IL. Oral presentation.
Marriott, E.E. and M.M. Wander. 2004. Soil organic matter in organic and conventional farming systems in long-term trials. Manuscript draft for SSSAJ.
Marriott, E.E. and M.M. Wander. November 2004. Particulate organic matter fractions in organic and conventional farming systems in long-term trials. Soil Sci. Soc. Am. Annual Meeting, Seattle, WA. Poster presentation.
Wander, M.M. Results from this noted in article (April 15, 2004) prepared for the New Ag Network http://www.ipm.msu.edu/new-ag/issues04/04-15.htm#2, and will be discussed in greater detail in a future article for the network.
Wander, M.M. June 2004, results presented in Research Summary of Organic Fertility at the SARE PDP program- “Organic Professional Development Opportunity on Organic Production” held in Springfield IL.
Areas needing additional study
Efforts to further refine fractionation methods to concentrate fractions that are predictive of soil N supply, disease suppression, and aggregation, are underway.
Soil testing strategies that rely on these measures will need to be tailored to fit intensive and extensive organic systems and inform management choices that address specific soil functions.