SCC083: Quantifying the Linkages Among Soil Health, Organic Farming, and Food
Statement of Issues and Justification
Organic farming is a vibrant and growing sector of the U.S. farm economy. The USDA Economic Research Service reported that U.S. sales of organic foods and beverages grew from $3.6 billion in 1997 to $21 billion in 2008. Growth in the organic food market has slowed recently because of domestic and global economic crises, but trend data suggest that market expansion will continue. Consumer marketing analysts at Packaged Facts predict at least strong single-digit growth in organic food and beverage markets through 2013.Scientific research directed at providing solutions to the problems encountered when growing crops organically is needed so that market demand can be met. Organic farmers articulated research needs to scientists during several sessions of the Scientific Congress on Organic Agricultural Research between 2000 and 2002. A summary of those sessions was provided in the National Organic Farming Research Agenda, a publication released by the Organic Farming Research Foundation in 2007. The report revealed that organic growers believe a healthy soil is essential for the sustainability of an organic farm. The farmers considered high soil organic matter (SOM) levels, lush crop canopies, reduced soil erosion, and earthworm presence to be among the indicators of healthy soils. Specific recommendations were generated from these surveys, including the need for research that would elucidate the relationship between crop nutrient content and soil health, and determine how organic farming systems can conserve SOM, enhance soil quality, protect soil from erosion, and sequester carbon to help mitigate global climate change. It is important to conduct the research across a diverse range of environmental conditions to extend the inference domain of the management tactics being investigated.
Organic amendments, crop rotations, and cover crops are used to improve soil health in organic farming systems, but the intensive tillage relied on for weed control in typical organic farming systems can compromise these gains. Conservation-tillage, organic farming systems are being explored, but the coordination of this research across a range of climates and farming systems is a key need to improve the environmental and economic sustainability of organic farming.
While comparisons of food quality between organic and conventional farming systems have been emphasized in response to consumer perceptions, specific factors contributing to enhancement of food quality within organic systems have not been examined thoroughly. In particular, little is known concerning the fundamental impact of soil biodiversity and biological activity on nutritional quality attributes and the health-promoting phytochemicals of food crops. This information is needed to determine the causal linkages between alternative organic farming practices and food quality.
Previous and ongoing research has quantified, or is quantifying, the impact of organic farming on soil health. These independent research efforts have made important contributions to our understanding of some of the linkages between soil health and organic farming at the local level. However, a more coordinated approach is needed to enhance our understanding of the relationship between soil health and specific practices used by organic farmers. This effort should include agricultural scientists with organic farming research experience representing an array of agricultural disciplines and agroecoregions. This project is designed to foster collaboration among organic farming scientists already involved, or planning to become involved, in research quantifying the soil health impacts of different organic farming practices. The project also will provide a mechanism for the engagement of food scientists in new organic farming research projects exploring relationships between food quality, soil health, and organic farming methods. In addition, the project will stimulate networking between new and experienced scientists engaged in understanding how organic farming practices affect soil health, and food quality.
This information will have broad and immediate application on most, if not all, organic farms since a common goal is to enhance the quality of the soil that is managed and the food that is produced. Project outcomes will address several SAAESD Priority Areas, including developing AN AGRICULTURAL SYSTEM THAT IS HIGHLY COMPETITIVE IN THE GLOBAL ECONOMY (Goal 1, A. Integrated and sustainable agricultural production systems), contributing to A HEALTHY AND WELL NOURISHED POPULATION (Goal 3, A. Nutritional quality of plant and animal food products), and creating GREATER HARMONY BETWEEN AGRICULTURE AND THE ENVIRONMENT (Goal 3, A. Air, soil, and water resources conservation and enhancement; B. Natural resource and ecosystem management; D. Environmentally benign agricultural operations; E. Nutrient management in agricultural systems; and F. Integrated pest management systems, including biologically-based tactics).
Related, Current, and Previous Work:
Organic farms are integrated, complex, and biologically diverse systems, capable of internal self-regulation of ecosystem processes, including optimized nutrient and carbon (C) cycling to maximize productivity and minimize environmental contamination (Stinner and Blair 1990; Reganold et al., 2001). Core elements of organic management are: 1) organic sources of nutrient inputs; 2) rotations of soil-building with soil-depleting crops, including cover crops, 3) non-chemical methods of weed control, and 4) reliance on biological control of arthropod pests and diseases (Lampkin, 1998). Nutrient Management
Soil fertility in organic systems is controlled through organic amendments, primarily composted animal manure, and forage/legume- based crop rotations (Gaskell et al., 2000). Nitrogen fertility is maintained through synchronization across space and time of N mineralization from soil organic N pools and plant uptake of inorganic N. Management of SOM to enhance soil quality and supply nutrients is a key determinant of successful organic farming. This involves balancing two ecological processes: mineralization of C and N in SOM for short-term crop uptake, and sequestering C and N in SOM pools for long-term maintenance of soil quality, including structure and fertility. The latter has important implications for regional and global C and N budgets, including water quality (Drinkwater et al., 1998; Kramer et al., 2006) and C storage in soils (Lal and Bruce, 1999; Robertson et al., 2000; Johnson et al., 2007). Organic management, especially longer crop rotations that include forage legumes and green manures, has been shown to improve soil physical properties (Reganold et al., 1993; Lal et al., 1994; Gerhart, 1997), decrease erosion (Lockeretz et al., 1978; Gantzer et al., 1991), reduce N leaching potential (Poudel et al., 2002; Kramer et al., 2006), improve SOM (Lockeretz et al., 1981; Reganold et al., 1993; Clark et al., 1998; Drinkwater et al., 1998; Liebig and Doran, 1999; Pulleman et al., 2000; Pimentel et al., 2005; Marriot and Wander, 2006a) and facilitate the production of competitive crop yields (Delate and Cambardella, 2004; Drinkwater et al., 1998; Teasdale et al., 2007). Cover crops
Cover crops planted in conventional farming systems have the potential to reduce herbicide reliance and minimize tillage while improving soil fertility (Decker et al., 1994), reducing soil erosion (Langdale et al., 1991), sequestering soil C (Sainju et al., 2002), increasing soil water infiltration and storage (Munawar et al., 1990) and suppressing weeds (Teasdale and Daughtry, 1993). Conventional no-till farmers use cover crops to suppress weeds, conserve moisture, and build soil tilth (Heer et al., 2006; Carerra et. al., 2004; McGuire, 2003; Pester, 1998). Cereal rye (Secale cereale L.) and hairy vetch (Vicia villosa Roth) are commonly used because of their winter hardiness and high biomass production (Hoffman et al., 1993; Wilkins and Belinder, 1996). Additionally, cereal rye is selected because of its residue persistence and flexible establishment date (Ruffo and Bollero, 2003), and hairy vetch for its capacity to fix large amounts of N (Abdul-Baki et al., 1997).
In an organic farming system, cover crops offer the greatest potential for weed management and enhancement of soil quality (Clark et al., 1998; Snapp et al., 2005). Effective cover crops for organic systems have included combinations of barley, rye, wheat, hairy vetch, and crimson clover, due to their quick establishment, competitiveness and ease of mechanical termination (Creamer et al., 1996; Nelson et al., 1991). Termination of cover crops through the use of traditional organic methods, such as plowing, disking or cultivating, constitutes a major challenge for organic growers as they attempt to build soil nutrient reserves with cover crops while using tillage to control weeds (Teasdale, 2007). Numerous tillage operations, implemented for weed control and seedbed preparation, stimulate SOM decomposition and can deplete biologically-active SOM pools that are critical for fertility in these systems. However, recent research comparing nine years of no-till management with organic management (including cover crops) suggests that organic farming systems can provide greater long-term soil improvement than conventional no-till systems, despite the use of tillage in organic systems (Teasdale et al., 2007).
In the absence of herbicides, cereal rye cover crops typically are terminated with tillage or with mowing when no-till is desired. In conservation tillage systems, mowing has several drawbacks including the risk of regrowth, accelerated residue decomposition, and patchy distribution of the surface residue (Wilkins and Bellinder, 1996; Creamer and Dabney, 2002). Uniformity of coverage of surface soil from cover crop residue is critical for optimizing weed suppression (Teasdale and Mohler, 2000). A roller/crimper represents a viable alternative to mowing and tillage. With this implement, the residue is deposited uniformly on the soil surface. In contrast to mowing, the resulting layer of rye residue persists for a longer period enhancing weed suppression, moisture retention, and soil conservation (Creamer and Dabney, 2002; Morse, 2001). However, recent research identified challenges that must be overcome before roller/crimper technology is adopted by many organic farmers. For example, cover crops are not killed consistently by rolling and crimping, unless the field operation is delayed until the cover crops reach a fairly advanced growth stage (Mirsky et al., 2009; Mischler et al., 2009).
Tillage
The benefits of conservation tillage (e.g. no-till) on surface soil physical and chemical (Elliott et al., 1987; Ismail et al., 1994; Karlen et al., 1994; Uri, 2000; Green et al., 2005), along with biological (Frey et al., 1999; Kennedy and Schillinger, 2006) properties have been well documented in the literature with the majority of studies from conventional systems using herbicide inputs prohibited in organic production. While no-till promotes C storage in the surface soil, incorporation of crop residues using conventional tillage (e.g., moldboard plow) can increase soil organic C content at or near the bottom of the plow layer (Angers et al., 1997). Tillage appears to affect the depth distribution more than net accumulation of soil organic C. Since the behavior of many indicators of soil quality (e.g., total N, particulate organic matter, microbial biomass) are biochemically and structurally linked to soil organic C, it is likely that tillage impacts on these soil properties will be similar to those observed for soil organic C.
Organic Farming and Soil Health
Organic practices have been reported to increase biologically available forms of SOM (Wander et al., 1994; Marriot and Wander, 2006b; Flie²bach et al., 2007), and increase the activities of beneficial soil microbes (Elmholt, 1996; Gunpala and Scow, 1998; Flie²bach and Mäder, 2000) and invertebrates (Werner and Dindal, 1990; Neher, 1999; Hansen et al., 2001). Organic systems have been shown to have more microbial biomass C, greater microbial community diversity, and higher microbial activity than conventional systems for a variety of grain, vegetable, and fruit production systems (Schjönning et al., 2002; Mäder et al., 2002; Diepeningen et al., 2006; Melaro et al., 2006; Monokrousos et al., 2006; Tu et al., 2006; Widmer et al., 2006; Esperschütz et al., 2007). The more highly diverse microbial communities have been shown to transform C from organic residues (Flie²bach and Mäder, 2000) into biomass at a lower energy cost (Fliebach et al., 2000), thus resulting in higher microbial biomass within the organic systems.
Organic Farming and Food Quality
Organic management strategies can improve soil physical, chemical, and biological properties and processes (Liu et al., 2007; Mäder et al., 2002; van Diepeningen et al., 2006). It seems reasonable to assume that the quality of the soil used to produce food crops influences the quality of that food. If so, then research indicating that organic farming practices can enhance soil quality (e.g., Liebig and Doran, 1999) suggests that food quality can also be improved following adoption of organic systems. Comparisons of nutritional quality between organic food and its conventional counterpart have produced inconclusive findings due to the lack of well-designed studies and the intrinsic complexity of farming systems (Bourn and Prescott, 2002; Magkos et al., 2003). Nevertheless, recently emerging evidence suggests that organically grown fruit and vegetables might contain higher levels of health-promoting phytochemicals (Mitchell et al., 2007; Olsson et al., 2006; Sousa et al., 2008). The essential role of phytochemicals in disease prevention has been well identified (Meskin et al., 2004). Phenolics are generally recognized as the largest group of phytochemicals with potent antioxidant activity (Manach et al., 2004). It has been estimated that organic vegetables may contain 10-50% higher defense-related secondary metabolites than conventionally grown vegetables (Brandt and Mølgaard, 2001).
Within the framework of organics, there are many production conditions, and they can significantly vary both within and among farms and regions. Without an understanding of the key components of organic farming systems with regard to their effect on food quality, it is impossible to make recommendations of the organic farming practices enhance phytochemical contents. The importance of determining if organic farming can improve food quality over conventional production is perhaps less critical than the need to identify specific contributing factors that can be manipulated in production systems to enhance food quality (Zhao et.al. 2006). Interdisciplinary efforts should take place to elucidate the interconnectedness of crop and soil health, along with food quality. It will be crucial to establish the best management practices for organic farming systems towards an integrated goal of improving crop productivity, food quality, profitability, and environmental stewardship.
Database Search
Several research projects exist in the CRIS database related to this project. A CRIS search in March 2009 using the keyword phrases organic farming and soil quality identified 93 different projects. Twenty of these projects still were active and considered relevant to this project. Karlen et al. (Project no. 3625-12000-012-00D) are using the soil management assessment framework (Andrews et al., 2004) to identify how different practices affect soil quality in organic and conventional systems. Ankumah et al. (Project no. ALX-SWQ) are quantifying the impact of organic and conventional systems on soil quality, with the focus on microbial community dynamics. Erich (Project no. ME08814-08H), Cogger et al. (Project no. WNPO7725), and several others (Gliessman et al. [Project no. CALW-2004-05136]; Harrington [Project no. MICL03480]; Hue and Valenzuela [Project no. HAW00875-06G]; Reeve [Project no. UTA00308]; and Zibilske et al. [Project no. 6204-12660-001-00D]) are quantifying the impact that soil amendments and nutrient levels have on soil quality in organic systems. Cogger et al. also are evaluating the impact that cover crops and reduced tillage have on soil quality, as well as incorporating pasture into a rotation with vegetable crops. Similarly, Hooks and Brust (Project no. MD-ENTO-0814) are determining the effect that cover crops have on soil quality in organic vegetable production, as is Mankolo et al. (Project no. ALAX-011-107), while Snapp et al. (Project no. MICL02132) are focusing on field crops. Mohler et al. (Project no. NYC-125569) are quantifying the interaction between soil quality and pests. Others investigating soil quality and pest management include Cardina et al. (Project no. OHO00991-SS), Kotcon et al. (Project no. WVA00447), and Williams (Project no. KY011031). Drinkwater and Wolf (Project no. NYC-145305) are quantifying the impact that cropping system selection has on soil quality, with particular focus on C sequestration. Green (Project no. ARK02156) is comparing the impact of organic and conventional farming systems, including some dedicated to biofuel production, on soil quality, while Delate et al. (Project no. IOW03801) and Posner (Project no. WIS04378) are comparing conventional and organic farming systems for their impact on soil quality in two long-term studies.
Considerable resources have and continue to be directed at quantifying the relationship between organic farming and soil health. This work is necessary and should be encouraged. Still, there is a growing need to coordinate these activities across multiple environments and regions so that information exchange is improved, resource allocation is optimized, and fundamental principles relating to soil health enhancement in organic systems can be identified and demonstrated, along with an understanding of how environment affects the mechanisms involved. This project is a response to that need and provides the vehicle for multi-disciplinary teams of scientists from north central, northeastern, southern, and western U.S. regions to work jointly on defining the linkages between production practice, soil and food quality, and environment when farming organically.
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