On-farm assessment of organic matter and tillage management on vegetable yield, soil, weeds, pests, and economics in California
Introduction
Participatory research with farmers on commercial fields provides a unique opportunity to study the diverse impacts of management practices on yield, pests, environmental quality, and economics. Since research is performed in situ, findings are representative of typical ecological responses by the populations and communities of organisms within the actual agroecosystem, as compared to research station experiments that often cannot replicate the abiotic and biotic environment of real farms (Witcombe, 1999, Wander and Drinkwater, 2000). Also, the results of on-farm research can be valuable to farmers who are stakeholders in the design and management of experiments (Goma et al., 2001). This is especially true if treatments are conducted at the scale of operations used on the farms.
Ideally, farmer participatory research employs a systems approach that addresses and integrates several aspects of production, biology, economics, social and sustainability issues. In terms of ecology, this might involve investigation of the autecology of organisms, population and community dynamics, and/or biogeochemical flows, e.g., of nutrients and water, as well as the aspects of human ecology that affect decision-making and resource use. From an agronomic perspective, this emphasizes the evaluation of management practices to increase yield and economic gain, and with efficient use of resources.
Trade-offs exist between the benefits and drawbacks of management practices designed to increase soil quality, which concerns the effects of soil management on agricultural productivity, and on the characteristics of soils that contribute to environmental quality (Karlen et al., 1997, Liebig and Doran, 1999). For example, reduced tillage is known to increase soil organic matter (SOM) (Silgram and Shepherd, 1999), but can decrease the productivity of some crops (Carter, 1991, Sims et al., 1998), as well as increase the incidence of some diseases (Jackson et al., 2002). Cover crops decrease the leaching of nitrate (NO3−-N) below the root zone (McCracken et al., 1994) and can increase N availability and crop yield (Paustian et al., 1992), but a drawback of certain cover crops is as alternate hosts for diseases that can then infect the subsequent cash crop (Koike et al., 1996). Higher financial costs from deeper tillage can be compensated by higher yields and net returns (Popp et al., 2001, Wesley et al., 2001), although higher fuel use contributes to greenhouse gas production (Robertson et al., 2000). Systems research attempts to account for these varied outcomes.
Intensive production for crops such as lettuce, broccoli, and celery (Apium sp.) occurs in the Salinas Valley of coastal California, USA, which is a major supplier of these vegetables nationwide. The mild climate and the high inputs of irrigation and fertilizers allow the production of two or three crops per year. Large NO3−-N leaching and denitrification losses occur in these cropping systems (Jackson et al., 1994, Ryden and Lund, 1980), and NO3−-N exceeds the public health standard (10 mg N l−1) in nearly half of the wells in the upper aquifer. Very little OM is returned to the soil after vegetable harvest, but use of cover crops and compost has recently increased. Tillage occurs frequently, ranging from single passes with cultivators for weed control, to disking, subsoiling, and leveling a field between crops.
The impact of reduced tillage and increased OM inputs on vegetable production and soils was evaluated in a participatory on-farm experiment with a Salinas Valley grower, using management regimes that were viable possibilities for farmers, with the use of large plots to represent operations at the farm-scale. Conventional tillage (subsoiling, disking, and surface mulching) was compared with minimum tillage that disked the surface layer of semi-permanent beds, and shanked the furrows. Organic matter was added as both cover crops and compost, with the purpose of incorporating both readily labile and more resistant sources of C. Cover crops increase the active fraction of the SOM for a few weeks to months after incorporation (Crozier et al., 1998, Jackson, 2000, Schutter and Dick, 2002). Manure and compost may contribute more C to slow versus active pools of SOM, partly depending on compost maturity (Paustian et al., 1992, Drinkwater et al., 1998). The objectives of the 2-year experiment (four crop phases) were to compare the effects of alternative tillage and OM management by: (1) monitoring changes in crop yield and nutrient uptake, soil microbial biomass, and N availability; (2) documenting effects on weeds, pathogens, and insect pests; and (3) evaluating the total costs and net returns as a means of assessing the economic viability of adopting practices conducive to increasing soil quality.
Section snippets
Soils and management practices
The field trial was established in April 1998 on an 8.3 ha site in the Salinas Valley of California. The Salinas silt loam is a fine-loamy, mixed, thermic Pachic Haploxerolls (FAO Haplic Phaeozems) (Table 1). The coastal Mediterranean-type climate has mild, rainy winters, and foggy, cool, rain-free summers. Rainfall was 44.35 cm from 4 April 1998 through 31 March 1999 and 34.93 cm from 1 April 1999 through 26 April 2000. The field was in long-term use for irrigated cool-season vegetable (e.g.,
Soil organic matter and bulk density
In this silt loam soil, total C and N concentrations (g kg−1) in the surface 0–15 cm layer were higher after 2 years of addition of cover crops and compost, compared to non-amended soils (Table 3). Tillage treatment did not have a significant effect on either total C or N concentrations, nor were there significant tillage×OM interactions. The addition of organic amendments caused a decrease in bulk density in the surface (0–6 cm) layer, but not at the lower depth (47–53 cm) (Table 3). No effects
Discussion
Managing a cropping system to promote soil quality has implications for crop yield and economic returns. Addition of cover crops and compost increased soil MBC, and reduced the potential for NO3−-N leaching loss. These inputs were occasionally associated with lower weed density and corky root disease, which has previously been shown to be suppressed by a rye cover crop (vanBruggen et al., 1990). Although OM inputs often resulted in higher marketable lettuce yields, this did not necessarily
Conclusions
Addition of cover crops and compost, and a combination of minimum and conventional tillage methods appear to be the most attractive management option to farmers for coping with various production, economic, and soil quality tradeoffs. Although OM inputs increased some attributes of soil quality (higher MBC and MBN in the surface layer, lower bulk density in the surface layer, and less propensity for NO3−-N to leach below the rootzone), and resulted in some production benefits (reduction in
Acknowledgements
We thank Tanimura and Antle, Inc. for their donations of time, labor, and materials, and for their excellent cooperation and attention to experimental details. Many people helped with the project, especially Martin Burger, Amy Clymo, Paula Ellison, Hung Kieu, and Kerri Steenwerth. Funding came from the USDA Western Regional Sustainable Agriculture Research and Education Program (95-COOP-1-2414), the Kearney Foundation of Soil Science, and the California Integrated Waste Management Board.
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