Crop Rotation Effects on Soil Quality at Three Northern Corn/Soybean Belt Locations

Do extended crop rotations that include forages improve soil quality and are they profitable? Our objectives were to determine (i) how crop rotation affected soil quality indicators, (ii) if those indicator changes were reflected in soil quality index (SQI) ratings when scored and combined using the Soil Management Assessment Framework, and (iii) how SQI values compared with profitability. Soil samples were collected from three long-term studies in Iowa and one in Wisconsin. Bulk density (BD), soil pH, water-stable macroaggregation, total organic C, total N, microbial biomass C, extractable Pand K, and penetration resistance were measured. The indicator data were scored using nonlinear curves reflecting performance of critical soil functions (e.g., nutrient cycling, water partitioning and storage, and plant root growth). Profit was calculated by subtracting costs of production from potential income based on actual crop yields and the 20-yr average nongovernment-supported commodity prices. Extended rotations had a positive effect on soil quality indicators. Total organic C was the most sensitive indicator, showing significant measured and scored differences at all locations, while BD showed significant differences at only one location (Kanawha). The lowest SQI values and 20-yr average profit were associated with continuous corn, while extended rotations that included at least 3 yr of forage crops had the highest SQI values. We suggest that future conservation policies and programs rewardmore diverse and extended crop rotations, as is being done through the Conservation Security Program. THE DOMINANT agricultural land use throughout the northern Corn/Soybean Belt became a 2-yr corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] rotation during the last half of the 20th century. This occurred for several reasons including simplicity and similar equipment requirements as farm size increased, commodity programs that emphasize short-term profit, public and private research and development efforts devoted to genetic improvement of corn and soybean, and increased food and industrial uses for both corn and soybean oils and various by-products (Karlen, 2004). It also coincided with major changes in the livestock industry that decreased demand for oat (Avena sativa L.) and alfalfa (Medicago sativa L.). The merits of extended crop rotations that include forage or pasture crops have been debated for centuries (Karlen et al., 1994). Key benefits include increased C retention in the surface horizon and a more even distribution of labor needs and risk due to climate or market conditions than those involving only grain or fiber crops (Magdoff and van Es, 2000). Despite those benefits, the infrastructure developed and devoted to corn and soybean has resulted in a 500% increase in harvested area and 800% increase in soybean production between 1950 and 2003 (USDA-NASS, 2004). During that same period, oat production declined by 90%, and although hay production increased because of better yields, the land area devoted to it decreased more than 15%.Expansion of the simplified corn–soybean system has tremendous economic and world trade benefits because of the many products and materials developed from those crops, but what impact has it had on soil resources, water quality, biodiversity, wildlife corridors, and rural communities? Some would argue that externalities of agricultural development are impossible to quantify and that the loss in crop diversity is no different now than during the 1950s when an average of 10Wisconsin dairy farms went out of business each day for an entire decade (Apps, 1998). Others emphasize that to help shape a sustainable future, it is important to know where we are and how we got here (Flora, 2000; Randall, 2003). Effects of extended crop rotations on soil quality have been discussed in general terms (Karlen et al., 1994) and specifically with regard to aggregate size distribution and stability (Kay, 1990; Robinson et al., 1994) and other indicators (e.g., yield, profit, N balance, etc.), but not with regard to soil quality per se. We acknowledge that soil quality assessment is in its infancy and that several valid concerns regarding its potential use and deficiencies have been raised (Letey et al., 2003; Sojka et al., 2003; Sojka and Upchurch, 1999). However, because shortsighted or improper soil and crop management decisions can lead to resource degradation and deleterious changes in soil function, tools and methods to assess and monitor soil quality are needed. Soil quality effects of alternative vegetable production systems in northern California were quantified using the Soil Management Assessment Framework (SMAF) to combine various biological, chemical, and physical indicator measurements for an overall assessment (Andrews et al., 2002a, 2004). It was also used for an on-farm assessment of supplemental C management practices in California’s Central Valley where it scored an established organic farming system higher than an adjacent conventional system (Andrews et al., 2002b). Recently, D.L. Karlen, C.A. Cambardella, and D. Meek, USDA-ARS, Natl. Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011-4420; E. Hurley, North Central Tech. College, 1000 W. Campus Dr., Wausau, WI 54401; S.S. Andrews, USDA-NRCS, 200 E. Northwood St. Suite 410, Greensboro, NC 27401; and M.D. Duffy, Dep. of Agric. Econ., and A.P. Mallarino, Dep. of Agron., Iowa State Univ., Ames, IA 500111070.Received4Apr. 2005. *Corresponding author ([email protected]). Published in Agron. J. 98:484–495 (2006). Soil Quality and Fertility doi:10.2134/agronj2005.0098 a American Society of Agronomy 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: BD, bulk density; C, corn; CSP, Conservation Security Program; ISU, Iowa State University; Om, oat/meadow; M, meadow; MB-C, microbial biomass carbon; Sb, soybean; SMAF, Soil Management Assessment Framework; SQI, soil quality index; TN, total nitrogen; TOC, total organic carbon; WSA, water-stable macroaggregation. R e p ro d u c e d fr o m A g ro n o m y J o u rn a l. P u b lis h e d b y A m e ri c a n S o c ie ty o f A g ro n o m y . A ll c o p y ri g h ts re s e rv e d . 484 Published online April 11, 2006