Animal manure can be an important resource providing soil available N for plant needs, but determining the nutrient availability resulting from such amendments is difficult. A study was conducted to examine changes in electromagnetic induction (EMI) soil conductivity and available N levels during four growing seasons in relation to manure or compost application and use of a green winter cover crop. With simultaneous soil samples, a series of soil conductivity maps of a research cornfield were generated using a global positioning system (GPS) and EMI methods. The Clay Center, NE, site was treated during a 10-yr period with a winter wheat (Secale cereale L.) winter cover crop (1CC) and no-cover crop (2CC). The site was split for subtreatments of manure and compost at rates matching either the P or the N requirements of silage corn (Zea mays L.). Differences between the 1CC and 2CC treatments for values of NO 3 -N and water-filled pore space (WFPS), as estimated by apparent electrical conductivity (EC a ), were compared for each year. Differences in profile weighted soil conductivity explained 79.5, 98.0, 93.4, and 98.4% of the variability due to NO 3 -N differences, and only 20.5, 2.0, 6.6, and 1.6% of the variability due to WFPS differences for years 2000, 2001, 2002, and 2003, respectively. Sequential measurement of profile-weighted soil electrical conductivity (EC a ) was effective in identifying the dynamic changes in plant-available soil N, as affected by animal manure and anhydrous ammonia fertilizer treatments during four corn growing seasons.S USTAINABLE AGRICULTURE requires innovative and practical tools to optimize farm economics, conserve soil organic matter, and minimize negative environmental impacts (Johnson et al., 2003). Electromagnetic induction soil conductivity sensors may provide one such tool. Profile weighted EC a can provide an indirect measure of important soil properties (Davis et al., 1997). The EMI instrument is sensitive to factors that influence soil conductivity, including (i) soil moisture content, (ii) amount and type of salts in solution, and (iii) the amount and type of clays present (Brune and Doolittle, 1990). Electromagnetic techniques are well suited for mapping soil conductivity to depths useful for agriculturalists (McNeill, 1990). Electromagnetic induction soil conductivity has been shown to be a very useful tool in locating seepage from animal waste lagoons (Ranjan et al., 1995); Eigenberg and Nienaber (2003) found values to correlate well with salt levels at an abandoned composting site. Sudduth and Kitchen (1993) used EMI methods to estimate clay pan depth in soil. Electromagnetic methods have been used to map soil salinity hazards (Williams and Baker, 1982;Corwin and Rhoades, 1982). Electrical conductivity methods have been shown to be sensitive to high nutrient levels (Eigenberg et al., , 2000 and have been used to detect ionic concentrations on or near the soil surface resulting from field application of cattle feedlot manure. Electrical conductivity has gene...