MODIFIED CENTER PIVOT SYSTEM FOR PRECISION MANAGEMENT OF WATER AND NUTRIENTS

Spatial yields since 1985 in a corn-wheat-soybean rotation at Florence, S.C., show little correlation of yield data with expected yields for soil map units. Research suggests that spatial yield variability for the southeastern Coastal Plain may be caused primarily by water relations. This causes difficulties in scheduling irrigation for conventional center pivot irrigation systems, which are not capable of applying variable depths of water to small areas of variation within the total system. Thus, the objectives of this work were to design and construct a site-specific center pivot irrigation system that could independently apply variable rates of water and chemicals to 100-m2 areas within the irrigation system. A commercial center pivot system was modified by adding three 9.1-m manifolds in each of 13 segments along the truss. Nozzles were spaced 1.5 m apart along each manifold, and both manifolds and nozzles were sized to provide 1x, 2x, and 4x nominal application rate at a given tower speed. All combinations of the three manifolds provided up to 7x nominal depth, which was 12.7 mm, in 1.8-mm increments when the outer tower traveled at 50% of full speed. A programmable, computer-controlled management system was installed near the pivot on the moving portion of the center pivot system. This controller obtained the position from the center pivot controller via a radio frequency modem and switched on the appropriate valves to obtain the application rate for a specific area. During 1995 and 1996, the system applied water and N fertilizer in a fixed-boundary field experiment. Measurements and observations of water and N application uniformities were acceptable; however, more extensive evaluation will be required before definitive conclusions can be reached regarding N application. Surface temperatures measured with an integral infrared thermometer system produced encouraging results that may be useful in management of water and nutrients. Using experience gained with this system, a second commercial center pivot system is being modified for site-specific water, nutrient, and pesticide management on a field with soil variation (irregular boundaries) typical of the Coastal Plain.