Submitted to: International Conference on Precision Agriculture Abstracts & Proceedings
Publication Type: Proceedings
Publication Acceptance Date: February 1, 1997
Publication Date: N/A
Interpretive Summary: It is easy for someone in an automobile or, particularly, an airplane, to see variation in soils in the Southeastern Coastal Plain, especially during periods when the soil is clean after tillage. Carolina Bays, which are shallow, circular depressions of varying sizes and debated origin, dot fields throughout the region. Farmers who try to grow crops in such fields must decide what plant population, fertilizer amounts, and pesticide applications should be made, although these are not usually the same for each soil. The emerging science of precision agriculture allows site-specific adjustments to equipment, changing, for example, nitrogen rate to match what is needed for the particular spot rather than just applying a uniform amount to the whole field. Because water stress appears to be an important cause of yield variation, the USDA-ARS in Florence, SC, built a site-specific center pivot to adjust irrigation amounts for varying soils. Initial testing has been done on a conventional small-plot experiment, where different amounts of irrigation and fertilizer were applied to each plot. The first year of tests indicates that the machine works as planned, thus proving feasibility of site-specific irrigation on variable soils. Research with the new machine will determine what parameters must be adjusted to overcome soil variability.
Technical Abstract: Observations of spatial yields for a conventional corn-wheat-soybean rotation in a test field since 1985 suggest that crop water relations may cause much of the spatial variability in yield for the Southeastern Coastal Plain. Mechanistic modeling indicates that for normal weather years, the final yield is particularly sensitive to variations in soil water, caused by sandy surface soil and limited rooting volume. These findings, plus difficulties in scheduling irrigation under a center pivot on variable soils, prompted the design and building a site-specific center pivot capable of differentially irrigating 100 sq m areas. A 3-tower commercial center pivot was modified by adding 39 9.2-m manifolds in 13 sections, 3 to a section. Manifolds and nozzles were sized 1x, 2x, and 4x, so that octal combinations would provide up to 7x the minimum application depth for a given outer tower speed. At 50% speed, the depths are 0 to 12.5 mm in 1.8-mm steps. A programmable controller was attached near the pivot end of the boom, clear of the pivot control. Each manifold was controlled by the programmable controller, which obtained pivot position and other information via radio modem link with the pivot control panel. Water and nitrogen application has been accomplished using this system on a replicated field experiment. Experience gained during this phase will guide modification of a similar pivot for site-specific water, nutrient, and pesticide management on a typically variable Coastal Plain field.