SPATIAL VARIATION OF CORN RESPONSE TO IRRIGATION

Abstract

Lack of basic knowledge about spatially varying crop response to irrigation hinders optimal irrigation management and economic analysis for site–specific agriculture. The objectives of this research were to measure the mean response of corn to irrigation amounts on 12 soil map units and compare variation in the response within and among soil map units. This experiment was implemented from 1999 through 2001 with a center–pivot irrigation machine that had been modified to enable site–specific irrigation on small plots within a representative, highly variable Coastal Plain field. Four irrigation treatments (0%, 50%, 100%, and 150% of a base rate designed to hold soil water constant) and two N treatments (135 and 225 kg/ha, the recommended rainfed and irrigated rates) were imposed in 2 U 4 factorial randomized complete blocks on eight soil map units, plus randomized incomplete blocks on four additional map units. The water treatments had consistently significant main effects in the analysis of variance (ANOVA) in both linear and quadratic forms at the 1% level, and the variation within soil map units was significant at the 5% level in the latter two years and at the 1% level in 1999. Variation in yield among soil map units at any point on the response curves approximated 25% of the maximum yield in all three years. Variation in mean irrigation amounts to produce maximum yield in the eight most common map units was 61%, 61%, and 120% of the base rate amount in the three years. These data, the first such known for any soil, crop, or location in the world, have significant implications for the design, management, and economic profitability of irrigation on spatially varying soils. While the obvious design and management change would be from whole–field to site–specific approaches, even under whole–field situations, designers should consider more strongly the management zone size, range of application rates, and need for documentation.