Effects of long-term water management in a humid region on spatial variability of soil chemical status

Water-management practices for crop growth require knowledge of the needs of the crop, optimum timing of irrigation applications, and the quality of the irrigation water. To maintain productive soils, the effects of irrigation with poor-quality water on soil properties should be considered. Many studies have recognized the impact of poor-quality water on soil characteristics, yet few have reported on the spatial variability of these soil properties as a response to irrigation in a humid region. Three fields that had been cropped in rice and soybeans in 3-yr rotations for over 30 yr were sampled for soil pH on a 61.5-m rectangular grid. Spatial variability of pH was quantitatively shown by the construction of linear semivariograms. The pH values were estimated on a 20-m grid using universal kriging of orders 1 and 2, as well as punctual kriging. The resulting kriged values were then plotted as contours of constant pH and compared. The mappings were all similar when compared within a single field, leading to the conclusion that for these data, elimination of drift was not necessary. All observed and mapped values were considerably greater than the pH of uncultivated soil of the region, thus demonstrating a significant accumulation of calcium carbonate in the soil surface due to precipitation from the irrigation water. Higher pH values were observed close to the water inlet, and progressively smaller pH values were seen with increasing distance from the inlet. This spatial distribution clearly followed water-flow patterns and suggested that year-to-year water-management alternatives to reduce localization of calcium carbonate precipitate are desirable within operational limits.