Spatial Variability of Soil Chemical Properties in Grazed Pastures

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Abstract
Excretal deposition concentrates nutrients in animal watering and resting areas of pastures, and including these areas in soil sampling may bias fertilizer recommendations. Our objective was to determine the magnitude and extent of nutrient enhancement near a water source in each of two experimental tall fescue (Festuca arundinacea Schreb.) pastures. The rectangular 0.44‐ha pastures were in southcentral Iowa and included three soils: a fine, montmorillonitic, mesic Typic Argiaquoll, a fine‐loamy, mixed, mesic Typic Argiudoll, and a fine, montmorillonitic, mesic Aquic Argiudoll; pH 6.8 to 7.0. Before grazing began in 1976, extractable P (Bray and Kurtz P‐1) and exchangeable K ranged from 3 to 9 and 87 to 156 mg kg−1, respectively, at 0 to 75 mm. After 5 yr of grazing by cattle, soil was sampled at 0 to 75 and 75 to 150 mm on a grid pattern starting at the water source and analyzed for extractable P and exchangeable K, total N, and organic C. Chemical values between the grid points were interpolated by kriging, and contour plots were constructed for each variable. Extractable P and exchangeable K exhibited large accumulations near the water source, with many P and K measurements exceeding 100 and 1000 mg kg−1, respectively. The zone of enhanced P and K levels extended to approximately 10 to 20 m from the water source. Total N declined from 4.2 to 2.1 g kg−1 with progressive distances from the water; however, the range of values was proportionately less than for P and K. Organic C concentrations showed less distinct trends in relation to distance from the water source. The results showed a distinct zone of nutrient enhancement within 20 m of the water source that should be either avoided or sampled separately when sampling pastures for fertilizer recommendations.
Funding Information
  • Iowa State Univ. Graduate Student Senate

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