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Soil Fertility & Plant Nutrition

Post-Harvest Soil Nitrate in Irrigated Corn

Variability Among Eight Field Sites and Multiple Nitrogen Rates

^a Dep. of Crop and Soil Sciences, Michigan State Univ., Plant and Soil Sciences Bldg., East Lansing, MI 48824
^b USDA-ARS, Building 3702, Curtin Rd., University Park, PA 16802
^c Dep. of Plant and Soil Sciences, Oklahoma State Univ., 368 Agricultural Hall, Stillwater, OK 74078
^d Dep. of Agronomy, Kansas State Univ., 2004 Throckmorton Plant Sciences Center, Manhattan, KS 66506

^* Corresponding author (gehlr{at}msu.edu)

Elevated post-harvest soil NO₃ is an indicator that N fertilizer was applied in excess of the amount required to obtain maximum corn (Zea mays L.) yield, and represents a quantifiable environmental risk if water percolates through the soil profile during the fallow season. The reliability of using post-harvest soil NO₃ as an indicator of NO₃ leaching potential was considered for various field sites with similar soil characteristics and slightly variable rainfall conditions. Six N treatments (surface broadcast) included: (i) 300 and (ii) 250 kg N ha^–1 applied at planting; (iii) 250 kg N ha^–1 split-applied at planting (1/2) and sidedress (1/2); (iv) 185 kg N ha^–1 split-applied at planting (1/3) and sidedress (2/3); (v) 125 kg N ha^–1 split-applied at planting (1/5) and sidedress (2/5, 2/5); and (vi) 0 kg N ha^–1. At one site, N treatments were represented in each of two irrigation treatments: 1.0x (optimal) and 1.25x (125% optimal). Soil samples were collected in 30-cm increments at preplant and post-harvest to a 240-cm depth. Sand content exceeded 0.8 g g^–1 within the 240-cm soil profile at every site except one; and distinct textural transitions were present within the soil profile at four sites. Maximum grain yield was obtained with <185 kg N ha^–1 at every site in both years. When less than average water was received at those sites with distinct textural transition (silt and clay to sand) in the upper soil profile, post-harvest soil NO₃ for N rates > 180 kg N ha^–1 often exceeded 60 kg N ha^–1 within a 30-cm sampling depth. When these same sites received additional rainfall, post-harvest results indicated that NO₃ had moved down the soil profile, past the textural transition, and perhaps beyond the 240-cm depth. For those sites with uniformly high sand content (0–240 cm), few differences in post-harvest NO₃ could be attributed to the N treatments exceeding 185 kg N ha^–1. Nitrate had probably moved beyond 240 cm by the end of the growing season. Slight differences in site characteristics (e.g., textural boundaries) can greatly influence conclusions derived from post-harvest soil sampling regarding the risk of NO₃ leaching.

Abbreviations: EONR, economic optimum nitrogen rate • IS, irrigation schedule or irrigation treatment • RCBD, randomized complete block design • SOM, soil organic matter