Post-Harvest Soil Nitrate in Irrigated Corn: Variability Among Eight Field Sites and Multiple Nitrogen Rates

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 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.