Post‐Harvest Soil Nitrate in Irrigated Corn

Elevated post‐harvest soil NO 3 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 NO 3 as an indicator of NO 3 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: 1.0× (optimal) and 1.25× (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 NO 3 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 NO 3 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 NO 3 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 NO 3 leaching.