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ARS Home » Northeast Area » University Park, Pennsylvania » Pasture Systems & Watershed Management Research » Research » Publications at this Location » Publication #186365
Title: POST-HARVEST SOIL NITRATE IN IRRIGATED CORN: VARIABILITY AMONG EIGHT FIELD SITES AND MULTIPLE N RATES

Author
item GEHL, R - MICHIGAN STATE UNIV
item Schmidt, John
item GODSEY, C - KANSAS STATE UNIV
item MADDUX, L - KANSAS STATE UNIV
item GORDON, W - KANSAS STATE UNIV

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/15/2006
Publication Date: 9/20/2006
Citation: Gehl, R.J., Schmidt, J.P., Godsey, C.B., Maddux, L.D., Gordon, W.B. 2006. Post-harvest soil nitrate in irrigated corn: variability among eight field sites and multiple n rates. Soil Science Society of America Journal. 70(6):1922-1931.

Interpretive Summary: When N fertilizer is applied at rates greater than necessary to achieve optimum crop growth, this excess fertilizer can move through the soil as nitrate and become a significant problem in ground and surface waters. Measuring soil nitrate after harvest has been used to determine if too much fertilizer had been applied; however, a conclusion depends on the assumption that N applied as fertilizer did not move through the soil and below the sampling depth during the growing season. The objective of this study was to evaluate this assumption, does after harvest soil nitrate indicate whether too much N fertilizer had been applied to a corn crop? The same N fertilizer treatments were applied during two growing seasons to eight different fields situated on sandy soils along Kansas’ main rivers. Soil nitrate was measured to an 8-ft depth prior to and after each growing season. In one year, after harvest soil nitrate ranged between 95 and 350 lb N per acre among these eight fields, despite applying a similar amount of N fertilizer and removing a similar amount in the harvested grain. Differences in soil nitrate levels prior to the growing season could account for 126 lb N per acre of this 255 lb N per acre discrepancy; however, almost 130 lb N per acre was unaccounted for at the site that had only 95 lb N per acre leftover after harvest. Because these are sandy soils, the most likely explanation is that nitrate moved below the 8-ft depth during the growing season. Considerable N losses during the growing season are a serious problem on these sandy soils when too much water and too much N have been applied. After season soil samples may not be a good indicator of whether too much N had been applied, especially if excess water (as rainfall and irrigation) during the growing season has already moved nitrate to below sampling depths.

Technical Abstract: Despite considerable evidence in the scientific literature demonstrating that soil nitrate accumulates under corn (Zea mays L.) when N is applied in excess of crop requirements, there is sparse evidence that contrasting results can be obtained for post-harvest soil nitrate, even under similar N fertilizer rates but slightly variable soil characteristics and growing season precipitation. The objective of this study was to evaluate the effects of N rate and timing on post-harvest soil nitrate content and distribution for sandy soils under irrigated corn production at multiple field sites in Kansas. Nitrogen treatments included 300 and 250 kg N per ha applied at planting; 250 kg N per ha applied at planting (1/2) and sidedress (1/2); 185 kg N per ha applied at planting (1/3) and sidedress (2/3); 125 kg N per ha applied at planting (1/5) and sidedress (2/5, 2/5); and 0 kg N per ha. Nitrogen treatments were duplicated at one site for each of two irrigation treatments: 1.0X (optimal) and 1.25X (25% > optimal). Grain yield was determined for all plots, and soil samples were collected in 30-cm increments at preplant and post-harvest to 240 cm and to 60 cm at the V6 growth stage. Mean post-harvest soil nitrate (0 to 240 cm) in 2001 ranged between 85 and 313 kg N per ha for the Rossville and St. John sites, respectively, despite receiving similar N fertilizer treatments and having similar mean grain yield. Although preseason soil nitrate was 113 kg N per ha greater at St. John, slightly greater clay content between 90 and 210 cm (.08 vs. 0.22 g per g) at St John probably prevented nitrate movement. Post-harvest soil nitrate-N concentration in the 30- to 60-cm depth at the Pretty Prairie East site (2001) was > 100 mg per kg for the three greatest N treatments. In 2002, this site received 14 cm additional rainfall, and post-harvest soil nitrate-N (0- to 60-cm depth) was less than 50 mg per kg for all N treatments and exceeded 50 mg per kg at the 210- to 240-cm depth and greatest N treatment. Slight variations in soil texture within the soil profile or in precipitation seemed to have a dramatic impact on post-harvest soil nitrate concentrations among these field sites.