Wednesday, November 7, 2007 - 10:30 AM
318-5
Effectiveness of Illinois Soil Nitrogen Test (ISNT) in Corn after Sod.
Several states have investigated the performance of the Illinois soil nitrogen test (ISNT) as a tool for identifying corn fields that require additional N and/or predictor for optimum N rates and their studies have shown mixed results. Earlier work in New York based on 32 field trials conducted from 2002 to 2004 showed that soil samples taken to 20-cm depth and analyzed for both ISNT and loss-on-ignition organic matter (LOI-OM) could be used to predict the need for additional N for corn beyond starter fertilizer over a wide range of soil types. Between 2005 and 2007, 37 N rate trials were conducted to study the impact of grass and/or legume sod incorporation or prior year soybean on N needs and the ability of the ISNT x OM curve to predict if sidedress N was needed. No extra N (beyond a 34 kg N ha-1 starter) was needed for 1st year corn (16 trials). So far (2007 results excluded), with samples taken at corn sidedressing time, the ISNT x OM curve accurately predicted if additional N was needed for 11 of 12 of the 2nd year sites (5 responsive and 7 non-responsive sites) and two corn after soybean sites. For sites continued over 2 years, ISNT values in year 1 were closely related to those in year 2 (ISNTyear2 = -14.1 + 1.1 ISNTyear1, R2=0.97***). This is consistent with laboratory and field trials that showed ISNT trends over time mimicked ammonium concentrations with a peak 2-4 weeks prior to sidedressing. Nitrate levels peaked 4-8 weeks after manure application or sod turnover. These results could explain the lack of correspondence between ISNT and optimum N rates found in dairy states that commonly grow corn in various rotations with grasses and legumes and point towards the importance of standardization of sampling time.
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