Evaluating the UV-Absorbance of NaHCO3 Extraction for Predicting Soil Nitrogen Supply to Corn Crop in Ontario, Canada.

The UV absorbance of 0.01 M NaHOC₃ (UV-NaHCO₃) have been successfully used as an indicator of plant available nitrogen (N) for corn crop. However, there have not been any studies to assess the options for improving the methodology.  We assessed the NaHCO₃ method as an indicator of potentially available N to corn crops using varying concentrations, shaking times and wave lengths on soils collected from corn fields representing a variety of soil characteristics and N availability across Southern Ontario. Three concentrations of NaHCO₃ solution (0.01, 0.05, and 0.1 nm), three shaking time (15, 30, and 60 min) and four wavelengths (200, 205, 210, and 260 nm) were evaluated. The results of the NaHCO₃ experiment were compared with water and KCl extractable N, and the relationship between relative yield and total N uptake were determined. Plant N uptake ranged from 85 to 275 kg N ha^-1 and had the greatest regression coefficients with the 0.05M NaHCO₃ absorbance at 200, 205, and 210 nm wavelengths, and 30 min shaking time (R² = 0.58, 0.64, and 0.68, respectively). Relative yield had slightly lower R² values, but still showed significant relationships with 0.01M NaHCO₃ absorbance at 205 nm, and 15 min shaking time (R² = 0.57) and 0.01M NaHCO₃ absorbance at 210 nm, and 30 min shaking time (R² = 0.57). ANOVA results showed significant interactions between concentration and shaking time at all the wavelength except 200 nm. There was an overall pattern in all three wavelengths, as concentration and shaking time increased the absorbance also increased with the highest readings at 0.1M NaHCO₃ with 60 min shaking time combination. For the 200 nm readings it was found that there was no significant difference between 0.01M and 0.05M NaHCO₃ but both were significantly different from 0.1M NaHCO₃. Similarly, 15 min shaking time was significantly different from 60 min shaking time, but 30 min shaking time was not significantly different from either 15 or 60 min. For each variation of the NaHCO₃ method that showed a significant relationship with a field-based index, critical value was estimated.  The 0.05M NaHCO₃ at 30 minute shaking time had critical values of 0.17, 0.57, 0.49, and 0.2 absorbance (abs) when read at 200nm, 205nm, 210nm, 260nm, and  respectively.  Extractions at 0.01M NaHCO₃ with a 15 minute shaking time had a critical value of 2.23 abs at 205nm and 0.01M NaHCO₃ with a 30 minute extraction time had a critical value of 0.55 abs at 210nm.