Alkaline Hydrolyzable-Nitrogen Changes with Soil Depth: Implications for Fertilizer Correlation and Calibration.

Alkaline hydrolyzable-N quantified by the Illinois Soil Nitrogen Test (ISNT) or Direct Steam Distillation (DSD) may index potentially mineralizable-N in the soil. Limited success has been achieved utilizing the ISNT to predict corn (Zea mays L.) yield response when soils are sampled 15 or 30 cm deep and a strong correlation of ISNT to total N (TN) has raised concerns over the method’s sensitivity to potentially mineralizable-N. A study was implemented to determine the effects of site and soil depth on alkaline hydrolyzable-N. Soil samples were collected from 0-60 cm in 15 cm depth increments and analyzed for alkaline hydrolyzable-N (ISNT and DSD) and TN. Analysis of variance for alkaline hydrolyzable-N and TN each showed a significant site by soil depth interaction. Alkaline hydrolyzable-N ranged from 22 to 280 mg N kg soil^-1 and the highest values were not always in the top 15 cm of the soil. Total N ranged from 191 to 1542 mg N kg soil^-1 and the highest values were always in the top 15 cm. Soil TN  from the 15-60 cm depths was generally uniform among 15-cm increments within each soil, but alkaline hydrolyzable-N was not. Alkaline hydrolyzable-N accounted for 11-38% of soil TN and was variable across sites and was variable across sites and depths. Variation in alkaline hydrolyzable-N and the fraction of TN quantified as alkaline hydrolyzable-N with site and soil depth indicates the importance of proper sampling depth for correlation and calibration of crop response using either the ISNT or DSD.  Significant changes in alkaline hydrolyzable-N with depth could influence crop available N and thus, a test’s ability to predict crop response to N-fertilization.