Comparison of Near-Infrared Spectroscopy and Other Soil Nitrogen Availability Quick Tests for Corn

Our ability to predict N fertilizer needs for corn (Zea mays L.) is improving, but more accurate and convenient tests are still needed. This work compared a new quick test for soil N availability using a near-infrared spectrophotometer (NIRS) with three published quick tests for predicting soil N-supplying capability (NSC) and relative corn grain yield. The other tests were the pre-sidedress nitrate test (PSNT), nitrate concentration (at-plant NO₃), and absorbance at 200 nm of a 0.01 M NaHCO₃ extract (UV-200 test) of 0- to 20-cm soil samples taken at planting. Soil samples taken at planting from 95 field experiments in Pennsylvania were analyzed at reflectance wavelengths from 400 nm to 2500 nm with NIRS. The coefficients of determination were the same (R² = 0.49) for both linear and quadratic regressions of NSC and NIRS test values. The abilities of the four tests to predict NSC and relative corn grain yield were compared using data from 90 of the 95 experiments. The R² values for linear and quadratic regressions between soil test values and NSC ranged from 0.49 to 0.58 for the NIRS, PSNT, and UV-200 tests; for the at-plant NO₃ test, R² was lower (=0.40). Eliminating sites where corn directly followed a legume, R² values for quadratic regressions between test values and NSC increased to =0.60 for the NIRS, PSNT, and UV-200 tests. The PSNT test was slightly better than the other tests in predicting a grain yield response to N fertilizer, but this advantage lessened when first-year-after-legume sites were eliminated. No test could accurately predict relative grain yield or NSC in fields responding to N fertilizer (R² = 0.08−0.36). The NIRS test offers a convenient, rapid, and inexpensive alternative to the PSNT for predicting whether humid-region corn fields will respond to N fertilizer.