Estimating Soil Nitrogen Supply in Soils with a History of Organic Amendments.

In Atlantic Canada, organic amendments such as manure and compost are important sources of soil nitrogen (N). A large portion of N (>50%) in such amendments exists in organic form, a portion of which will be released through mineralization during the growing season. The remaining N can accumulate to be released in succeeding years. As a result, a history of organic amendment application can significantly change the nature and release rate of the active organic N fraction. Currently in Atlantic Canada, manure recommendations rates are estimated using literature data and there is no soil N test available. Therefore, there is an urgent need to develop a soil N supply (SNS) test for organically amended soils to improve manure recommendations. This research investigated the effectiveness of different soil N tests in predicting N uptake by different crops on long-term organically amended soils with the goal of determining the soil test(s) with the most accurate predictions. The effectiveness of hot KCl extractable ammonium, SNS test (KCl extractable mineral N after 2 wks aerobic incubation at 25^oC), Illinois Soil Nitrogen Test (ISNT), Ion Exchange Membrane nitrate and ammonium (IEM), hot water extractable ammonium and total soluble N (80 ^oC for 16 h), and UV absorbance of NaHCO₃ extract at 205 and 260 nm tests were investigated. Five sites with long-term history of organic amendment application under cereal, corn or forages in Atlantic Canada were sampled prior to fresh amendment application. Results of each test were correlated against plant N uptake and dry biomass yield. After linear regression between soil test and plant N uptake, the most accurate tests were hot water NH₄-N (R² = 0.58), followed by SNS test (R² = 0.54), Mineral N + SNS test (R² = 0.51), and hot water total soluble N (R² = 0.51). Linear regression between the dry biomass and the N tests was not as strong, with the strongest relationship resulting in hot KCl NH₄-N (R² = 0.23). Overall, the most accurate soil N test investigated in this study was hot water extraction.