An important aspect for estimating optimal N applications is to predict the soil N supply capacity (ability to supply plant-available N) each year. Since fertilizer applications supplement crop N requirements that the soil system cannot meet, understanding potential plant-available N originating from the soil can aid in setting N fertilization rates. General differences in plant-available N and fertilization need between fields are commonly accounted for through effects of crop rotation. An example of further refinement is the grouping of similar N responsive soils by yield potential (Vanotti and Bundy, 1994), as is done in Wisconsin for corn N fertilization. Determining soil N supply (N responsiveness) through soil testing has received considerable attention over the years. Most commonly used tests measure soil nitrate, either nitrate already present in the soil (post-harvest profile nitrate or preplant profile nitrate - PPNT) or a combination of nitrate present in the soil and that mineralized in the early spring (presidedress soil nitrate – PSNT or late spring soil nitrate – LSNT). For various reasons (time of sampling, sample depth, sampling difficulties, reliability) these tests have not been widely used in the north-central USA (Fox et al., 1999). Many other soil tests, based on various chemical extraction or biological incubation methods, have been studied over the years as potential indicators of mineralizable-N (examples in Bundy and Meisinger, 1994). However they have either not been successful in quantifying potential plant-available N or are not suitable for routine analysis.