Determination of an optimum fertilizer application rate involves various influential factors, such as past management, soil characteristics, weather, commodity prices, cost of input materials and risk preference. Spatial and temporal variations in these factors constitute sources of uncertainties in selecting the most profitableapplication rate. Therefore, a decision support system (DSS) that could help to minimize production risks in the context of uncertain crop performance is needed. This paper presents a newly developed framework for a dynamic DSS, called NumericAg, which seeks to estimate the probability of achieving expected profits under specific growing conditions. The proposed system includes a database, a user interface, and a numeric engine for computation of profit space. The online web interface (www.numericag.com) allows a user to specify production conditions (e.g., previous crop, tillage system, soil type, organic matter content, rainfall, and crop heat unit), and to accept or modify the price of grains and fertilizers. The database stores the results of previously recorded fertility trials. The profit space computation engine was designed to estimate the probability of achieving different levels of net return over the cost of nitrogen fertilizer for every potential application rate. The profit space engine considers over 20,000 potential quadratic-plateau fertilizer response functions in combination with 49 cost scenarios evaluated against every fertility trial weighted according to the growing conditions specified by the user. Consequently, probability of different levels of potential net return over cost of fertilizer was estimated using fertilizer response observations that relatively closely match the growing conditions specified by the user. A sensitivity analysis was used to show DSS response to changes in specified growing conditions. Thus, when comparing different growing conditions, a smaller probability of achieving relatively high profits was found with sandy soils, relatively low crop heat units and water availability, or low levels of nitrogen contribution from previous crops. Although the rate that maximized the expected net return over cost of nitrogen did not change substantially, the rate of profit decline due to under application of nitrogen fertilizer was different for different growing conditions.