Banding of N fertilizers below the seed is a common practice in direct seeded wheat dominated dryland agricultural systems, yet the placement of fertilizer below the seed has been shown to decrease yields and damage the root system architecture (RSA) in canola (Brassica napus). However, there is a lack of understanding as to which rates and sources of fertilizers might be safe for seedling root systems. The goal of this study was to compare the effect of three different N sources over a gradient of application rates on canola RSA. The effect of the fertilizer band on RSA has previously been imaged using rhizobox methods. However, the research presented here describes a novel technique for comparing sources and rates simultaneously while using the existing imaging technology. Canola seedlings were exposed to increasing rates of urea, ammonium sulfate (AS), and urea ammonium nitrate (UAN) from 0 mg N cm(-1) to 40 mg N cm(-1) placed 50 mm below the seed row. Canola showed significant changes in RSA as a result of the increasing ammoniacal-N fertilizer rate. Dose response curves were developed for urea, AS, and UAN, and median lethal doses (LD50s) for tap roots were estimated at 4.7, 9.7, and 20.6 mg N cm(-1), respectively. Comparisons between multiple measurements of the dose response curves revealed a consistent ranking of relative potencies among urea, AS, UAN (most to least toxic).