Soil NO3– affects microbial processes, plant productivity, and environmental N losses. However, the ability to measure soil NO3– is limited by labor‐intensive sampling and laboratory analyses. Hence, temporal variation in soil solution NO3– concentration is poorly understood. We evaluated a new potentiometric sensor that continuously measures soil solution NO3– concentration with unprecedented specificity due to a novel membrane that serves as a barrier to interfering anions. First, we compared sensor and salt extraction‐based measurements of soil NO3– in well‐controlled laboratory conditions. Second, using 60 d of in situ soil NO3– measurements every 10 s, we quantified temporal variation and the effect of sampling frequency on field estimations of mean daily NO3– concentration both within and across days. In the laboratory, sensors measured soil NO3– concentration without significant difference from theoretical adjusted soil NO3– concentration or conventional salt extraction‐based methods. In the field, the sensors demonstrated no within‐day pattern in soil NO3– concentration, although individual measurements within a day differed by as much as 20% from the daily mean. Across days, when soil solution NO3– was dynamic (early spring) and sampling frequency was >5 d, estimates of mean daily NO3– concentration were >20% from the actual mean daily concentration. In situ soil sensors offer potential to improve fundamental and applied sciences. However, in most situations, sensors will measure soil properties in a different manner than conventional salt‐extract soil sampling‐based approaches. Research will be required to interpret sensor measurements and optimize sensor deployment.