The soils of a barley producing area in the Andean highlands of Tungurahua province in Ecuador were investigated for their nitrogen (N) and phosphorus (P) supplying power. Phosphorus was found to be more limiting to barley production than N, possibly because crop rotations included legumes and additions of livestock manures which maintained adequate N levels in these high organic matter soils. A series of field studies were established in farmers' fields to assess the predictive capability of soil tests for barley yield response to added fertilizer N and P. Of the methods evaluated at six locations, a mineralization incubation for N and soil solution P were best correlated with the relative yield increase to applied nutrients. The estimated critical N mineralization level was 9 mg kg−1 day−19 mg kg−1 day−1 to achieve a barley yield that was 80 to 90% of the potential yield at each site when both N and P supply were sufficient. The critical P concentration in soil solution was estimated to be 0.03 mg L−10.03 mg L−1. A multiple regression equation including soil test N and P levels accounted for 81% of the variation in yield across nine experimental sites. Although further studies that include several cropping seasons and more sites will be required to establish soil fertility recommendations, the results from this study indicate the potential for predicting the severity of nutrient limitations on these Andean highland soils.