The lowest acceptable pH values for spring wheat (Triticum aestivum. cv. Waverly), winter wheat (cv. Stephens) and lentils (Lens culinaris. cv. Chilean) were determined in two greenhouse studies using a Latahco silt loam (fine‐silty, mixed, frigid Argiaquic Xeric Argialboll). The soil pH was adjusted to values ranging from 3.7 to 7.0 with additions of elemental S or Ca(OH)2prior to planting. Parameters evaluated included germination rate, stand vigor, N and P plant uptake, and plant biomass production for winter wheat and lentils in study 1, and seed germination rate, stand vigor, number of seed, seed yield, nodulation status, root mass, and stem diameter of spring wheat and lentils in study 2. Significant correlations were observed between evaluated parameters and soil pH for lentils in both studies. Although not always statistically significant, good correlations between evaluated parameters and soil pH were observed for both spring and winter wheat. Nitrogen and P availability did not limit plant growth at pH values at or above 3.7. Germination rates were not affected by soil pH. Growth of winter wheat, spring wheat, and lentils was severly reduced, as indicated by stunted growth and necrotic leaves, at pH values below 3.7, 3.7, and 4.5, respectively. Nodulation of lentil roots byRhizobium leguminosarumwas affected by pH with highest numbers of nodules observed above pH 5.5 and no nodulation observed below pH values of 4.0. The lowest acceptable pH values for lentil and wheat crops were similar to reported values obtained in field studies in northern Idaho (5.6 for lentils; 5.2 for wheat). In these greenhouse studies, low soil pH may have reduced biomass production by interfering with plant nutrition and by probably increasing plant disease problems. The data presented in this paper are important because they show that the acidic surface 30 cm of agriculturally important soils in northern Idaho control potential crop yield, and that the less acid subsoil is of only minor importance