Technical Abstract: Insufficient plant available soil water limits dryland crop yields on the semiarid Southern Great Plains. Deep plowing to eliminate dense subsoil layers may increase soil water by increased infiltration and rooting, but the duration of treatment effects must be sufficiently long to recoup plowing costs. Our objective was to quantify the long-term effects of deep plowing a Pullman clay loam (fine, mixed, superactive, thermic Torrertic Paleustoll) on soil bulk density, penetration resistance, ponded infiltration, and crop yield. In fall 1971, paired 24 x 460 m level conservation bench terrace plots at the USDA-ARS, Conservation and Production Research Laboratory, Bushland, Texas (35 degrees 11' N, 102 degrees 5' W) were moldboard plowed to 0.7 m or untreated and, subsequently, maintained with sweep tillage. Grain sorghum [Sorghum bicolor (L.) Moench] was grown during the next 34 years for 15 paired yield comparisons. Compared with the control, deep plowing decreased bulk density (BD) and penetration resistance (PR) and increased water infiltration in 2002. Long-term mean annual grain yield of 2.87 Mg ha-1 with deep plowing was greater (p < 0.05) than the 2.61 Mg ha-1 yield in control plots; however, four of the 15 paired sorghum crops accounted for ~ 85 percent of the cumulative yield benefit with deep plowing. We attributed this yield benefit to improved internal drainage that prevented crop injury due to rain flooded plots. Thirty years after soil profile modifying deep plowing, our measurements show improved water intake and that flow limiting subsoil layers did not redevelop. The sustained deep plowing benefit extended the period to recoup 1971 deep plowing costs of $160 ha-1.