Long-term effects of profile-modifying deep plowing on soil properties and crop yield

Increasing precipitation storage as plant-available soil water increases dryland crop yields on the semiarid southern Great Plains. The dominant soil in this region is a Pullman clay loam (fine, mixed, superactive, thermic Torrertic Paleustoll), which features dense subsoil layers that limit infiltration and root proliferation. Deep (similar to 0.7 m) plowing may eliminate these layers, but the duration of treatment efficacy must be sufficient to recoup costs. Our objective was to quantify the long-term effects of deep plowing on soil bulk density (BD), penetration resistance (PR), ponded infiltration, and crop yield. In fall 1971, paired 24- by 460-m level conservation bench terrace plots at the USDA-ARS Conservation and Production Research Laboratory, Bushland, TX (35 degrees 11'N, 102 degrees 5'W) were moldboard plowed to 0.7 m or untreated and, subsequently, maintained with conventional sweep tillage. Grain sorghum [Sorghum bicolor (L.) Moench] was grown intermittently during the next 34 yr for 15 paired yield comparisons. Compared with the control, deep plowing significantly decreased BD and PR and increased water infiltration. Long-term mean grain yield of 2.86 Mg ha(-1) with deep plowing was greater (P < 0.05) than the 2.61 Mg ha(-1) yield in control plots. Approximately 89% of the 15-yr 3.75 Mg ha(-1) cumulative yield increase with deep plowing occurred during 4 yr with large early growing season rains. We attributed this yield increase with deep plowing to greater internal drainage that reduced crop injury during infrequent floods and not greater root growth through the dense subsoil. Thirty years after deep plowing, high water intake suggests that flow-limiting subsoil layers had not yet redeveloped. The sustained deep-plowing benefit extends the period to recoup the 1971 plowing costs of US$160 ha(-1).