RESIDUAL DEEP PLOWING EFFECTS ON IRRIGATION INTAKE FOR PULLMAN CLAY LOAM

Pullman clay loam (fine, mixed, thermic Torrertic Paleustoll) and rela ted soils predominate in the southern High Plains of the USA. They are slowly permeable but respond to deep tillage with increased irrigatio n water intake, A one-time deep moldboard plowing to 0.4-, 0.6-, or 0. 8-m depths was performed in 1966 to evaluate the inversion and mixing of the slowly permeable Bt1 horizon with the Ap horizon. We hypothesiz ed that these deep tillage effects would still be present after 25 yr, We report irrigation intake effects with 5 yr (four growing seasons) of cropping to winter wheat (Triticum aestivum L.) beginning in 1988. After moldboard plowing to 0.2 m to restore surface tillage layer perm eability, residual effects from the 1966 deep plowing caused an averag e increase in intake of 26% (129-163 mm) for the 0.4-m plow depth, com pared with the 0.2-m check, for the first irrigation after tillage fro m 1988 to 1992, Irrigation intake increased 40% (52 mm) with 0.6-m dee p plowing; however, there was no additional increase for 0.8-m plowing . Grain yields increased from 4.2 to 5.0 Mg ha(-1) (19%) for the 0.4-m or deeper plowing. The 1966 deep tillage also increased deep soil wat er storage between the 1- and 2.3-m depths during 1988 to 1992. Water use efficiencies were approximate to 8% greater for the deep plowing t reatments.