Deep Tillage of Irrigated Pullman Clay Loam— A Long-Term Evaluation

Abstract

ABSTRACT PULLMAN clay loam (Torrertic Paleustolls) is the most extensively irrigated soil type in the United States. It is slowly permeable to water, but is responsive to deep tillage for increasing water intake and profile storage. A one-time deep moldboard plowing to 0.4-, 0.6-, and 0.8-m depths was tested in 1966 for modifying the slowly permeable B2t horizon and compared with conventional tillage to 0.2 m. The 0.6-m plowing depth fully penetrated the slowly permeable B2t, while the 0.8-m depth loosened and mixed an increment of more permeable clay. This paper reports long-term residual ef-fects for eight crops during 1972-79. After moldboard plowing to 0.2 m to restore surface-layer permeability, residual effects of deep tillage in-creased irrigation water intake from 120 to 240 mm in 1975 and from 170 to 280 mm in 1978, indicating no diminishment of the effect with time. After primary tillage, irrigation and wheel traffic caused surface soil consolidation, which greatly reduced residual deep-tillage effects on water intake during the growing season. During some periods of the study, deep-tillage had little to no effect on intake, but the effect was restored by con-ventional moldboard plowing to 0.2 m. During two seasons of sorghum {Sorghum bicolor L. Moench) and two of corn (Zea mays L.), residual deep tillage increased sorghum grain yields in the range of 880 to 1,250 kg/ha, and corn yields in the range of 1,280 to 1,630 kg/ha. The increased water intake and deeper profile wetting of deep-tillage plots delayed the beginning and reduced the severity of plant water stress. We concluded that deep-tillage is a practical method for increasing water intake of a slowly permeable structured clay and associated crop yield, particularly on the normally drier, lower parts of graded furrow irrigated fields where it can be used in association with reduced tailwater runoff.