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Yields of irrigated alfalfa (Medicago sativa L.) are lower on Pullman clay loam than on more permeable soils in the same climatic area. The objectives of this study were to determine (i) the residual effect of thoroughly mixing the soil profile on alfalfa growth and production, and (ii) the amounts and timing of irrigations required for maximum alfalfa production on modified and unmodified soil profiles.Alfalfa was grown under three profile modification treatments (thoroughly mixing the soil profile to 0‐, 90‐, or 150‐cm depth) on Pullman clay loam. For the first 3 years, all plots were either irrigated similarly (1970 and 1972) or unmodified plots were irrigated twice between harvests when necessary (1971). During the next 3 years, we expanded the study to include three irrigation treatments (two 10.2‐cm, one 17.8‐cm, or two 15.2‐cm irrigations between harvests). During the first 3 years, when single irrigations were applied between harvests, profile modification increased dry‐matter yields 40% (90‐cm modified) to 60% (150‐cm modified) over those for the unmodified check. However, with two irrigations between harvests, effects of modification were less marked.In the second phase of the study, with two 10.2‐cm irrigations between harvests, profile modification increased yields 30% (14.1 to 18.4 metric tons/ha). On 90‐cm modified soil, 3‐year average yields were 16.6, 18.3, and 20.1 metric tons/ha with one 17.8‐, two 10.2‐, or two 15.2‐cm irrigations between harvests, respectively. Respective yields on 150‐cm modified soil were 17.4, 18.5, and 18.6 metric tons/ha.Yields, water intake rates, bulk densities, and surface elevations showed that profile modification treatments made in 1964 were still effective 12 years later.

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