Persistence of Loosened Horizons and Soybean Yield Increases in Bolivia

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Abstract
Soil compaction is a major factor limiting soybean [Glycine max (L.) Merr.] production in tropical eastern Bolivia. An experiment was conducted from 1985 through 1989 on a fine‐loamy, siliceous, isohyperthermic Aquic Ustropept with a compacted horizon (0.12–0.33 m) to determine the persistence of subsoil loosening by deep tillage and its effect on soybean yield responses. Four tillage treatments were investigated: conventional tillage consisting of disk harrowing to 0.12‐m depth (CN), subsoiling in 1985 only to 0.40‐m depth (SS/85), disk plowing in 1985 only to 0.30‐m depth (DP/85), and annual subsoiling (SS/YR). All three deep tillage treatments completely loosened the compacted horizon, reducing penetrometer resistance (PR) from a maximum of 3.05 to 2.01 MPa. From December 1987 onward, which coincided with an exceptionally wet period, subsoil reconsolidation occurred, and by June 1989 the PR of CN, DP/85, and SS/85 exceeded 3 MPa. Reconsolidation was attributed to a close‐packing rearrangement of soil particles under wet conditions. Coupled with increasing subsoil strength, a relatively greater reconsolidation occurred in SS/85 and DP/85, resulting in reduced “loosened” zones of 0.07‐m and 0.04‐m thickness, respectively. The pattern of relative reconsolidation suggests it occurred from above downward and from below upward, the latter perhaps being related to a high water table. In contrast SS/YR maintained a much thicker “loosened” horizon. overall subsoil reconsolidation, soybean yield responses continued for 3 yr with SS/85, and for 4 yr with DP/85 and SS/YR. Although substantial differences in relative reconsolidation existed between DP/85, SS/85, and SS/YR, few significant differences in yield were found when soil water was limiting.