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Persistence of Loosened Horizons and Soybean Yield Increases in Bolivia

British Tropical Agricultural Mission, Santa Cruz, Bolivia, c/o F.C.O. (La Paz), King Charles Street, London, SW1A 2AH, UK

*Corresponding author.

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.

Received for publication February 5, 1993.

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