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.

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This paper examines the soil degradation processes that are threatening the productivity of the mechanized annual cropping areas in the Santa Cruz lowlands of tropical eastern Bolivia. The dominant process is that of machineryinduced and natural compaction, which has resulted in an estimated 50 per cent of the soils in the Central Zone being moderately to severely compacted, causing serious root restriction and the loss of both transmission and waterstorage pores. Degradation has made the soils increasingly susceptible to moisture stress due to the combined effect of (i) restricted rooting as a result of compaction and the hardsetting characteristics of the soils, (ii) reduced rainfall infiltration due to the loss of transmission pores and surface crusting, and (iii) a decrease in available soil moisture caused by the loss of storage pores, the incorporation of wind-blown fine sand deposits, and soil organic matter losses due to accelerated decomposition rates. The loss of transmission pores has also made the soils more prone to waterlogging in periods of high rainfall. The degradation tendencies of these soils are exacerbated by the greater variability of seasonal rainfall during the last 20 years that has led to a greater frequency of extremely high or extremely low rainfall events than hitherto.

Potassium futation was measured for 13 Kenyan soils with a range of applied potassium (0 to 1000 mg kg-' ). The fixed potassium was measured, and hence is defined in this study, by the difference between the amount of applied K and the increase in the extractable K after equilibration, using a single 'equilibrium' extraction procedure with 0.25 M CaCI, as the extractant. At application rates of 50 and 1000 mg kg-I the fvted potassium varied from -52% to 64% and 7% to 39% respectively. A correlation was found between the K fvtation and % clay content at the 1000 mg kg-' application rate.Estimates of the % fixation by the clay fractions gave 53 to 80% for montmorillonites, 4 8 to 66% for amorphous clays and 19 to 32% for kaolinites. It is suggested that the amorphous clays may fix potassium by a 'sieve' mechanism.

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