BYPASS FLOW AND ITS ROLE IN LEACHING OF RAISED BEDS UNDER DIFFERENT LAND-USE TYPES ON AN ACID SULFATE SOIL

A better understanding of leaching processes in raised beds is useful in assessing management options for acid sulphate soils. Field and lab oratory studies were carried out to quantify the effects of soil physi cal properties and bypass flow on leaching processes of new, 1-year-ol d and 2-year-old raised beds for yam and pineapple cultivation in a Ty pic Sulfaquept in Tien Giang, Vietnam. The methylene blue staining tec hnique was used to characterize the water-conducting pores in terms of number, stained area, and total pore perimeter at 10 cm depth interva ls of six 1 x 1 m subplots. Undisturbed 20 cm x 25 cm soil cores taken from the raised beds were subjected to three 30 mm h(-1) rains. Volum e, aluminum and sulphate concentration of the outflows were monitored. Consolidation with time decreased the area and perimeter of water-con ducting pores in 2-year-old pineapple beds to about a third, and bypas s flow rate to about 80% of those in newly constructed beds. Consolida tion did not affect macropore network geometry in yam beds because the y were subjected to annual tillage and yam tubers were uprooted regula rly. Al3+ and SO42- concentrations in the outflows of the newly constr ucted and 1-year-old raised beds were higher in pineapple, while those in 2-year raised beds were higher in yam. (C) 1997 Elsevier Science B .V.