The study area is located at planted forest on the hills of Chittagong University and ranged in elevation from 11 to 48 m above mean sea level. The climate of the study area is warm (sub-tropical) with a mean annual air temperature of 25.70C, a mean annual precipitation of 2735 mm and potential total evaporation of 1427 mm. The soil moisture regime is udic and soil temperature regime is isohyperthermic. The study area is predominantly covered with different types of tree species, herbs and shrubs.
In this study, four soil profiles formed on different elevation of hill forests were investigated. The physiography of the investigated area comprises hill summit, upper piedmont and lower piedmont. Two profiles were dug on hill summit: one hill summit site was narrow landscape with gently sloping to moderately sloping and another hill summit had broad and flat landscape . The profiles were also opened at higher piedmont and lower piedmont.
At each site, soil samples were collected in triplicates from different successive horizons using a scoop. A total of 45 soil samples (3 replicates × 15 horizons) were collected for analysis.
The morphological properties of the profiles were described during soil samples collection in the field using the criteria of the soil survey manual of soil survey division staff and the guidelines for soil profile description.
In addition, core samples were also taken by using bulk density corer from the center of each horizon for soil density analysis.
The undisturbed soil samples were packed in precleaned air tight plastic bags labeled and transferred to the laboratory for further processing and analysis.
The soil samples were air dried ground and screened using a 2 mm sieve. Particle size distribution of the soils was determined by hydrometer method. Soil color was measured by Munsell soil color chart at both moist and dry conditions of soils. Bulk density was determined by the core method. Cation exchange capacity (CEC) was determined after extraction of the cations with ammonium acetate. The PH values were measured by mixing soil with deionized water at a ratio of 1:2.5 (soil: water) followed by shaking the suspension for 1 h and using corning glass electrode pH-meter. The pHKCl of the soils was measured with corning glass electrode pH-meter using a soil solution 1:2.5 (soil: 1 M KCl) after shaking for 1 h. Organic carbon was determined by wetoxidation method of Walkley-Black. Available calcium, magnesium, potassium and sodium were extracted with 1N NH4OAC and determined by an atomic absorption spectrophotometer. Percent base saturation (PBS) was calculated using the formula BS% = [(Σ of exch. Ca, Mg, K, and Na)/CEC] × 100.
Exchangeable acidity was determined by titration of 25 ml KCl extract with 0.5 N NaOH, using 0.1% phenolphthalein indicator (titration from colorless to pink). Then, the concentration of exchangeable Al3+ was obtained by back-titration of the same KCl extract, previously used, after the acidification with a drop of HCl and addition of 4% NaF, with 0.5 N HCl (titration from pink to colorless).
The effective cation exchange capacity (ECEC) was calculated as the sum of exchangeable Ca, Mg, K, Na and exchangeable acidity (Al3+ + H+). Percent Aluminum saturation (Als) was calculated using the formula: Als (%) = (exchangeable Al/ECEC) × 100.
The horizons from each profile were condensed into surface and subsurface soils for statistical analysis. A and E horizons of profile I and profile II were considered as surface layers and Bt as subsurface layers. For profile III, A and Bw1 + Bw2 were regarded as surface and subsurface layers, for profile IV, A and C1 as surface and subsurface layers respectively. Statistical analyses were done by using Minitab program.