Abstract:
The excavated unsuitable soil of the Southern Expressway Extension project has
been dumped in various sites. Some sites show poor re-vegetation and there are
concerns about the impacts of unsuitable soils in the dumping sites. This study
aimed to identify the basic properties and problems of these unsuitable soils and
potential solutions to the problems. Main experiment examined the soil properties
and existing problems. Three sites were selected for the main experiment based on
the period of storage: Magahenawatta (45 months), Paragahawatta (42 months)
and Miriswatta Walawwa (36 months). Soil samples were collected from nine
points (in a 3 × 3 grid) at 0-15, 15-30 and 30-45 cm depths. Soil pH, EC, Organic
Matter (OM) and bulk density (BD) were measured. The second experiment (pots)
examined the possibility of managing the extreme acidity observed in the soil of
Ruppewatta site, which was actual acid sulfate soil (AASS). Two treatments with
two controls were arranged in completely randomized design. The 15 pots of the
first treatment (Ø - 11.8 cm and depth-11.5 cm) were filled with acid soil and redyellow
podzolic subsoil as four alternating layers (layer thickness: 2 cm), keeping
top layer AASS. The second treatment (9 pots) had, five alternating layers, where
the top layer was subsoil. Daily water application was 75 ml. The pH of each layer
was measured once a month (5 replicates, destructive sampling) for three months
and the pH of the drained water was measured every week. The Magahenawatta
had been filled with subsoil which has low fertility, while Paragahawatta site had
been filled with gley soils that does not show strong acidity. The Miriswatta
Walawwa site had soil close to AASS yet pose no threat of extreme acidity. The
organic matter content (9.58%) and the sulfate content (1540 mg/ kg) were
significantly lower than the Ruppewatta site. A large variability exists in the studied
unsuitable soils. Therefore, their disposal or implications of disposal should not be
generalized. Characterization of the original excavation sites could help to decide
the ultimate uses of the unsuitable soils. In the pot experiment, when the AASS layer
is exposed to the air, the pH increased slightly, from initial value of 3.2, to 3.5 and
3.4 in treatment 01 and the control having AASS, respectively. The pH of the
concealed layers increased up to 3.7 in treatment 01 and, up to 4.2 and 4.1 in the
first and third concealed layers of the treatment 02. Accordingly, covering of soil
layers could reduce further acidification of AASS and raise the pH considerably.
However, long-term and scaled-up studies are necessary.
