The Thickness of the Shallow-Depth Hardpans on Soil Hydraulic Characteristics

Abstract

Soil-water relations in terrestrial ecosystems are characterized by the set of attributes collectively referred to as soil hydraulic characteristics. Soil hydraulic characteristics play an important role in determining the capacity of soil to serve the crop growth and ecosystem. Subsurface compaction in soil or shallow depth hardpan development, which can vary from a few millimeters to meters, is one of the crucial factors that determine soil hydraulic characteristics. However, information lacks on how the thickness of the hardpan (THP) influences the soil hydraulic characteristics of the soil. Therefore, this study was aimed to assess the effects of THP on the initial infiltration rate (Ii), unsaturated hydraulic conductivity (K_UNSAT) and sorptivity (Sw) of soil using moderate thickness levels of 1 (THP₁), 3 (THP₃) and 5 cm (THP₅), prepared using 1.7 gcm^-3 bulk density and with 0 cm as the control (without hardpan). The soil used in the study was collected from the field in the Faculty of Agriculture, University of Ruhuna (6° 03 '29"N 80° 34' 13"E). The area is in the low country wet zone (WL2) agro-ecological region. PVC tubes with 10.5 cm inner diameter and 30 cm in height were used to prepare the soil columns used in the experiment. The readings were taken in triplicates, using Mini disk infiltrometer. The depth to the hardpan from the surface was kept at 3 cm. All soil columns with hardpans showed significantly low Ii, K_UNSAT and Sw values compared with the control. The highest Ii, K_UNSAT and S_W showed in the control, and it was about 60 cmh^-1, 21 cm h^-1 and 0.14 cms^-1 respectively. The Ii, K_UNSAT and S_W in soil columns gradually decreased with the increase of THP. Ii of soil columns with THP₅ showed approximately 4.5% reduction than the THP_3. The K_UNSAT of soil columns with THP₃ showed approximately 29.5% reduction and 42% increment than those soil columns with THP₁ and THP₅ respectively. The results revealed that the presence of soil hardpans has a negative influence on Ii, K_UNSAT and S_W, even THP₁ can significantly reduce the water movements in the soil matrix. Furthermore, it was clear that THP₃ and THP₅ can reduce water movements to almost negligible levels, which would create subsurface water-logging conditions.

Keywords: Hardpan, Thickness, Subsurface, Infiltration, Hydraulic conductivity