Effects of an imbedded gravel–sand layer on reclamation of coastal saline soils under drip irrigation and on plant growth

Governmental goals for the development of new municipalities in areas of very saline soils near shorelines of the Bohai Gulf, in north China, include planting landscape trees and shrubs along streets and highways, and in parks in residential areas. The soils are saline because of the presence of shallow groundwater (45–138 cm) with salinity levels of >24 dS m−1. Prevention of upward water movement from the saline groundwater could be an important factor in facilitating reclamation of the soil above the water table, as well as the long-term sustainability of any reclamation that is achieved. The possibility of placing a 20-cm thick layer of gravel–sand at a depth of 80 cm to block upward water movement was evaluated in a three-year field experiment conducted during 2009–2011. Soil salinity and growth of landscape plant species were measured. Irrigation water was the local deep groundwater with electrical conductivity of 1.7–2.1 dS m−1 and SAR (sodium adsorption ratio) of 4.3 mmol L−1. Results indicated that the extent of desalinization (the ratio of salt content leached to the initial salt content) for the imbedded gravel–sand layer (GL) was larger than that for the non-gravel–sand layer treatment (NGL). Less salt accumulated in the surface soil during winter in GL compared to NGL. The mean electrical conductivity of saturated paste extracts (ECe) were <5 dS m−1 in root distribution zones (0–40 cm) for NGL compared to 3 dS m−1 for GL; the corresponding pH of saturated paste extracts (pHe) were 8.5 and 8.7. The influences of time, treatment and time × treatment on soil ECe and pHe were significant. At the end of the treatment, the salinity levels in root zones for both treatments were suitable for the growth of plants that were moderately sensitive to salinity (3–4 dS m−1). The results provide theoretical and technological guidance for salt leaching and landscaping of highly saline lands located along the coast of the Pacific Ocean.