Transient salinity and subsoil constraints to dryland farming in Australian sodic soils: an overview

Abstract

More than 60% of the 20 million ha of cropping soils in Australia are sodic and farming practices on these soils are mainly performed under dryland conditions. More than 80% of sodic soils in Australia have dense clay subsoils with high sodicity and alkaline pH (>8.5). The actual yield of grains in sodic soils is often less than half of the potential yield expected on the basis of climate, because of subsoil limitations such as salinity, sodicity, alkalinity, nutrient deficiencies and toxicities due to boron, carbonate and aluminate. Sodic subsoils also have very low organic matter and biological activity. Poor water transmission properties of sodic subsoils, low rainfall in dryland areas, transpiration by vegetation and high evaporation during summer have caused accumulation of salts in the root zone layers. This transient salinity, not influenced by groundwater, is extensive in many sodic soil landscapes in Australia where the watertable is deep. ‘Dryland salinity’ is currently given wide attention in the public debate and in government policies, but only focusing on salinity induced by shallow watertables. While 16% of the dryland cropping area is likely to be affected by watertable-induced salinity, 67% of the area has a potential for transient salinity not associated with groundwater and other subsoil constraints and costing the Australian farming economy in the vicinity of A$1330 million per year. A different strategy for different types of dryland salinity is essential for the sustainable management and improved productivity of dryland farming. This paper discusses the sodic subsoil constraints, different types of salinity in the dryland regions, the issues related to the management of sodic subsoils and the future priorities needed in addressing these problems. It also emphasises that transient salinity in the root zone of dryland agricultural soils is an important issue with potential for worse problems than watertable-induced seepage salinity.

A locus for sodium exclusion (Nax1), a trait for salt tolerance, mapped in durum wheat

“…Over 6% of the world's land is affected by either salinity or sodicity (FAO 2004). A large proportion of the Australian wheat belt is at risk of salinisation due to rising water tables (NLWRA 2004) and a further and larger part has soils that are sodic, and underlain with subsoil salinity (Rengasamy 2002). This subsoil salinity is formed in semi-arid zones (with annual rainfall less than 450 mm), and is transient in nature as it moves in and out of the root zone according to soil wetting and drying cycles (Rengasamy 2002).…”

High clay contents, dense soils, and spatial variability are the principal subsoil constraints to cropping the higher rainfall land in south-eastern Australia

“…Plant-available Al and Mn increase in acid soils in the presence of waterlogging (Khabaz-Saberi et al 2005). In alkaline subsoils, Na, B, Al, and carbonate toxicities have been identified (Rengasamy 2002). In addition to the major chemical constraints identified above, several subsoil constraints have been identified.…”

“…More commonly, transient salinity, associated with poor permeability of subsoils, may occur in the root-zone unrelated to any groundwater influence (Rengasamy 2002). Transient salinity is associated with reduced subsoil infiltration common in texture-contrast soils, particularly Sodosols.…”

“…The impact of sodicity on crop production has been extensively reviewed Rengasamy 2002;Surapaneni et al 2002). Soil sodicity limits plant growth indirectly through poor physical conditions and directly as a consequence of sodium toxicity.…”

“…In addition to the major chemical constraints identified above, several subsoil constraints have been identified. Limited nutrient availability has also been identified as a subsoil constraint to plant growth and root exploration of the soil (Gourley and Robson 1992;Rengasamy 2002). …”