Achieving a salt balance—Should it be a management objective?

The long-term sustainability of irrigated agriculture depends on protecting land and water resources from salinity. Generally, a favourable salt balance (mass out ≥ mass in) is considered necessary for sustainable irrigated agriculture. This concept has been used in irrigated agricultural systems to satisfy varying objectives. In this paper we discuss the application of the salt balance concept at various spatial scales including root-zone, farm, irrigation-region and catchment scales. The relationship between the spatial scales, the governing processes and the time to reach salt equilibrium is discussed. Recognised management options for achieving a favourable salt balance are questioned with regard to (a) the uncertainties in salt balance estimation caused by a mismatch of surface and sub-surface hydrological boundaries, (b) the large subsoil salt store, (c) impacts on downstream water quality and (d) the implications of achieving a favourable salt balance. It is argued that aiming for a favourable salt balance is critical and achievable at a root-zone scale for maintaining production. For scales larger than the root-zone, managing for a favourable salt balance is often not practical nor necessarily beneficial. Management of the water balance is more important at the larger scale to prevent the formation of a shallow watertable and the remobilising of salt stored in the subsoil. The management of the water balance is likely to be more effective in managing productivity and the off-site impacts of irrigation than managing a farm or regional salt balance.