A dynamical study on sodium accumulation and soil reclamation in secondary salinization

Irrigation with saline water, together with poor leaching of the salts in the root zone, are the major causes of human-caused soil salinization, or secondary salinization. About 20% of irrigated land are salt-affected, and nearly four million acres of farmland are estimated to be lost to excessive salt every year. The development of a high percentage of sodium in the soil exchangeable complex is especially problematic, changing the soil structure and greatly reducing its hydraulic conductivity. This condition, called sodicity, mainly affects arid and semi-arid areas, where precipitation is not adequate to leach salts.

We investigate the coupled water and salt balance equations in the root zone irrigated with saline water. By describing the fractions of sodium (Na) and calcium (Ca) cations in the irrigation water, soil water, and exchangeable complex, we determine the dynamics of the total salt concentration in the soil water (C) and of the Exchangeable Sodium Percentage (ESP). We found that the dynamics of the ESP can be one order of magnitude slower than that of C: while the typical time scale of convergence of C to its steady-state value is of the order of 1 year, the ESP evolves in a typical time scale of 1 decade and more.

These results provide useful indications for the reclamation of sodic soils, which can be accelerated through the replacement of Na cations by Ca ions in the exchangeable complex, often by means of the application of gypsum to the irrigation water. We show that a two-phase managing scheme can shorten the time scale of reclamation, when compared to irrigation with same volumes of water of better quality (low C levels): first irrigation water with high salt concentration and low fraction of Na brings the ESP to low levels in relatively short time scales; then good quality irrigation water (low C) can be applied, decreasing total salinity levels. We also explored the effects of high sodicity on the soil hydraulic conductivity by adding a feedback between ESP and infiltration rate. Finally, the extension to stochastic inputs of water of different quality yields a probabilistic description of C and ESP, with special attention given to the influence of a changing climate to secondary salinization process.