CONCENTRATION CHANGES OF SODIUM AND CHLORIDE ASSOCIATED WITH ROOTS IN SALINE ENVIRONMENTS

Rami  A. ZREIK

Salinity is one of the world's oldest agricultural problems. The yield of most crops is reduced as the concentration of salt, mainly sodium and chloride, increases in the soil solution. Water loss by evapotranspiration can cause the salinity of the soil solution to increase markedly in cropped fields. There is some evidence to support that in specific situations, when ion absorption is slower than supply, processes in the soil‑root environment can also cause a build‑up of ions in the rhizospheric soil, especially at the soil‑root interface. An increase in salt concentration at the soil‑root interface could be crucial in determining the yield and survival of crops. Monitoring the extent of this accumulation is therefore essential for the management of agriculture in saline areas.

There does not yet exist an accurate technique for the determination of the concentration of ions in the soil solution at very close distances from the root surface. By developing a mechanistic model of the relevant processes in the soil‑root environment, it should be possible to predict these concentration changes over a wide range of soil conditions and management practices.

In this thesis, a numerical solution to a model of solute transport by diffusion and convection to a cylindrical root element is developed The model is used to predict the concentration changes of sodium and chloride near the root in saline environments. The model has eight input parameters the initial concentration of sodium and chloride, the initial water content the water flux at the root surface, the solute Flux at the root surface the salt diffusion coefficient of sodium and chloride in solution, the relationship between moisture content and the diffusion impedance factor, the root radius, and the radius of influence of the root The model's predictions generally agree with predictions from other models and with earlier reports of salt accumulation near plant roots.

The model's validity is tested in specifically designed experiments, using a novel technique developed for this purpose. The technique uses thin pure cellulose and clay-impregnated cellulose strips rolled into cylinders and imbibed with a saline solution labeled with the appropriate radio- isotope. The concentration-distance curves predicted from parameters measured from several experiments, with cellulose and clay-cellulose cylinders, agreed well with the experimentally determined profiles. Thus, it appears that all important processes have been accounted for in the model.

The potential occurrence and extent of sodium and chloride concentration gradients in saline agricultural soils under common agronomic practices are also investigated. Plant parameters are measured for maize seedlings grown in solution cultures at a range of external sodium chloride concentration and at two transpiration levels. Soil parameters are obtained from the literature for a sandy loam soil. It appears that under these conditions steep gradients will only be established below the soil water levels at which crop growth normally takes place. The effect of sodium chloride salinity and of external humidity on plant growth and uptake is also discussed.