This PhD project aims to determine the mechanisms, by which soil sodicity impacts on the uptake of nutrients and growth of cotton crops, in order that soil sodium levels can be factored into crop nutritional management decisions.

The mechanisms, by which sodium impacts on cotton nutrition include;

• Influencing the ability of plant to access nutrients by affecting soil structure and hence PAWC, waterlogging and root growth.

• Altering the availability of nutrients to plants through soil pH, exchange equilibrium and oxidation-reduction potential changes

• Interacting directly with nutrients at the membrane surfaces throughout the plant.

An understanding of these factors has important practical implications for crop nutritional management and may lead to the development of appropriate tools for the diagnosis of sodicity-induced nutritional imbalances.

The nature of the impact of sodicity on the growth and nutrition of cotton in the field was determined through a program of plant measurements and tissue analysis in a cotton crop grown on a field with varying levels of sodicity in different parts of the field. The results showed that as the level of sodium in the soil increased, there was a corresponding increase in the uptake of sodium and decrease in the uptake pf phosphorus ad potassium by the crop. These results are illustrated in the attached graph (Dodd_SodicityGraph).

The quantitative relationship between the level of sodium in the soil and the growth and nutrition of the cotton plant is currently being determined in a glasshouse experiment, in order that the variability present in a field situation can be removed. Sodic soils were artificially created by leaching large volumes of solutions with different cation concentrations through given volumes of soil. In this way, the ESP of the soil could be manipulated while keeping other factors, such as soil electrical conductivity, mineralogy and nutrient levels constant. Analysis was carried out in order to determine that the synthetic soil conditioning agent polyacrylamide has no impact on the availability of nutrients to the plant in the soil solution. The effects of the physical and chemical properties of sodic soil were then separated in the glasshouse experiment using polyacrylamide treatments. The progress of this experiment is illustrated in the attached photograph (Dodd_GlasshousePhotograph).

It is hypothesized that the relationship between the level of sodium in the soil and the performance of the cotton plant is affected by a number of factors, especially the presence of salinity and high levels of magnesium in the soil profile. Following the completion of the above glasshouse experiment, it is envisaged that any chemical effect of sodicity on the performance of the cotton plant and its interaction with salinity and magnesium will be further investigated through solution culture experiments. This data will then be related to soil test data through soil solution analysis. Additionally, the physical effect of sodicity on the performance of cotton and its interaction with salinity and magnesium will be further investigated by quantifying how these factors influence soil physical structure.