Magnesic soils: first principle soil management strategies for increased productivity

Magnesic soils: first principle soil management strategies for increased productivity

Irrigation waters containing considerable amounts of magnesium (Mg) are increasingly used in farming, due to the scarcity of good quality irrigation water. This increases the concern for Mg accumulating in soil systems. Together with naturally formed magnesic soils, irrigation induced Mg affected soils have been reported in many countries. In magnesic soils – for which there is limited information – the combination of poor soil structural properties, calcium deficiency and magnesium toxicity have been suggested as stress factors for plants. Unfortunately, there is a paucity of conclusive information detailing Mg effects on soil structure, with existing literature suggesting Mg has a specific effect. This thesis is focused on the effect of Mg, either Mg in irrigation water or soils, on the soil structural stability. This is one of the prerequisites in decision making for soil management that maintains and/or increases soil productivity.

To investigate the effect of Mg on soil structural stability, fundamental studies based in the laboratory were conducted in both soil clay suspensions and aggregated systems. Other factors influencing structural stability were also considered, including soil pH, electrolyte concentration, clay mineralogy, organic matter, texture and mechanical stress. To quantify clay dispersion, a rapid and inexpensive method was developed via use of a turbidimeter. The measured turbidity (NTU) of dispersed clay in suspension, can be easily and quantitatively converted to the dispersed clay quantity (mg/L, % of soil). This method was used in all experimentations throughout the whole thesis for the investigation of the effect of Mg on soil structural stability.

The dispersive behaviour of Mg-saturated soil clay with contrasting mineralogy was studied at pH 3–8. The results demonstrated that, at low pH, Mg clays flocculate, although different clay types had different flocculation ranges; at neutral pH, Mg clays had higher dispersed clay (%) than Ca clays under similar conditions. This study further showed that Mg clays have greater net negative charge (more negative zeta potential value) and smaller mean particle size than Ca clays at similar conditions. Nevertheless, zeta potential for Mg and Ca clays are >-30 mV, suggesting Mg has a flocculative effect, rather than a dispersive effect, on the soil-clay suspensions, whereby these are the same forces that would enact within an aggregated system; i.e. irrespective of Mg concentrations within the pH range of this study, the zeta potential results indicate that an aggregated system would be expected.

Permeability is one of the most important indicators of soil structural stability. In exploring the effect of Mg on saturated hydraulic conductivity (Ksat), four contrasting soils were leached with a range of concentrations of Mg solutions. When the electrolyte concentration of leaching solution was below the critical flocculation concentration (CFC) of the soil clays, the Ksat of all soils reduced rapidly. However, the extremely low turbidity (<15 NTU) of leachate indicated Mg caused negligible clay dispersion (<0.016% dispersed clay of soil). There was no visual dispersion occurring in the aggregate stability test on Mg soil aggregates. As compared to Ca, Mg has a disaggregation effect on soil structural stability by reducing Ksat and increasing desorption of dissolved organic carbon from the clay surface. The increased disaggregation in Mg systems, due to enhanced inter-crystalline and intra-crystalline swelling, was the identified hydraulic reduction mechanism, meaning clay domains remain non-dispersive.

The Cation ratio of soil structural stability (CROSS) and exchangeable dispersive percentage (EDP) have been recently proposed to replace the traditional sodium absorption ratio (SAR) and exchangeable sodium percentage (ESP) for predicting soil structural stability. CROSS and EDP have incorporated the dispersive coefficients a[K]=0.556 and c[Mg]=0.037 and flocculative coefficient b[Mg]=0.60, transforming dispersive effect of K and Mg to equivalent Na as well as the flocculative effect of Mg to equivalent Ca. However, these coefficients, a[K]=0.556, b[Mg]=0.60 and c[Mg]=0.037, in the generalised CROSS and EDP are derived from the flocculating power of Na, K, Mg and Ca based on a limited number of soil clays. To further explore the effect of Mg on different soil types and re-examine the coefficients a[K], b[Mg] and c[Mg], the CFC of seventeen contrasting soils were studied. Significant differences (P<0.05) in the CFC values were found for the same homoionic clay system, i.e. Na, K, Mg and Ca. The dispersive coefficients of a[K] and c[Mg] and flocculative coefficient of b[Mg] were calculated for each soil. These derived coefficients were further used in calculating specific CROSS and specific EDP. Through a sensitivity simulation, both the variation in the coefficient a[K] and b[Mg] for studied soils were found to significantly affect the absolute error between specific and generalised CROSS. In regards to EDP, the dispersive coefficient a[K] proved to be playing the primary role in determining the difference between the specific and generalised EDP, while the dispersive coefficient c[Mg] had negligible impact in predicting soil structural stability. The work recommended the removal of the Mg term from the EDP, and reinforced that the soil-specific coefficients should be used in the determination of soil structural relations.

The different effects between Mg and Ca on soil dispersive behaviour are due to their inherent difference. Despite both Ca and Mg being divalent valence, the covalency degree of Ca-clay bond was stronger than that of Mg-clay, in all circumstances, as indicated by ionicity indices. This mechanism was confirmed by the higher dispersed clay (%), more negative zeta potentials, and smaller particle size of Mg clays. Consequently, the ionicity index, which describes the ionicity (and covalency) degree of the clay–cation bond, together with solution pH, electrical conductivity, zeta potential and particle size has been used to generate a model to predict the dispersive behaviour of clay suspensions. This pedotransfer function is promising, but requires more work for general use and should be the focus of future investigations in this field of study.