Organic acids behavior in calcareous soils
Organic acids have been hypothesized to be important in mobilizing nutrients in calcareous soils, however, evidence to support this was lacking. This thesis investigates the effect of increasing concentrations of H-citrate, K-citrate, H-oxalate and K-oxalate on the solubility and uptake of P, Fe, Mn, Zn and Cu in plants growing in calcareous soils. In this study, the fate and capacity of organic acids as mechanisms for solubilizing mineral nutrients was investigated by measuring the sorption, and biodegradation reactions of citrate and oxalate in soil. This study has shown that the sorption reactions of oxalate and citrate in soil are rapid and that the degree of sorption is organic acid specific and to a lesser extent soil type specific. In addition, the amount of organic 'acid sorption was strongly dependent upon the cation balancing the organic acid anion (i. e. H+ or K+). I also demonstrated that CaCO3 is a binding site for oxalate and to a lesser extent citrate in soil. The biodegradation of oxalate and citrate was fast in soil although citrate tended to be mineralized faster. When investigating the sorption and desorption reactions of organic acids in calcareous soils, the sorption of oxalate was much greater that of citrate. The amount of organic acid desorbed from the solid phase was dependent on ionic strength and composition of the desorption solution. In comparison to citrate, more oxalate could be desorbed from the soil, depending upon the initial amount of oxalate added to the soil. In addition, the test soils were extracted with organic acid solutions of different concentrations for different time periods and the amount of Fe, Mn, Zn, Cu, P and Ca mobilization into solution measured. The results showed that the presence of organic acids significantly affects the concentration of metals and anions in soil solution. Generally, organic acid concentrations >1 mM were required to mobilize nutrients into solution. Citrate tended to be more effective than oxalate at mobilizing nutrients from the soil. In most cases the mobilization was not transient and elevated levels of nutrients were seen in solution for up to 6 h after addition of the organic acids. The effects of three different organic acid application methods on the availability and uptake of P and micronutrients in wheat was also investigated. The study showed that wheat plants treated with organic acids accumulated significantly higher concentrations and amounts of micronutrients than the untreated control plants. Foliar application of organic acid without any Zn treatment proved the most effective method of increasing Fe uptake into wheat leaves. The effects of organic acids on the availability and uptake of P from the wheat rhizosphere was studied to determine the importance of organic acid type and concentration on 33P uptake by shoots. The results showed that elevated concentrations of organic acids in the rhizosphere significantly enhanced shoot 33P accumulation, while citrate was much less effective at stimulating 33P uptake. In the absence of microorganisms, both organic acids were capable of mobilizing P from the soil particularly at concentration >1 mM. Citrate mineralization by soil microorganisms proceeded at a higher rate than oxalate mineralization. Overall, the presence of organic acids significantly affects the concentration of metals in soil solution and this mobilization is dependent upon the type of organic acid, its ionic form, its concentration, reaction time and the type of soil. The response of wheat plants to different methods of organic acid addition proved variable. Organic acids did result in an increase in metal mobilization and plant uptake. In particular, oxalate resulted in a significant enhancement in plant P. This work presents evidence to show that organic acids may be important in nutrients acquisition in calcareous soils.