ROLE OF ROOT DERIVED ORGANIC-ACIDS IN THE MOBILIZATION OF NUTRIENTS FROM THE RHIZOSPHERE

The role of organic acids in the mobilization of plant nutrients from the rhizosphere was assessed in seven contrasting soil types. The resu lts indicated that malate was poor at mobilizing micronutrients from a ll the test soils, whilst citrate was capable of mobilizing significan t quantities. Citrate was also capable of mobilizing P from one soil w hich possessed a large Ca-P fraction. This mobilization of P was due t o both the complexing action of the citrate anion and due to the disso lution properties of the protons released from citric acid upon equili brium with the soil solution. The reaction of citrate with cations was found to be near instantaneous with significant absorption to the sol id phase in some soils at low concentrations. Soil decomposition studi es indicated that citrate was rapidly broken down in organic soils but was more resistant to degradation in subsoil horizons. It was conclud ed that organic acids can be expected to be of little consequence in n utrient mobilization from high pH soils, whilst in acid soils they may be involved both in a more general mechanism for micronutrient uptake or as a potential Al detoxification mechanism.x