Phosphate rock dissolution and transformation in the rhizosphere of tea (Camellia sinensis L.) compared with other plant species

Tea (Camellia sinensis L.) is generally grown in highly weathered acidic Ultisols of the humid tropics. The low pH, large P fixing capacity and moisture content of these soils favour the dissolution of phosphate rock. Plant species differ widely in their ability to take up P from phosphate rock, and we have compared phosphate mobilization in the rhizosphere of tea with that under calliandra (Calliandra calothyrsus L.), Guinea grass (Panicum maximum L.) and bean (Phaseolus vulgaris L.) by studying the changes in the concentration of P fractions at known distances from the root surface in an acidic (pH in water 4.5) Ultisol from Sri Lanka treated with a phosphate rock. Plants were grown in the top compartment of a two-compartment device, comprising two PVC cylinders physically separated by a 24-μm pore-diameter polyester mesh. A planar mat of roots was formed on the mesh in the top compartment, and the soil on the other side of the mesh in the lower compartment was cut into thin slices parallel to the rhizosphere and analysed for pH and P fractions. All plant species acidified the rhizosphere (pH water] difference between bulk and rhizosphere soils was 0.17-0.26) and caused more rock to dissolve in the rhizosphere (10–18%) than in the bulk soil (8–11%). Guinea grass was most effective, though the rate of acidification per unit root surface area was least (0.02μmol H⁺ cm^?2) among the four species. Tea produced the largest rate of acidification per unit root surface area (0.08μmol H⁺ cm⁺²). All species depleted P extracted by a cation–anion exchange resin and inorganic P extracted by 0.1 M NaOH. All except tea depleted organic P extracted by 0.1M NaOH in the rhizosphere. The external P efficiencies (mg total P uptake) of Guinea grass, bean, tea and calliandra in soil fertilized with phosphate rock were 4.82 ± 0.42, 4.02 ± 0.32, 1.06 ± 0.02 and 0.62 ± 0.02, respectively, and the corresponding internal P efficiencies (mg shoot dry matter production per mg plant P) were 960 ± 75, 1623 ± 79, 826 ± 33 and 861 ± 44. This study showed that the various crops cultivated in tea lands differ in their rates of acidification, phosphate rock dissolution and P transformation in the rhizosphere. This requires testing under field conditions.