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 acid ic Ultisols of the humid tropics. The low pH, large P fixing capacity and moisture content of these soils favour the dissolution of phosphat e rock. Plant species differ widely in their ability to take up P from phosphate rock, and we have compared phosphate mobilization in the rh izosphere 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 d istances 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 cylin ders physically separated by a 24-mu m pore-diameter polyester mesh. A planar mat of roots was formed on the mesh in the top compartment, an d 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 a nd P fractions. All plant species acidified the rhizosphere (pH [water ] difference between bulk and rhizosphere soils was 0.17-0.26) and cau sed 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 aci dification per unit root surface area was least (0.02 mu mol H+ cm(-2) ) among the four species. Tea produced the largest rate of acidificati on per unit root surface area (0.08 mu mol H+ cm(-2)). All species dep leted P extracted by a cation-anion exchange resin and inorganic P ext racted by 0.1 M NaOH. All except tea depleted organic P extracted by 0 .1 M NaOH in the rhizosphere. The external P efficiencies (mg total P uptake) of Guinea grass, bean, tea and calliandra in soil fertilized w ith phosphate rock were 4.82 +/- 0.42, 4.02 +/- 0.32, 1.06 +/- 0.02 an d 0.62 +/- 0.02, respectively, and the corresponding internal P effici encies (mg shoot dry matter production per mg plant P) were 960 +/- 75 , 1623 +/- 79, 826 +/- 33 and 861 +/- 44. This study showed that the v arious crops cultivated in tea lands differ in their rates of acidific ation, phosphate rock dissolution and P transformation in the rhizosph ere. This requires testing under field conditions.