The aluminon method of Hsu for the determination of aluminium in soil has been examined for interference from the presence of iron and copper. Only iron(II1) interfered. The use of thioglycollic acid not only eliminated this interference but bleached the colour of the aluminon reagent and caused a reduction in sensitivity. Ascorbic acid, however, eliminated it without bleaching the colour of the reagent, the addition of 1 ml of a 0.50 per cent. solution of ascorbic acid suppressing the interference from up to 500p.g of iron(II1). When 200 pg of copper were added to aluminium standards no interference occurred.
Root exudate of young tea plants contained appreciable quantities of malic acid. Malic acid was capable of solubilising P, Fe and A1 from rock phosphate fertiliser. Tea soils incubated with added malic acid released appreciable quantities of P, Fe and Al. In tea soils where most of the P added as the rock phosphate is immobilised as the iron and aluminium phosphates, malic acid may prove to be useful in chelating these elements and so releasing the phosphates for utilisation by the tea plant.
IntroductonTea plants thrive on acid soils and most of the soils in which tea is grown are predominantly kaolinitic. Field experiments have shown that tea plants did not respond to applications of rock phosphate beyond 33.6 kg P~O~/hectare per annum. Subsequent fractionation of the soil for phosphorus carried out by the method of Chang and Jackson1 indicated that most of the phosphorus applied as the rock phosphate fertiliser was fixed as the relatively insoluble iron and aluminium phosphates.Many investigators have reported that certain organic substances and particularly hydroxylated organic acids normally produced in soils by the action of microorganisms are effective agents in releasing phosphates that are rendered insoluble as the iron and aluminium phosphates. Garretsen2 showed that insoluble phosphates were solubilised in the rhizosphere either by the action of microorganisms or plant root exudates.Malic acid has been detected in tea roots.3 The object of this study was to investigate whether malic acid was capable of releasing the phosphates rendered insoluble as the iron and aluminium phosphates so that the plants can utilise the phosphates so released.
Materials and methodsOne year old tea plants of the clone TRI 2025 established in polythene bags were used. The roots were cleaned and washed thoroughly with demineralised water. The plants with their root systems fully immersed were allowed to remain in 250 ml Erlenmeyer flasks containing 100 ml demineralised water for 48 h in the laboratory at room temperature (23°C). At the end of this period, the plants were removed and the water 1895
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