A kinetic method is described and investigated for determining exchangeable A1 in acid soils using M NH,Cl solutions the pH and leaching rate of which was critical for obtaining accurate and reproducible values. Molar concentrations of ammonium acetate displaced at least part of the A1 chelated to functional groups in the soil organic matter.The adsorption kinetics of K and A1 from I O -~ M chloride solutions on the NH,-saturated forms at the original soil pH of these, under leaching conditions similar to the 'extraction' method, showed that, when the atom ratio K/A1 > I in the equilibrating solution, initially more K was adsorbed than at equilibrium, although A1 did not 'over-equilibrate' when K/AI < I. At least 24 h were required for attaining equilibrium in K:AI equilibria work with soils and clays.The kinetics of desorption (extraction method) and adsorption of K and A1 obeyed the first-order and parabolic rate respectively. This is attributed to the large difference in anion concentrations in the two methods.

Tea seedlings were grown for up to I 0 months in the glasshouse in an acid soil, to which solutions of varying KIA1 concentration rdlios Were given weekly. During growth, the % K in the leaf dry matter increased with the labile K concentration but was not influenced by the A1 concentration in the soil.However, the largest level of A1 in soil decreased plant growth and % Kin the dry matter. A f this level of A1 in soil, plant height, number of leaves, dry matter and % K in the dry matter increased linearly with the K given to the soil. The % A1 in the plant was not aflected by the labile K and A1 contents of the soil.A highly significant linear relationship was observed between A1 and P uptake by parts of the plant and by the whole plant, even though the soil was not treated with P as a basal nutrient. The Al/P ratios in the first mature leaf, total leaves andstems were 3 . 4 , 3 . 0 and 0.8, respectively, after 6 months growth. Uptake mechanisms are proposed to explain this.

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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