Abstract
Phosphorus Sorption Characteristics of Red Soils from Lampung, West- and Central Java (S.A. Siradz and R.J. Gilkes): Red soil is a common name used to identify any soil showing yellowish to reddish colour, these soils mainly occur in tropical and subtropical regions. The term is frequently used to convey an image of red, infertile, acid soils in the tropics and is associated with tropical and subtropical climates which are characterized by high temperature and humidity. Major soil constraints to crop production may include low pH, low available P and high P fixation capacity, toxicity due to Al and sometimes Mn and Fe, deficiencies of N, Ca, Mg, K, and micronutrients e.g. Mo, Zn and Cu, low CEC and low base saturation, and very low content of weatherable minerals. The aim of this study was to determine the phosphorus sorption characteristics of red soils from Lampung, West Java and Central Java. Soils were sampled at depth of 0-20 cm from minimally disturbed sites after discarding the uppermost few cm consisting of an accumulation of litter. The samples were air-dried, crushed using a ceramic mortar and pestle and then passed through a 0.5 mm stainless steel sieve. Measurements of P sorption were conducted by equilibrating 200 mg of air dry soil (<0.5 mm) in 20 mL 0.2 M KCl containing 0-30 mg PmL-1. P concentrations in filtrate were determined by the molybdate blue method. Phosphorus sorption data for soils were fitted to simple Langmuir and Freundlich isotherm equations and both equations described the data equally well. Langmuir P sorption maximum (xm) for soil ranged from 719-2747 µg Pg-1, with median values of 1.825 µg Pg-1. There were systematic decreases in the values of Langmuir sorption maximum (xm) for soils from Lampung > West Java and Central Java. Values of pH(NaF), Ald, and exchangeable Ca were most predictive of P sorption in these soils.
Highlights
Red soils occupy about 29% of arable land in Indonesia and more than 80% of them are on flat to undulating areas where agricultural expansion is most favourable
Each isotherm was characterized by a large increase in the amount of P sorbed for equilibrium solution
The shapes of isotherm curve for each samples were remarkably similar despite the great differences in the amount of P sorbed at a particular P concentration in solution
Summary
Red soils occupy about 29% of arable land in Indonesia and more than 80% of them are on flat to undulating areas where agricultural expansion is most favourable. A major problem of these soils for plant growth is severe to high deficiency of P. Severe deficiency of P and the low eficiency of P fertilization is generally attributed to fixation of P by iron and aluminium oxides sensu lato. The iron and aluminium oxides are capable of adsorbing much phosphate on a unit weight basis but are only minor constituents of the soils whereas alumino-silicate clay is a major constituent and may provide an important contribution to P sorption. Further it has been proposed that the mechanism for sorption was the same for the various clay minerals and there are two proposed sorption mechanisms: exchange of phosphate ions with the hydroxyl group linked to the gibbsite layer, and as exchangeable anions which counter a positive charge developed by the adsorption of protons. At low concentrations P was sorbed in accordance with the Langmuir adsorption isotherm for all exchange cations and clay minerals. Clay saturated with divalent ions sorbed more P than clay saturated with monovalent ions
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