Reduction of Phosphate Adsorption by Ion Competition with Silicate in Soil

Abstract

287 levels of solution P following addition of Si to soils high in accumulations P result from replacement of P by Si 9). The enhancement of P availability would be a key measure in reducing the amount of phosphate fertilizer and consequently decreasing the P accumulation in the soil. Although many researchers have shown that silicate competes with phosphate for adsorption sites on soil, however, the quantitative information about the efficiency of Si in reducing P adsorption as function of pH is limited. The objectives of this study were to determine the effect of Si concentration as functions of pH (5.0-9.0) and the order of P and Si addition on P adsorption on soil. MATERIALS AND METHODS Soil selection and analysis Samples (0-15 cm) collected from an unfertilized alpine soil were air-dried and crushed to pass a 0.5-mm sieve. The physical and chemical properties of the soil were determined as follows: pH (1:5 water extraction), organic matter content by the Walkley and Black method as described by Allison 10) , available P content (5 g of soil were extracted with 20 ml of 0.33 M CH3CHOOH, 0.15 M lactic acid, 0.03 M NH4F, 0.05 M (NH4)2SO4 and 0.2 M NaOH at pH 4.25) 11). Ammonium acetate (pH 7.0)-exchangeable Ca, Mg, and K were determined by atomic absorption spectrophotometry. Particle size distribution was analyzed by the pipette method 12) and cation exchangeable capacity (CEC) was determined by the 1 N CH3COONH4 method 11). Oxalate-extractable iron and Al oxide contents were measured using the McKeague and Day 13) method. The surface area of the < 2 m-fraction was measured by sorption of μ N2 at liquid N2 temperatures 14). Total P was determined after digestion with conc. HClO4 using the ascorbic acid method 15) The soil selected was of silt loam texture (SL, 21% clay, 61% silt and 18% sand). Available P content (4.0 mg kg-1) was below the optimum range (130-220 mg kg-1) for plant cultivation 1) and indicated a virgin soil not affected by fertilization (Table 1). The content of total P was 325 mg kg-1 and water soluble P was below the detection limit by the ascorbic acid method 15). P and Si single adsorption at different pHs Silicate adsorption studies were carried out at pH 5.0, 7.0 and 9.0 using 1 g soil samples that were placed in 50 ml polypropylene centrifuge tubes with 20 ml of silica at concentrations in the range of 0 ~ 2.3 mM Si L-1 using Na2SiO3 in 0.1 M NaCl, whose pH was previously adjusted with 0.1 M HCl or 0.5 M NaOH. Phosphate adsorption was characterized at concentrations in the range of 0 ~ 5.0 mM P L-1 with KH2PO4 in 0.1 M NaCl, whose pH was previously adjusted with 0.1 M HCl or 0.5 M NaOH. The suspensions were shaken for 12 h at 20°C on a reciprocating shaker. P and Si competitive adsorption at different pHs The experiments on the competition between P and Si were carried out at pH 5.0, 7.0, and 9.0 (i) by adding to the soil mixtures of P (0~5 mM L-1) and Si (0, 0.5, 1.0 and 2.0 mM L-1) and (ii) by adding 0 to 5 mM P L-1 to the soil equilibrated previously by each concentration of Si (0, 0.5, 1.0, and 2.0 mM L-1