Phosphorus-Sorption Characteristics and Phosphorus Buffer Coefficients of Some Important Soils in Lao PDR

Abstract

An estimated 97 percent of the soils in Laos are characterized by low phosphorus (P). This characteristic, together with high acidity, constrains food crop production. The P status, sorption, and associated properties were evaluated for fifteen important agricultural soils from the uplands. Soil pH values ranged from 4.5 to 5.9. Soil organic carbon (C) varied from 7.0 to 22.9 g kg⁻¹. Soil clay varied from 179 to 709 g kg⁻¹. The cation exchange capacity (CEC) also varied from 4.30 to 32.1 cmol_c kg⁻¹. Extractable P levels of thirteen of the fifteen soils were P deficient with medium to very high P sorption, indicating substantial fertilizer P requirements. Dithionite and oxalate aluminum and iron predicted P sorbed at 0.2 mg P L⁻¹. The extractable P increase per unit added P, P buffer coefficient (PBC), was low, also indicating high P sorption. Field studies are needed to confirm predictions of P requirements.

Keywords:

• Highly weathered soil
• phosphorus buffer coefficient
• phosphorus sorption curves

Acknowledgments

The authors deeply thank both Lao and Thai colleagues, especially Nivong Sipaseuth and Khonpany Dounphady for their help in soil selection and soil sampling and Sunan Kunaporn for great help with the classification of the selected soils in this study.