Fractionation and Availability of Calcium and Boron in Lateritic Soils of Kerala, India

Abstract

This study investigated the fractionation patterns of calcium and boron in lateritic soils across four agro-ecological units of Kerala to assess their availability in relation to soil properties. The laboratory-based fractionation study with sequential extraction procedures was conducted at the Department of Soil Science and Agricultural Chemistry, College of Agriculture, Kerala Agricultural University, Vellanikkara, between 2023 and 2024. Forty soil samples from four lateritic agro-ecological units (AEU 8, 9, 10, and 11) of Kerala were analyzed. Calcium was fractionated using Mokwunye and Melsted (1972) method into water-soluble, exchangeable, organic complexed, acid soluble, and mineral-bound forms. Boron was fractionated following Datta et al. (2002) into readily soluble, specifically adsorbed, oxide-bound, organically bound, and residual forms. Correlation analysis was performed between fractions and soil properties. Calcium fractions ranged from 3.57 to 11.33 mg kg⁻¹ for water-soluble, 298.71 to 1895.43 mg kg⁻¹ for exchangeable, 18.59 to 268.53 mg kg⁻¹ for organic complexed, 68.74 to 195.98 mg kg⁻¹ for acid soluble, and 89.64 to 635.01 mg kg⁻¹ for mineral calcium. Total calcium varied from 348.33 to 3425.44 mg kg⁻¹ across AEUs. The contribution of calcium fractions followed the order: Ex. Ca > Min-Ca > Ac-s. Ca > Or-c Ca > WS. Ca. Boron fractions ranged from 0.41 to 1.86 mg kg⁻¹ for readily soluble, 0.12 to 1.83 mg kg⁻¹ for specifically adsorbed, 0.05 to 0.59 mg kg⁻¹ for oxide-bound, 0.41 to 2.87 mg kg⁻¹ for organically bound, and 44.42 to 90.23 mg kg⁻¹ for residual boron. Residual boron constituted the largest pool (54.98-60.08 mg kg⁻¹ across AEUs). All calcium fractions showed highly significant positive intercorrelations (P < .001). Available boron fractions (RS-B, SA-B, OX-B, OR-B) exhibited strong negative correlations with soil pH (r = -0.78 to -0.97, P < .001) and positive correlations with available iron (r = 0.72 to 0.91, P < .001) and exchangeable aluminum (r = 0.74 to 0.92, P < .001). A distinct antagonistic relationship was observed between calcium and available boron fractions, with exchangeable calcium showing the strongest negative correlation with all available boron forms (r = -0.60 to -0.76, P < .001). Conversely, residual boron and total boron exhibited highly significant positive correlations with all calcium fractions (r = 0.52 to 0.83, P < .001). The dominance of exchangeable and mineral-bound calcium fractions indicates moderate calcium availability in Kerala lateritic soils. The predominance of residual boron fraction with strong pH-dependent availability suggests potential risk of boron deficiency under acidic conditions typical of lateritic soils. The strong negative correlations between calcium and available boron fractions reveal significant Ca-B antagonism, necessitating balanced fertilization strategies including integrated organic matter management and split application of amendments. Soil pH, organic carbon, clay content, and exchangeable aluminum emerge as key factors governing calcium and boron fractionation patterns. These findings provide a scientific basis for developing AEU-specific nutrient management recommendations for lateritic soils of Kerala.