Aluminum and Iron Toxicity Mitigation Strategies for Sustainable Pineapple Cultivation on Acid Sulfate Soils

Abstract:

Can toxic concentrations of aluminum and iron in acid sulfate soils be lowered enough to support commercially viable pineapple orchards? Acid sulfate soils, characterised by pH values below 4.0 and exchangeable aluminum exceeding 2 cmol kg⁻¹, pose a persistent barrier to root expansion and nutrient acquisition in Ananas comosus. This research assessed six amendment strategies dolomitic lime at 2tanα ⁻¹, rock phosphate at 1.5 t ha⁻¹, gypsum at 1tana⁻¹, rice-husk biochar at 5tana⁻¹, a lime-plus-biochar combination, and an unamended control arranged in a randomised complete block design with four replications on a Typic Sulfaquept near Chiang Mai, Thailand, from March 2020 to September 2021. Soil pH, exchangeable Al³⁺, extractable Fe²⁺, root colonisation by arbuscular mycorrhizal fungi (AMF), and above-ground biomass were recorded at 3, 6, 12, and 18 months after transplanting. The lime-plus-biochar treatment raised soil pH from 3.47 to 5.31, cut exchangeable A | ³⁺ by 74.1 % (from 8.34 to 2.16 cmol kg⁻¹), and lowered extractable Fe²⁺ by 65.4 %. Root AMF colonisation in the combined treatment reached 68.3 %, compared with 21.7 % in the control. Above-ground dry biomass at 18 months was 2.87 kg plant⁻¹ in the combined treatment versus 1.14 kg plant⁻¹ in the control. These outcomes point to a synergistic liming-biochar approach as a practical, cost-efective path toward rehabilitating acid sulfate soils for pineapple production in tropical lowlands.