Effects of lime and wood ash on yields and cadmium uptake by red and green amaranth.

Abstract

Cadmium (Cd) contamination in agricultural soils threatens food safety particularly in the leafy vegetables such as amaranth which accumulate heavy metals in the edible tissues. This study compared the effectiveness of agricultural lime and wood ash in reducing Cd bioavailability and uptake by red and green amaranth varieties. A pot experiment was conducted at Makerere University Agricultural Research Institute, Kabanyolo (MUARIK) using two Amaranth varieties under four treatments with two replicates: control, agricultural lime, 5t/ha wood ash and 10t/ha wood ash. Soil chemical properties, plant biomass, plant height and total and bioavailable Cd were measured. Both agricultural lime and wood ash significantly reduced shoot Cd concentration compared to the control. The highest shoot cadmium concentration occurred in the control 0.101mg/kg, while lime treated soils recorded 0.036mg/kg, 5t/ha wood ash 0.038mg/kg and 10t/ha wood ash the lowest value at 0.033mg/kg. Bioaccumulation factors (BAF) followed the same trend, decreasing from 1.04 in the control to 0.558 in lime and reaching the lowest values under 5t/ha and 10t/ha wood ash. Bioavailable soil Cd also declined sharply with amendment use, from the highest levels in the control to as low as 0.032mg/kg in the lime treatment. Wood ash notably enhanced soil fertility, increasing exchangeable Mg to 5.82cmolc/kg, K to 1.098cmolc/kg and available P to 10.326mg/kg in the 10t/ha treatment. This led to higher biomass with fresh shoot weight rising from 40.4g (control) to 71.6g and fresh root weight from 4.27g to 8.13g. Plant height also increased consistently with the tallest plants observed under 10t/ha wood ash. Agricultural lime improved soil PH and reduced Cd availability but produced lower biomass gains due to limited nutrient provision. No significant varietal differences were observed between the red and green amaranth varieties in the Cd uptake or growth responses. These findings demonstrate that wood ash, especially at higher application rates, is a highly effective, low-cost amendment for immobilizing Cd while enhancing soil fertility and crop performance, offering a practical solution for managing Cd-contaminated acidic soils. Further research is recommended to assess the long-term stability of Cd immobilization and the purity of wood ash sources to ensure safety in agricultural applications.