Exchangeable aluminum as a measure of lime requirement of Ultisols and Alfisols in humid tropical Western Ethiopia

Abdenna Deressa, Markku Yli-Halla, Muktar Mohamed and Lemma Wogi

Net Journal of Agricultural Science
Published: September 18 2020
Volume 8, Issue 3
Pages 46-58

ABSTRACT

Acid soils with pH-H₂O < 5.5 are widely distributed in humid tropical Western Ethiopia. Liming is used to amend soil acidity and exchangeable aluminum (Al_ex) toxicity through precipitating Al_ex and increasing pH. Liming requirement (LR) for humid tropical Western Ethiopia on the basis of KCl extractable Al_ex has not yet been established but there is a strong need for this recommendation as a rapid and reliable method for reclaiming soil acidity. The objectives of this study were to determine LR of Ultisols and Alfisols, establish relationship between LR and Al_ex, and further verify the results through a greenhouse pot experiment. Soil samples were collected from Ultisols and Alfisols from 0 to 20 cm depth. Seven samples with Al saturation ranging from 7 to 75% and pH from 4.7 to 5.2 were screened and advanced to a 60-day incubation experiment. The LR was determined from intercepts of regression equations of lime (CaCO₃) rate versus Al_ex. The LR ranged from 1.72 to 9.24 cmolc CaCO₃ kg^-1 which is equivalent to 1.54 to 11.41 ton CaCO₃ ha^-1. These LR recommendations contradict with the 3 ton CaCO₃ ha^-1 blanket recommendation developed 35 years ago. The LR and Al_ex of experimental soils fit to an inverse quadratic relationship, where the LR (i.e., CaCO₃ needed) and Al_ex are expressed as cmolc kg^-1 soil. Aluminum saturation becomes zero at pH 5.1 to 5.2. The linear model of LR is expressed as LR = 1.47∗ Al_ex (cmolc CaCO₃ kg^-1 soil), (R² = 0.72), and the quadratic one as LR = 2.56*Alex-0.19∗ (𝐴𝑙ex)² (cmolc CaCO₃ kg^-1 soil) (R² = 0.90). The quadratic model fitted well with results of LR of previous studies and predicted properly LR of test soil used for verification. The LR of Ultisols and Alfisols of humid tropical Western Ethiopia can be reliably predicted by use of this quadratic model of LR determination on the basis of exchangeable Al.

Keywords: Aluminum toxicity, laboratory incubation, linear and quadratic models.

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