Estimating cation exchange capacity and clay content from agricultural soil testing data

Mari Räty,Markku Yli-Halla,Riikka Keskinen,Juha Hyvönen,Helena Soinne

doi:10.23986/afsci.111107

Abstract

Clay content and the ability to reversibly retain cations affect many essential chemical and physical properties of soil, such as pH buffering and carbon sequestration. Cation exchange capacity (CEC) and base saturation are also commonly used as criteria in soil classification. However, determination of CEC and particle-size distribution is laborious and not included in routine soil testing. In this study, pedotransfer functions including soil test cations (STCat; Ca2+ + Mg2+ + K+), pH and soil organic carbon (SOC, %) as explanatory variables were developed for estimating CEC, titratable acidity (TA; H+ + Al3+) and clay content (clay, %). In addition, reference values for potential CEC and its components were determined for Finnish mineral and organic soils. The mean of potential CEC extracted by 1 M ammonium acetate at pH 7.0 ranged from 14 (range 6.4−25) in coarse soils to 33 (21−45) cmol(+) kg-1 in heavy clay soils, and from 42 (24−82) in mull soils to 77 (25−138) cmol(+) kg-1 in peat soils. The average CEC of clay and SOC were 27 and 160 cmol(+) kg-1, respectively. Titratable acidity occupied 53% and around 40% of the CEC sites in organic and mineral soils, respectively, evidencing that it is a prominent component of the potential CEC in these predominantly acidic soils. STCat, pH and SOC explained 96% of the variation in potential CEC. STCat and pH can be used in estimating the clay content especially for soils containing over 30% clay. In coarse textured soils, in contrast, SOC hampers the STCat based estimation of clay content.

Highlights

Cation exchange capacity (CEC) is one of the basic properties commonly reported when characterizing soil in a scientific context
Pedotransfer functions were developed for predicting the soil clay content, potential CEC and titratable acidity (TA)
The estimation of potential CEC was based on 148 mineral and 67 organic soil samples collected from plough layers from various parts of Finland, which were further grouped into different soil types

Summary

Introduction

Cation exchange capacity (CEC) is one of the basic properties commonly reported when characterizing soil in a scientific context. Bache 1976, Ross and Ketterings 1995) This phenomenon is linked to soil nutrient stock and supply, mobility of elements and pH buffering, all of which are important factors in agricultural land use. Predicting the CEC value from more readily available soil data via pedotransfer functions is a credible alternative for direct laboratory measurement (McBratney et al 2002). Clay and organic matter contents explain the majority of variability in the CEC values in many studies (e.g. Bell and van Keulen 1995, Parfitt et al 1995, Krogh et al 2000, Fooladmand 2008, Sulieman et al 2018, Mishra et al 2019). Due to spatial variation in clay mineralogy and composition of organic matter, different pedotransfer functions are needed in different regions (Seybold et al 2005)

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