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Effect of Lime on Exchangeable Magnesium in Variable Surface Charge Soils¹

ABSTRACT

Early work with commercial-grade limestones containing small quantities of MgCO₃ (2–10%) demonstrated that exchangeable Mg was reduced as the soil pH was increased in several soils. An incubation experiment was conducted to more precisely determine the extent of the Mg "fixation" and to evaluate several hypotheses regarding the mechanism behind this pH-dependent phenomenon.

Five acid topsoils were used in this study. The clay fractions of four of these were dominated by constant-surface potential colloids. Five levels of lime (CaO) and three levels of Mg from two sources (MgCO₃ and MgSO₄) were applied in complete factorial combination. Exhaustive extraction with 0.2N NH₄Cl was used to determine exchangeable Mg and K. Cation exchange capacities were obtained by extracting the NH₄⁺ saturated samples with 0.2N KNO₃.

The Mg-fixation reaction was essentially complete in 24 hours. Lime application reduced NH₄Cl-extractable Mg in all soils, although not until the soil pH was raised to 5.1 (1N KCl). Effective CEC increased with lime addition but was unaffected by Mg treatment. It appears that the Mg was lost to a surface-solid phase involving Al. For the soils with clay fractions dominated by constant-surface potential colloids, Mg recovery by NH₄Cl was inversely related to the quantity of exchangeable Al per unit mass of clay found in the acid-end member of each soil. The lack of change in effective CEC and exchangeable K with Mg addition at higher soil-pH values indicated that a charge blocking brucite-like interlayer was not formed.

¹ Contribution from the Agronomy Dep., College Stn., Univ. of Georgia, Athens, GA 30602. Part of a dissertation submitted by the senior author in partial fulfillment of the requirements for the Ph.D. degree. Presented in part before Div. S-4, Soil Sci. Soc. Am., Aug. 1979, Ft. Collins, Colorado.

² Graduate Research Assistant, Professor, and Visiting Professor, Agronomy Dep., Univ. of Georgia, Athens, GA 30602, respectively. Address of third author is Dep. of Soil Sci., Massey Univ., Palmerston North, New Zealand.

Received for publication October 7, 1980. Accepted for publication January 14, 1981.