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Ion Exchange in Soil-Plant Root Environments: III. The Use of Mg²⁸ for the Measurement of Magnesium Absorption by Plants¹

ABSTRACT

The availability of the Mg²⁸ isotope having a half-life of 21.3 hours has presented the possibility of its use in the measurement of the available magnesium in soils. Data are presented to show the relative proportions of fertilizer and soil magnesium adsorbed by cation-exchange resin sheets over time periods up to 96 hours. The percentage of magnesium adsorbed from the tagged magnesium fertilizer remained relatively constant for exchange periods from 48 to 96 hours; being of the order of 76.6, 57.8, and 53.9% for soils with 50, 150, and 250 pounds per acre of exchangeable magnesium, respectively.

Four-week-old corn plants adsorbed adequate Mg²⁸ during a 96-hour period for radioactive analysis of the nonadsorbed, adsorbed magnesium on corn roots, and permitted the analysis of the absorbed magnesium in the roots and tops. Increasing the rate of magnesium fertilizer increased the percentage of magnesium derived from the fertilizer in each of the different stages of nutrient uptake. Conversely, at any one fertilization rate, increasing the level of soil magnesium reduced the percentage of magnesium derived from the applied fertilizer.

These data indicate that the Mg²⁸ isotope is usable up to periods of at least 7 half-lives for nutrient uptake and cation-exchange studies.

¹ Published with the permission of the Director of the Arkansas Agr. Exp. Sta. Presented before Div. IV, Soil Science Society of America, Atlanta, Ga., Nov. 21, 1957. This investigation was largely supported by a grant from the International Minerals & Chemicals Corp.

² Associate Agronomist, University of Arkansas; and former Graduate Assistant now with Agronomy Department, Texas A. & M. College, respectively.

Received for publication January 10, 1958. Accepted for publication March 25, 1958.