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Availability of Exchangeable and Nonexchangeable Mg in Sandy Coastal Plain Soils¹

ABSTRACT

The chemical forms of Mg in five sandy Coastal Plain soils were determined. The total Mg ranged from 0.73 meq. to 2.48 meq/100g. The percentage of the total Mg in the exchangeable form varied from 3.8 to 9.4% and generally increased as the total increased. Soils containing an expansible lattice clay had the highest Mg content per gram of clay.

Consecutive extractions with 1N NaOAc solution of pH 1 removed almost twice as much Mg as consecutive extractions with a solution of 1N NaOAc pH 7. This indicates that a portion of the nonexchangeable Mg is quite soluble in a dilute acid. The soils which released the highest amount of nonexchangeable Mg per gram of clay also were the ones containing some expansible clay minerals.

The exchangeable Mg in the range 0.05 to 0.23 meq/100 g of soil appeared to be equally available as indicated by plant uptake. In four of the soils a large part of the Mg taken up by corn (Zea mays L.) came from the nonexchangeable form.

¹ Paper no. 2494 of the Journal Series of the North Carolina State University Agr. Exp. Sta., Raleigh. Taken in part from the Ph.D. dissertation of the Senior author. Presented before Div. S-4, Soil Science Society of America, August 1966.

² Former graduate student, now Agronomist, Olin Mathieson Chemical Corporation, Little Rock, Arkansas, and Professor of Soil Science, respectively.

Received for publication October 3, 1967. Accepted for publication January 15, 1968.