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Noncrystalline Clay Distribution in the Major Land Resource Areas of Mississippi¹

ABSTRACT

Selected horizons from 45 Mississippi soils have been examined for noncrystalline clay content as determined by 0.5M NaOH and 0.2M ammonium oxalate (pH 3). The soils represent the major soil resource areas of the state. Parent material varied in age from Cretaceous to Holocene. Only small amounts of amorphous clay were extracted from the weakly developed Inceptisols and Entisols, from calcareous soils of the Blacklands and from the alkaline Natraqualfs of the Coastal Plains. Under these nonacid conditions crystalline clay minerals are the more stable phase. The strongly developed Ultisols released large amounts of noncrystalline material with a low silica/alumina molar ratio. The low molar ratio is indicative of gibbsite; however, the large quantity of silica extracted suggests that silica may be adsorbed on the crystalline minerals.

When only soils high in smectite are considered, the following trends in noncrystalline clay formation were observed. The silica/alumina ratio increases with smectite content. The quantity of noncrystalline clay increases with increased soil acidity. The poorly drained soils contain less amorphous clay indicating that smectite is the more stable mineral in this environment.

Extraction of the clays with acid ammonium oxalate resulted in an average increase in cation exchange capacity of 14 cmol ( Ca²⁺) kg^–1 due to the removal of surface coatings blocking exchange sites.

¹ Contribution from the Dep. of Agronomy, Mississippi Agric. and Forestry Exp. Stn., Mississippi State Univ., Mississippi State, MS 39762. Approved by the Director as Journal Paper no. 5299.

² Professors of Agronomy and Graduate Research Assistant, respectively.

Received for publication October 25, 1982. Accepted for publication July 22, 1983.