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Chemical Factors Affecting Root Growth in Subsoil Horizons of Coastal Plain Soils¹

ABSTRACT

Lack of root growth into the acid subsoils of the Coastal Plain of the southeastern U.S. is a recognized yield-limiting problem for many crops, but little has been done to identify root-limiting factors by soil horizon. In an effort to delineate Al toxicity and Ca deficiency by horizon, representative profiles of six major soil series of Alabama's Coastal Plain were sampled by horizon to a depth of 1 m. Horizon samples were subdivided and treated with one of the following: (i) check, (ii) CaSO₄, (iii) MgO, and (iv) Ca(OH)₂. Growth rate of primary roots of cotton (Gossypium hirsutum L.) into each soil material was measured, and visual symptoms of Ca deficiency and Al toxicity on roots were noted. Five untreated soil horizons grew roots showing only symptoms of Al toxicity (all were Bt horizons, previously designated as B2t); five grew roots showing only Ca-deficiency symptoms (all were E, EB, BE horizons, previously designated as A2, A3, and B1, respectively); three grew roots showing both Al toxicity and Ca deficiency (all were Bt horizons); five horizons grew normal-appearing roots (E, BE, and Bt horizons). Calcium deficiency occurred when soil-solution Ca activity was 0.27 mM or less and Ca saturation was 17% or less. Aluminum toxicity occurred in some Bt horizons with soil-solution Al at <0.4 µM but not in E, EB, and BE horizons with Al ranging between 9 and 134 µM. This apparent contradiction in toxic levels of Al was probably the result of solution Al being chelated in eluviated horizons but not in illuviated horizons.

¹ Contribution from the Dep. of Agronomy and Soils, Auburn University, AL 36849. Published with the approval of the Director of the Alabama Agric. Exp. Stn. as Jour. Series no. 3-82239.

² Professor of Soil Science and former Graduate Research Assistant, now District Conservationist, USDA-SCS, Auburn, Ala.

Received for publication June 16, 1982. Accepted for publication August 23, 1982.