Aluminum has long been recognized as a major limiting factor for root growth in acid subsoils, but little has been done to delineate toxic and nontoxic forms of soil‐solution Al. In an effort to determine if the presence of organic acids in soil solutions affected Al phytotoxicity, short‐term, split‐root experiments were conducted with cotton (Gossypium hirsutum L.) taproots as the growth indicator. Based on pure solution experiments, short‐chain, carboxylic acids can be divided into three groups as Al detoxifiers: (i) strong (citric, oxalic, tartaric), (ii) moderate (malic, malonic, salicylic), and (iii) weak (succinic, lactic, formic, acetic, phthalic). The Al detoxifying capacities of these acids were positively correlated with the relative position of OH/COOH groups on their main C chain, positions that favored the formation of stable 5‐ or 6‐bond ring structures with Al. In addition, analyses of soil solutions from several eluviated acid horizons (E, EB, BE) revealed the presence of several organic acids whose concentrations were generally higher in forested than in cultivated soils. Based on these concentrations, total solution Al (as measured by ICAP) was partitioned into monomeric Al (Al3+ + hydroxy‐Al species) and complexed Al (Al‐organic acid complexes). The latter accounted for 93 and 76% of the total solution Al concentrations of the two acid subsoils into which elongation rate of cotton taproot was studied. Root growth was significantly correlated with monomeric Al but not with total Al in soil solutions.
Four soils, ranging in texture from loamy sand to clay, were fertilized differently and equilibrated moist for several days. Soil solutions were then separated by column‐displacement, by simple centrifugation, and by immiscible displacement with CCl4 via centrifugation. The ionic compositions of soil solutions were unaffected by the method used to obtain the solutions.
Pure‐solution and soil‐solution experiments were conducted in which the solubility products of synthetic basaluminite and alunite in pure solutions were compared with the ion‐activity products of Al4(OH)10SO4 and KAl3(OH)6(SO4)2 in soil solutions of sulfate‐treated soil. Solution experiments were conducted in which 0.05M Al2(SO4)3 was titrated with NaOH, KOH, or Ca(OH)2 to an OH/AI mole ratio of 2.0. The initial precipitate was amorphous, with the chemical composition of basaluminite, Al4(OH)10SO4·5H2O. Aging the amorphous precipitates under different conditions transformed them to compounds of different composition and crystallinity: amorphous precipitate remained unchanged when aged in their mother solutions at 25°C; crystalline basaluminite formed in Ca mother solutions when aged at 50°C; crystalline alunite formed in Na and K mother solutions when aged at 50°C; crystalline alunite formed in K mother solutions that had been seeded with alunite or bentonite when aged at 25°C. The solubilities of the amorphous and crystalline precipitates were determined in dilute NaClO4 solutions and gave the following average calculated pKsp values: 116.0 for amorphous basaluminite; 117.7 for crystalline basaluminite; 79.7 for Na‐alunite; and 85.4 for K‐alunite. Similar Ksp values for basaluminite were found for soil solutions from a sandy loam soil that had been treated with different rates of K2SO4. However, the soil solutions were supersaturated with respect to alunite. When the SO4‐treated soil was subsequently treated with a high rate of KH2PO4, the soil solution remained supersaturated with respect to alunite, but became undersaturated with respect to basaluminite. It is proposed that sulfate retention by acid soils is a consequence of the solubility of basaluminite and/or alunite.
The exchangeable acidity determined by neutral 1N NH4OAc was compared to that measured in a buffered solution on 348 soil samples from profiles of Red‐Yellow Podzolic soils in Alabama. The soil pH values ranged from 4.1 to 6.5 and cation‐exchange capacities from 0.8 to 13.0 me. per 100 g. The methods measured comparable amounts of acidity. An equation was calculated by the method of least squares for the relationship between soil pH and percent base unsaturation for all samples. The amount of CaCO3 needed to bring soil pH to desired level was calculated by estimating percent base unsaturation from soil pH values in water and by measuring exchangeable acidity of soil by pH values in a buffered solution. The validity of this procedure was tested by incubating soils with increments of Ca(OH)2 and measuring the change in soil pH.
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.
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