Phosphorus Distribution in Five Highly Weathered Alabama Soils

Abstract

A major nutritional problem to crops grown in highly weathered Alabama soils is phosphorus (P) deficiency linked to their low availability and the capacity of the soils to fix P in highly insoluble forms. This work, examines factors that might influence the distribution, availability, and adsorption of P in five typical highly weathered Alabama soils. The total P contents range from 199 to 543 mg P kg⁻¹ soil. Soil inorganic P fractions were determined by a sequential-fractionation procedure (NH₄Cl–P, NH₄F–P, NaOH–P, CDB–P, and H₂SO₄–P). The comparative rankings of the various inorganic P fractions on the average were in the order: NaOH–P>NH₄F–P>H₂SO₄–P>NH₄Cl–P>CDB–P. Most of the inorganic P occurred in the least available NaOH–P form. The following soil test P on the average, extracted P in this order: Mehlich 3 (16.78 mg kg⁻¹), Bray 1 (14.72 mg kg⁻¹), Mehlich 1 (9.01 mg kg⁻¹), Olsen (8.09 mg kg⁻¹), H₂O (1.55 mg kg⁻¹), and CaCl₂ (0.39 mg kg⁻¹). The correlation results suggest that the extractants removed easily soluble P as well as P from Al oxides. The P adsorption rate coefficient (k _d ) obtained, correlated with the clay content ( r=0.77;p<0.01). The magnitude of the k _d values was not affected by cultivation practices. The possible soil factors affecting P availability in these soils are iron (Fe) and aluminum (Al) oxides, cations, and clay minerals. Because of the low total P content, low labile P fraction, and the relatively low fixation capacity of these soils, moderate amounts of P amendments would be necessary for sustained productivity.

Acknowledgments

Contributed by the Agricultural Experiment Station, Alabama A&M University. Research partially supported by grant # 2-280-14-3189.