Phosphorous availability as influenced by chemical and mineralogical properties of Sergipe state soils, Brazil

Abstract

Sergipe state is located Northeast of Brazil between the parallels 9(DEGREES)31'20" and 11(DEGREES)34'12" south latitude, and between the meridians 36(DEGREES)24'27" and 38(DEGREES)11'20" by longtitude west of Greenwich. Soil samples from ten representative Sergipe state soils were collected to a 20 cm depth and used in this study. Tentatively these soils can be classified as Oxisols, Alfisols, Inceptisols, Entisols and Spodosols. It has been determined that phosphorus is the most limiting nutrient in these soils and that significant response to the nutrient has been observed in corn, beans, cassava, peanuts, cotton and forages. Since the mineralogy of these soils had not been investigated, that was an objective of the present study. Mineralogy of the coarse and fine clay fractions of these samples were determined by XRD, infrared, CEC and total dissolution. Phosphorus status was determined by plant P uptake, fractionation, and parameters of P availability (intensity, capacity and quantity). The mineralogical data revealed that the coarse clay fraction was composed of quartz, feldspars, kaolinite, mica and chlorite. The fine clay fraction was composed of kaolinite and mica. Smectite and chlorite also were detected. Phosphorus availability measured by plant P uptake in the greenhouse was not highly influenced by the chemical and mineralogical properties studied. However, P capacity calculated from sorption isotherms was highly influenced by Fe(,2)O(,3). Phosphorus fractionation data revealed that Al-P and Fe-P were the predominant sources of P. Nevertheless, a considerable amount of P was in occluded, reductant soluble and organic forms. Among the extractants studied 0.05 N H(,2)SO(,4) produced adequate results with the least analytical problems. The mineralogical data should provide more accurate interpretations and extrapolations about chemical and physical properties of these soils. In addition, the P data should contribute to upgrade management decisions on these soils.