Abstract

AUXTERO, E.; MADEIRA, M.  and  SOUSA, E.. P adsorption and desorption capacities of selected Andisols from the Azores, Portugal. Rev. de Ciências Agrárias [online]. 2007, vol.30, n.2, pp.55-66. ISSN 0871-018X.

Phosphorous adsorption maxima (P-ads max) and the amounts of P desorbed (P-des) using Langmuir equation and eight successive extractions with 0.01M CaCl2, were determined on sixteen horizons of Andisols from the Azores, Portugal. Results showed that the values of P-ads max and the proportions of P desorbed (P-des) were highly influenced by allophane, Alo, Ald, and Fed contents, and by the values of Alo + ½ Feo and ferrihydrite + allophane. P-des were negatively correlated with these constituents, but not with allophane. Vitrandic Haplustepts and Typic Udivitrands, and Typic Hapludands containing low amounts of allophane, Alo, Ald, and Fed showed low values of P-ads max, but high P-des values. Phosphate ions applied as fertilizer on these soils are highly available for plant use, but maybe easily lost through surface runoff, subsurface drainage and soil erosion. They may require restricted amounts of P fertilizer, and P losses should be controlled to minimize eutrophication of nearby water bodies. Conversely, Typic Placudands containing large amounts of allophane, and Acrudoxic Hydrudands and Typic Hapludands containing large amounts of organic C showed high values of P-ads max, but low P-des, indicating unavailability of P when retained by these soils. Large amounts of slow P-releasing fertilizer may be required for better crop use.

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