Capacidade de adsorção e de desorção de P em Andossolos dos Açores, Portugal

E. Auxtero¹, M. Madeira & E. Sousa

ABSTRACT

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.

RESUMO

A adsorção máxima de P (P-ads max) e a quantidade de P desorvido no solo (P-des), foram determinadas em 16 horizontes superficiais e subsuperficiais de Andossolos dos Açores, Portugal, usando equação de Langmuir e oito extracções sucessivas com cloreto do cálcio. Os valores de P-ads max mostraram uma correlação positiva com os teores de Alo, Ald, Fed, alofana, ferihidrite + + alofana, e com valores de Alo + ½ Feo. Por outro lado, os resultados de P-des, mostraram uma correlação negativa com os mesmos constituintes, excepto com alofana. Vitrandic Haplustepts, Typic Udivitrands e Typic Hapludands mostraram um valor de P-ads max baixo, mas com elevado teor de P-des. Iões fosfato adicionados como fertilizante neste tipo de solos ficam disponíveis para utilização pelas plantas, mas provavelmente, serão facilmente perdidos por drenagem e erosão do solo. Estes solos podem necessitar de quantidades limitadas de fertilizante de fósforo, com controlo das perdas deste elemento para minimizar a eutroficação dos cursos de água das proximidades. Por outro lado, os Typic Placudands com elevado teor de alofana e os Acrudoxic Hydrudands, com elevado teor de carbono orgânico, têm elevado valor de P-ads max e baixo teor de P-des, indicando indisponibilidade do P quando retido por estes solos. Neste caso, poderá ser preferível o uso de grandes quantidades de fertilizante de libertação lenta de P.

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