Avaliação do grau de saturação de P nos Andossolos dos Açores (Portugal) pelos métodos do oxalato de amónio e de Mehlich 3

E. Auxtero¹, M. Madeira & E. Sousa

ABSTRACT

Extractable aluminium (Al), iron (Fe), and phosphorus (P) were assessed on twenty eight soil horizons from a representative range of Andisols from the Azores, using the acidified ammonium oxalate method (Alo, Feo and Po) and the Mehlich 3 method (AlM3, FeM3 and PM3). The degree of P saturation (DPS) by these two methods (DPSo and DPSM3) was also estimated.

The amounts of Alo and AlM3 were positively correlated, while the amounts of Feo and FeM3 were negatively correlated. On the other hand, the amounts of Po and PM3 were not significantly correlated. Soils with high allophane content, showed higher values of Alo, AlM3, and Po, while non-allophanic Andisoils, containing large amounts of organic C, showed higher values of Feo than other studied soils.

Values of DPSo were greater than those of DPSM3 and highly correlated. DPSo values were negatively correlated with the contents of Al and Fe extracted by dithionite (Ald, Fed), by pyrophosphate (Alp and Fep), and also with P retention (PR) values. A negative correlation was observed between DPSM3 values and Fep and Ald contents, and PR values. High values of DPSo and DPSM3, observed in soils with low amounts of allophane, low amounts of organic C, and with less pronounced andic properties (Typic Haplustepts, Vitrandic Haplustepts and Andic Haplustepts), may indicate high P susceptibility to surface runoff and leaching. In contrast, P losses through water runoff and leaching may be negligible in Andisols with large amounts of extractable Al by the 1N KCl (Alic Hapludands) and high allophane content (Typic Placudands), and with low values of DPSo and DPSM3.

RESUMO

Observou-se uma correlação positiva entre as quantidades de Alo e AlM3 e uma correlação negativa entre as quantidades de Feo e FeM3. Por outro lado, as quantidades de Po e PM3 não foram significativamente correlacionadas. Os solos com elevado teor de alofana mostraram valores de Alo, AlM3, e Po mais elevados, enquanto que os solos não alofânicos e com elevado teor de car-bono orgânico, mostraram teores de Feo superiores aos dos outros solos estudados.

Os valores de DPSo foram bastante superiores aos de DPSM3, mostrando-se forte-mente correlacionados entre si. Os valores de DPSo mostraram uma correlação negativa com os teores de Al e Fe extraídos pelo ditionite (Ald, Fed) e pelo pirofosfato (Alp e Fep), e com os valores da percentagem de retenção de P no solo (PR). Do mesmo modo, os valores de DPSM3 também mostraram uma correlação negativa com aqueles constituintes, excepto com o Alp e Fed. Os valores mais elevados de DPSo e DPSM3 foram obtidos nos solos com baixos teores de alofana e de carbono orgânico, e com características ândicas pouco marcadas (Typic Haplustepts, Vitrandic Haplustepts e Andic Haplustepts), sugerindo uma elevada susceptibilidade de perda de P por escoamento superficial e lixiviação. Pelo contrário, nos Andossolos com elevado teor de Al extractável pelo KCl 1N (Alic Hapludands) e com elevado teor de alofana (Typic Placudands), que mostraram baixo valor de DPSo e DPSM3, o P será pouco susceptível aos efeitos do escoamento superficial e da lixiviação.

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¹ Departamento de Ciências do Ambiente, Instituto Superior de Agronomia, Tapada da Ajuda, 1349017 Lisboa e-mail: eauxtero@iol.pt