Abstract

AUXTERO, E.; MADEIRA, M.  and  SOUSA, E.. Predicting the degree of P saturation of selected Andisols from the Azores (Portugal) by the acidified ammonium oxalate and the Mehlich 3 methods. Rev. de Ciências Agrárias [online]. 2007, vol.30, n.2, pp.314-325. ISSN 0871-018X.

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.

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