Phairat Phimsirikul and Toru Matoh

pp. 423 - 434

Abstract

Although the available P extracted by Bray II method in tropical soil is low, most of tropical plants
can grow well. The objective of this study was to study P status and to evaluate the available P extracted by
EDTA-NaF method. Top soil and sub soil of 10 dominant soil series in Thailand were analyzed for some
chemical properties and characterization of the forms of phosphorus using EDTA-NaF extraction and
successive phosphorus extraction by the modified Sekiya method. The soil total P concentration was 38-1137
mg P₂O₅ kg^-1. The available Bray II-P was very low to high (1-76 mg P₂O₅ kg^-1), and it approximated 0.17-12%
of the total P. Iron and aluminum phosphates were the main fraction of inorganic P in acid soil, whereas Ca
phosphates were in calcareous soils. Organic P content accounted for 33-67% and most of them were bound
with Fe and Al in acid soils and Ca in calcareous soils. P extracted by EDTA-NaF reagent was obviously
larger than that of Bray II reagent. The EDTA-NaF extracted P [high molecular weight organic P (HMWP)+
inorganic P (EDTA ext P_i)] was 7-46% and 1-6% of total P in acid soils and calcareous soils respectively. The
EDTA ext P_i tended to be larger than HMWP except in Tk soil. The total amount of extracted P correlated
well with Al-P_i and Fe-P_i which were the main fraction of inorganic P. It also correlated with HMWP, but HMWP did not correlate with organic P determine by ignition method and Ca-P_o, Fe-P_o and Al-P_o. The
EDTA-NaF method may be suitable for P evaluation in the soils which have high amounts of Fe-P_i
, Al-P_i and organic P widely distributed in Thailand.