Method to evaluate "uplandization" in converted field from a paddy based on crystallinity of free iron oxides

Abstract

"Uplandization" is the process that changes the physical and chemical properties in a paddy field into those of an upland field. There is a transitional period when a paddy field is converted to an upland field. We propose a new method to evaluate the degree of "uplandization" based on changes in crystalunity of free iron oxides. The iron oxides influence soil physical and chemical properties such as soil microstructure or phosphate retention. Iron crystallinity is evaluated by the amount of iron extraction over 120 min by 1 mol L-1 pH 3.0 sodium acetate buffer, at solution to soil of 100 to 1 (Feac). Feac decreases in proportion to the period after conversion to upland cultivation. Feac is significantly correlated with phosphate retention properties and iron reducibility of soils under submerged conditions, but acid-oxalate-extracted iron, which is used to extract amorphous free iron generally, is not. For this reason, Feac is considered to be more suitable for evaluation of the history of uplandization.