Theoretical aspects on local alkalinization following hydrolysis of deep-placed urea in paddy soils under transplanting

Abstract

Urea broadcasting in rice cultivation often leads to ammonia volatilization because the hydrolysis reaction of urea quickly gives ammonium, ammonia and bicarbonate. Particularly ammonia (NH₃) is subject to emanation which can be avoided if urea is deep placed. This no loss optimistic scenario is realized by strict positioning of urea supergranules in the anaerobic layer of the paddy soil. In this study, we wanted to verify if the development of alkalinity in the site of placement could be predicted from the chemical equilibria in soil solution, neglecting the effect(s) of the soil matrix. Calculation of pH related to ammoniacal N concentration was performed considering the main acid-basic equilibria taking place within the soil solution. Data were compared to localized pH and ammoniacal N values obtained in two paddy fields following deep placement of urea supergranules. The pH never trespassed 9.25 which is the pK_a of ammonia, in theoretical terms as well in the localized point placement sites in the fields. The linear relationship between log of ammoniacal N concentration and pH seems to indicate a strong influence of the cation exchange capacity so far not taken into account by the model. Urea supergranules are compared to mini-bombs of NH₃; to prevent any leak to the floodwater (then to the atmosphere) they have to be buried with precision.

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Keywords: Point placement, urea supergranule, generated alkalinity, wetland soil, soil solution, pH