Ammonium fixation in some tropical rice soils

The NH4 + fixing capacity of 12 tropical rice soils was studied by treating 10 g soil samples with 50 mg NH4 + as (NH4)2 SO4 dissolved in 20 ml of water, equilibrating for 2 h by continuous shaking and then extracting NH4 + with 2 M KCl. The portion of NH4 + added which was not extracted by KCl was termed fixed. The NH4 + fixing capacity of the soils studied ranged from 3.8 to 7.7 m.e./100 g of soil. NH4 + fixation in soils was not related to pH, organic matter or clay content but correlated with the amount of active iron in the soils. The results of the study suggest that because of the reversible oxidation and reduction of iron oxides in rice soils, this mechanism of NH4 + fixation may be of special importance in sorption and desorption of NH4 + ‐ N.