AbstractComparative fixation of NH4and K by wetland rice soils in relation to clay mineral composition and treatment sequence is of agronomic interest, but information on the subject is scanty. An attempt has been made to clarify this relationship by employing the normally recommended doses of fertilizers, 3 application sequences, and 5 soil clays of divergent mineralogies. The results show that montmorillonitic clay is the greatest fixer of NH4, but least of K. X‐ray amorphous clay also prefers NH4over K. Vermiculitic and beidellitic clays strongly fix both NH4and K. Soil clay consisting of hydrous mica, chlorite, and halloysite fixes low proportions of applied NH4and K. The sequence in which NH4and K are applied does not appear to affect the relative amounts that are fixe