Filter-papers impregnated with iron oxides (“iron-strips”) are increasingly being used to estimate “bioavailable” phosphorus. This study shows that these strips, in addition to being able to adsorb phosphate ions, can also mediate the hydrolysis of a model organic phosphorus compound. It is proposed that the high correlation between bioavailable phosphorus and iron-strip available phosphorus is due in part to the strips' hydrolytic capability.