The adsorption of humic acid to mineral particles can be characterized by specific and electrostatic interactions and by adsorption fractionation. In this paper we focus on adsorption fractionation and the influence of the polydispersity (with respect to size) of humic acids on the adsorption process and the shape of the isotherms at different pH values. In the previous paper the effects of electrostatic interactions were described. The adsorption from a mixture of two humic substances, purified Aldrich humic acid (PAHA) and Laurentian fulvic acid, on nonporous hematite is studied. It is shown that, in time, the large humic acid molecules replace the readily adsorbed, small, fulvic acid molecules. Preferential adsorption of large humic molecules over small ones also affects the shape of the adsorption isotherm of a single humic acid. After a high affinity start, the adsorbed amount of PAHA gradually increases with increasing humic acid concentration. This is mainly due to an increasing adsorption fractionation. For a certain concentration in solution the effect of the fractionation increases with increasing adsorbed amounts. This explains why for PAHA the polydispersity effect is much stronger at pH 4 than at pH 9 where the adsorption is lower. To obtain a general impression of the effects of polydispersity on the adsorption of weak polyelectrolytes on a variably charged surface, calculations are presented using a self-consistent-field theory. The calculated adsorption of a mixture of two acidic polyelectrolytes, that (mainly) differ in size, onto an oppositely charged hematite type surface confirm the trends found for humics : adsorption fractionation occurs, and the effect of the fractionation on the isotherm increases with decreasing pH. The calculations also show that the area-to-volume ratio affects the isotherm and results in an adsorption/desorption hysteresis.