Determination of subsurface solute fluxes is central in critical zone (CZ) science because key processes such as bio-geochemical weathering, nutrient dynamics, and contaminant transport can be determined. With passive capillary fiberglass wick samplers (PCaps), solute, and water fluxes can be assessed; however, the presence of fiberglass can impact soil solution chemistry. To determine which solutes are suitable for sampling by fiberglass wick PCaps, flow-through experiments were performed where aqueous soil extracts were percolated through the wicks and changes in effluent solution pH, dissolved inorganic carbon (DIC), anions, major cations, and trace metals including rare earth elements (REE) were monitored. Results indicated dissolution of wicks releasing the glass constituents B, Na, Si, Ca, Mg as well as F− and DIC. Barium, K, and Sr were retained, likely due to exchange reactions with either glass constituents or interlayer cations of clay colloids. Stop-flow was included to mimic precipitation events revealing increased pulse-like release of glass constituents. Results of the full-scale experiment indicate substantial contribution from wick material (59 ± 20 for Si, 92 ± 7 for Na, 29 ± 19 for Mg, and −26 ± 32 for Ca, all values in percentage of total effluent concentrations) that cannot be corrected for, hence the use of PCaps for these solutes is not recommended. A great number of other solutes were however not impacted by the presence of wicks such as most anions (Cl−, NO3−, SO42−) and many trace metals (Al, Ti, Mn, V, Fe, Co, As, Y, Mo, Sn, Pb, U, and all REE).