Fig. 5. Factors evaluated as possible sources of interference from soil solution components regardless of the sampling method used: (A) organic acid standard mixture with various levels of Fe, NaCl, and oxalic acid; (B) soil solution sample with and without cation exchange membrane resin (CEMR) strips; and C) organic acid standard mixture with and without CEMR strips. Formic acid values were multiplied by 10 in graph B. Bars = ±1 standard deviation. Different letters represent significant differences by Tukey's HSD at = 0.05 where treatment differences within each organic acid were significant by ANOVA at the 0.05 level.