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Critical micronutrient concentrations in soils and plants have not been clearly determined for marginal soils where deficiencies are likely to occur. The objective of this study was to develop a reliable method for assessing micronutrient deficiency in soils and plants. Soybean plants [Glycine max (L.) Merr.] were grown in the A and B horizons of two Brazilian Ustoxes and watered with a complete nutrient solution (i.e., control) or solutions lacking one of the following micronutrients: Mn, Zn, B, or Cu. Soybean plants were repeatedly grown in the same soils until maturity. The cumulative frequency of deficient plants where the relative seed dry weight was significantly decreased was recorded. The percent relative cumulative frequency (PRCF) is calculated as: cumulative frequency + total number of tested plants x 100. A linear-plateau regression line was fitted to the PRCF as a function of the micronutrient concentrations in the soil, the uppermost mature leaf, or seed. Critical concentrations (mg [kg.sup.-1]) for deficiency indicated as transition points of regression lines are: 6.4 for Mn (reference), 1.0 for Zn, 0.2 for B, and 0.2 for Cu in the soil (Mehlich-I extraction for Mn, Zn, and Cu; hot 0.005 mol [L.sup.-1] Ba[Cl.sub.2] extraction for B); 63 for Mn, 34 for Zn, 25 for B, and 4 for Cu in the uppermost mature leaf; 55 for Mn, 42 for Zn, 14 for B, and 5 for Cu in seeds. Thus, this determination method can clearly identify the critical values for micronutrient deficiency in soils and in plant tissues. Abbreviations: PRCF; percent relative cumulative frequency. doi: 10.2136/sssaj2010.0158