Chemical analysis of plants is often done without paying attention to the ubiquitous natural presence of exogenous material on the plant surface. The analytical result will then be higher than the endogenous plant concentration. It is shown that this error can be related to three parameters: (i) the surface loading L, (ii) the enrichment factor (EF) of the exogenous material with respect to the soil, and (iii) the plant‐to‐soil concentration ratio (CR). These factors are considered for 19 essential and trace elements and the results indicate that the error can range from insignificant to disastrous. Experimental data are mostly for needles of spruce trees growing in a relatively clean surrounding, but the proposed relations can be extended to estimate the effect for other plant species and surroundings.