The simplest method to correct estimates of root nutrient concentration for contamination by adhering soil is to express concentration on an “ash-free” basis—total sample nutrient divided by the ash-free mass of the sample. Such ash-free corrections can seriously inflate estimates of root nutrient concentration. Equations exist to correct estimates of plant composition for soil contamination, but they require information on the composition of the contaminating soil. Such information is difficult to obtain because soil fractions adhering to roots after washing may differ from whole soil. We developed a statistical method to estimate indirectly the composition of adhering soil by eliminating the correlation between estimates of plant composition and the degree of soil contamination. This method was tested against mixtures of grass shoots with soil, and then applied to roots from field-collected soil cores for two grass species. The estimated properties of the contaminating material differed from whole soil, and the differences were great enough to seriously affect root N estimates. The nature of the contaminating material may have to be determined for each new combination of soil and plant species. Our approach requires enough replicate samples for regression analysis but does not rely on separate soil samples and should be applicable over a wide range of experimental conditions.
