Correcting estimates of root chemical composition for soil contamination

The simplest method to correct estimates of root nutrient concentration fur 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. S uch ash-free corrections can seriously inflate estimates of root nutrient c oncentration. 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 f ractions 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 co mposition and the degree of soil contamination. This method was tested agai nst mixtures of grass shoots with soil, and then applied to roots from fiel d-collected soil cores for two grass species. The estimated properties of t he contaminating material differed from whole soil, and the differences wer e great enough to seriously affect root N estimates. The nature of the cont aminating material may have to be determined for each new combination of so il and plant species. Our approach requires enough replicate samples for re gression analysis but does not rely on separate soil samples and should be applicable over a wide range of experimental conditions.