Results may then be expressed on a weight or a volume basis. Since this requires recording an extra data point for each sample, it would be most feasible for highly automated operations where weights can be captured and recorded electronically. Expression of analytical results in pounds of nutrient per acre of soil, using either subsampling system, is normally based on the assumption that the plow layer contains two million pounds of soil per acre. Both systems suffer from the common problem of not taking the volume or weight of coarse fragments into account. Ultimately, selection of scooping or weighing as the subsampling technique should depend primarily upon the method that was used in field calibration trials or yield-response studies conducted in the state or region.
Many factors affect the analytical results of a nutrient extraction procedure.
Where large numbers of samples are analyzed using automated or mass production procedures, contamination of laboratory equipment can be a significant source of analytical error. For example, ions adsorbed by extraction flasks or sample bottles in contact with extracted solutions may be a source of contamination for subsequent analyses using the same containers. Rinsing glassware with a cleaning solution is particularly important for the analysis of micronutrients and trace metals, provided that the acid anion is not one of the analytes. Acids have the disadvantage of being corrosive to equipment and potentially hazardous in the work environment. Aluminum chloride is also less corrosive and safer than most acid rinses, but may not be effective for removing trace elements to sub-part per million levels.