IN-FIELD RAPID ANALYSIS OF P: EXTENSION TO A BROAD ARRAY OF SOIL TYPES

Previously, a rapid field portable method for phosphorus determination in soil was developed for application to an archeological site (Smith et al. 2014). The method uses a 1 M sulfuric acid solution with a 15 minute extraction time. Phosphorus analyses are completed using the ascorbic acid method and a field portable spectrophotometer. Previous results on two Alfisols found the rapid method compared well with a standard laboratory method. In this study, we extend the study to evaluate its efficacy to a range of geographically diverse soils. Eleven different A horizon soil samples, representing 5 soil orders (Ultisols, Mollisols, Entisols, Vertisols and Aridisols) were analyzed using the rapid and standard methods. Results from the standard extraction found soil P values ranging from 88 to 540 mg kg^-1 P. Corresponding samples using the rapid extraction found soil P to range from below the field instruments detection limit to 283 mg kg^-1 P. Analysis of results found that correlation of the standard and rapid methods was significant at 95% confidence. Regression analysis was significant with an r² of 0.79. No apparent trend or relationship was found by soil order. It does appear that the range of variability decreases with increasing P concentration which is a function of the analytical instrumentation.