A Rapid and Accurate Field Test for the Analysis of Soil Phosphate.

A reliable, quantitative field test for phosphate would have numerous applications in research fields ranging from agriculture to archaeology.  Previous field-based tests have utilized the reaction of orthophosphate (PO₄^3-) with an acid-molybdate solution, which is then reduced to form a molybdenum-blue complex.  These tests suffer by requiring time-consuming extractions, which are then run on a portable colorimeter.  One approach, known as the Òspot test,Ó uses a phosphate-index derived from qualitative descriptions (e.g., color strength, timing of color appearance, etc.) of the near-instantaneous molybdenum-blue reaction on filter paper.  While this method has the advantage of being rapid by not requiring a field extraction, it does not yield quantifiable phosphate concentrations.  The objective of this project was to combine the spot test with reflectance spectrophotometry to rapidly quantify phosphate concentration in the field.  We conducted several experiments to test the application of this method.  First, triplicate filter papers were wetted with 19 solutions of varying concentrations of PO₄-P  (0 – 500 mg L^-1), reagents were added, and filter papers scanned with a portable, hand-held spectrophotometer capable of measuring reflectance wavelengths in the visible spectrum (400 – 700 nm).   This was done to assess the ability of the spectrophotometer to detect the molybdenum-blue reaction without soil interference.  Second, soil samples with negligible background P concentrations were ground to 3 particle-sizes (<2 mm, <0.59 mm, and <0.25 mm), spiked with 8 concentrations of PO₄-P  (0 – 500 mg kg^-1) and run with the same method.  Our results show a strong correlation between PO₄-P concentration and the intensity of the reflection peak at 450 nm.  The addition of soil to the filter paper did not appreciably interfere with results and effects of particle-size were statistically negligible.  These findings suggest our method may provide a very rapid and accurate determination of phosphate concentration in the field.