Extraction times and analysis methods influence soil test measurements of phosphorus and sulphur

The influence of time of shaking (5, 30 and 60 min) on the extraction of phosphorus (P), sulphur (S), aluminum (Al), iron (Fe), manganese (Mn), potassium (K) and magnesium (Mg) by five soil test solutions (water, sodium acetate, sodium bicarbonate, Kelowna, and Mehlich-3) were examined on six diverse south coast British Columbia soil samples. Inorganic P and S were measured by ion chromatography (IC) and inductively coupled plasma spectrophotometry (ICP) to obtain total P and S and the other five elements. In most cases, increased amounts of the elements were extracted as shaking time increased and the amounts varied among the samples. The exceptions were P and Fe in Kelowna and Mehlich-3 extracts, where in many of the samples, amounts of these elements decreased. The solutions extracted substantial quantities of organic S (ICP minus IC measurements), and relatively small quantities of organic P. In Mehlich-3 extracts, IC values for P were often greater than ICP P measurements, suggesting analytical interference. The interference was assumed to be in the IC measurement, and concluded to be most likely due to the chelate (EDTA) in the solution. The increased S extracted as time of shaking time was increased was largely increased organic S. Increasing the shaking time from 1 to 24 h increased extraction of inorganic P from soils by water and the amount of P that was bound during a P equilibration treatment. The amounts of the changes were relatively small, making 1 h shaking acceptable for routinely using these measurements for current and potential pollution risk by P in soils. However, the extraction times must be precise to enhance reproducibility. The results of these studies show the importance of shaking time on these measurements, with short times being preferred for theoretical and operational considerations. The significant and variable amounts of organic P and S in extract solutions show the importance of the quantification method for interpreting results for soil testing and for fractionation studies. Since the two factors (time of extraction and method of element quantification) examined in this study changed the results dramatically both from individual (main) and compounded (interaction) effects, it is apparent that the practical implications for any changes to soil test analysis procedures need thorough and systematic re-examination.