Plant available potassium and phosphorus in arable soil: a comparative study on testing methods

Abstract

Determination of soil fertility status is based on determination of plant available nutrients which occur in several forms. This strongly challenges the choice of extracting solution and detection technique. Deriving a mathematical model for data recalculation would allow relating the results of different methods without loss of agronomical validation data. This comparative study was aimed at assessing the relationship between two methods for determination of plant available potassium and phosphorus in arable soil. Soil K and P were extracted followed a diluted double acid (Mehlich 1) and an acetate/lactate methods (ALM). Potassium in the obtained extracts was determined by flame AES and phosphorus - by molybdenum blue spectrophotometry. To compare the results obtained by the both extraction methods, the uncertainty of measurement of each method was assessed following the single laboratory validation and quality control approach. The expended uncertainty was found to be 2 mg/100g as P2O5 and 7 mg/100g as K2O (at k=2). The results from the comparative analysis of 93 arable soil samples showed a correlation between Mehlich-1 and ALM extracted P and K within uncertainty limits (at k=3). Based on the recalculated values of extractable K and P, the fertility status of 60 % of soil samples were correctly classified, 30 % with acceptable error and 7 % could not be classified. The study showed that Mehlich-1 and ALM methods provided comparable results for studied range of soil samples. However, determination of soil fertility status based on recalculated results should be made with caution as it could be highly influenced by soil type.