comparison of three methods for determining calcium ion activity in soil solutions

Calcium ion activity, (Ca 2+ ), in 37 soil solutions was determined using species calculation, potentiometric (ion selective electrode), and colorimetric (tetramethylmurexide) methods. Agreement among the three estimates of (Ca 2+ ) was tested by comparing the potentiometric and colorimetric methods to the species calculation method. Good agreement was observed between (Ca 2+ ) estimates using the potentiometric and species calculation methods. Percent relative error between the two methods was less than 5% for 76% of the soil extracts and less than 10% for 85% of the soil extracts. Differences are thought to be related to interference from H + , Na + , K + , and Mg 2+ above certain concentrations. The colorimetric and species calculation methods agreed poorly in their estimates of (Ca 2+ ) in soil solutions. Relative error was less than 20% for only 39% of the estimates. Sodium interference is thought to be a major contributor to error in estimating (Ca 2+ ) using the colorimetric method. Key words: Calcium ion activity, methods comparison, aqueous species distribution, selective ion electrode, tetramethylmurexide