Soilco is often contacted by customers using home pH testers and getting very high and inaccurate readings for pH in Soilco organic soils. Theoretically, the indicator paper should be accurate, as it is a known and calibrated (to a degree) chemical reaction.
Field pH tests have chemical reactions with Barium Sulphate to change the colour of the indicator. It is thought that the organic matter in the enriched garden soil release ammonium that reacts with the Barium Sulphate. Ammonium has a very high pH, and is caught in the barium solution, resulting in a false high reading. Field test kits have been designed for natural soils and behave unreliably with manufactured mixes such as those containing compost or chicken manure.
Soil laboratories such as Soilco’s internal accreditted lab and SESL Australia use water extract methods and measure the soluble salts, in this case Hydrogen and Hydroxide, to obtain a true measure of pH. The lab uses reference samples with a known pH value to provide ongoing monitoring of the accuracy of the readings.
Laboratory quality controlled and externally certified pH tests are the only true and reliable way of assessing a soils pH. Soilco uses both internal and external testing including pH and EC regularly to monitor the consistency and quality of our mixes.
(Thanks to Ryan Jacka from SESL Australia for his assistance with this article).