A gasometric procedure to measure residual lime in container substrates

Abstract

Unreacted residual limestone in the container substrate is key in buffering pH change over time. Our goal was to develop a substrate test protocol to measure residual lime [in units of CaCO3 equivalent (CCE)] by applying a strong mineral acid (HCl) to a substrate sample and measuring the evolved CO2 gas with a gasometric method based on a Chittick apparatus. In one experiment, CaCO3 was added to a substrate that had previously been neutralized to pH 7.35 with Ca(OH)2 so that there would be minimal CaCO3 reaction with the substrate at this high pH. The gasometric method was then used to estimate residual CCE. Measured CCE and applied CaCO3 were similar, indicating reliable CCE estimation. In a second experiment, a pH titration method was used to quantify the relationship between substrate-pH and milliequivalents of reacted base and provided an additional validation of the estimated reacted and residual CCE. The gasometric method demonstrated declining residual CCE over time as a dolomitic limestone reacted to raise substrate-pH and increasing residual CCE as applied CaCO 3 concentration increased. Residual CCE in a substrate is an important property that should be considered for pH control and management in greenhouse crop production. Our results indicate that the gasometric system may be useful for optimizing lime application rate, lime source, or management of residual CCE during crop production.