Liming materials are incorporated into container substrates to neutralize acidity and to provide calcium (Ca) and magnesium (Mg). Liming materials differ in their reactivity rate, which in turn determines the proportion of base that remains as unreacted “residual” limestone in the substrate. Research has shown that most pH buffering in container substrates comes from residual lime (Argo and Biernbaum, 1996). However, a test method is currently lacking to quantify the concentration of residual lime [in units of calcium carbonate (CaCO3) equivalents (CCE)] in container substrates.
The most common liming materials used in greenhouse substrates are carbonate-based limestones: calcite (CaCO3 with 40% Ca), calcitic [CaMg(CO3)2 with greater than 30% Ca and less than 5% Mg], dolomitic [CaMg(CO3)2 with less than 30% Ca and greater than 5% Mg], and dolomite [CaMg(CO3)2 with 22% Ca and 13% Mg]. When carbonate-based limestones react with acid from proton sources such as acidic peat, then calcium (Ca2+) or magnesium (Mg2+), water (H2O), and carbon dioxide (CO2) gas result:
Our hypothesis, based on research measuring CCE in agronomic soils (Dreimanis, 1962; Hülsemann, 1966), was that the unreacted lime concentration (on the left side of Eqs. 1 and 2) could be determined in a soilless substrate by addition of a strong acid and subsequent measurement of released CO2 gas through volume displacement. The volume of CO2 gas evolved by the chemical reaction can be determined with a Chittick apparatus (or variants thereof), which is easily constructed from basic laboratory glassware and tubing (Fig. 1; Dreimanis, 1962). Once the device is calibrated by use of prepared CaCO3 standards, 1) media samples are introduced and reacted with HCl, 2) the volume of CO2 released is measured, and 3) the amount of residual lime is calculated with either a CO2-CaCO3 standard curve or the Ideal Gas Law with a known air temperature and air pressure.
Soil CCE content can be determined by either measuring the CO2 evolved when treated with acid or the use of empirical standard curves relating pH to known carbonate content. The first method includes several procedures differing in how evolved CO2 is quantified. The liquid displacement (i.e., Chittick) method on which our technique is based has been widely used (Dreimanis, 1962; Hülsemann, 1966) to measure the volume of CO2 evolved on application of strong acid. The gravimetric method for measuring CaCO3 (Allison and Moodie, 1965) is another well-known procedure and appears in other soil analysis manuals (Goh et al., 1993). In the gravimetric procedure for measuring CaCO3, a soil sample is combined with excess 4 M HCl and the loss in weight incited by CO2 evolution is converted to CCE using an equation based on formula weights. The gravimetric method uses an Erlenmeyer flask as the reaction vessel and involves periodically removing and replacing a rubber stopper while the flask is swirled to promote CO2 evolution. Loss of H2O vapor along with CO2 contributes to the loss in weight each time the stopper is removed and thus increases the calculated CCE content. A resolution problem can also occur because the flask and stopper together weigh well over 100 g along with a soil sample of 1 to 10 g, whereas the loss in weight for a typical soil is only tenths of a gram.
The second method involves the reaction of the soil carbonates with 0.4 M acetic acid and then measuring pH after allowing the acid-soil solution time to reach equilibrium. The pH value is converted to an equivalent CaCO3 content by means of an algorithm obtained in separate calibration runs with pure CaCO3 (Moore et al., 1987). The problem mentioned by Moore et al. (1987) is that appreciable CaCO3 content can be estimated even for acidic soils, apparently as a result of neutralization of dilute acetic acid by clay and organic matter.
Our goal was to develop and test a gasometric method for quantifying residual lime in a container substrate and describe a recommended protocol. To improve confidence that the gasometric method was a valid approach, specific objectives were to compare estimated CCE against 1) applied CaCO3 under high pH conditions, and 2) a pH titration method to relate substrate-pH with milliequivalents of reacted CaCO3. The method was then used to quantify the effect of 3) dolomitic limestone on residual CCE and substrate-pH over time, and 4) increasing applied CaCO3 on residual CCE and substrate-pH. Additional experiments were run to identify potential sources of measurement error. The conclusion of this article describes a recommended protocol.
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