Residual Lime and ph Buffering in Container Substrates

Residual Lime and ph Buffering in Container Substrates P.R. Fisher, Jinsheng Huang, W.E. Horner W.R. Argo and C.N. Johnson Blackmore Co. Environmental Horticulture Dept. 10800 Blackmore Ave. University of Florida Belleville, MI 48111 P O Box 110670, Gainesville FL 32611-0670 USA USA Keywords: calcium carbonate, Chittick, gasometric, greenhouse, growing media, peat, substrate-ph, titration Abstract Unreacted residual limestone plays a key role in buffering of ph change over time in container substrates. Different methods for quantifying residual alkalinity in substrates were evaluated. A gasometric method based on a Chittick apparatus quantified residual carbonate and bicarbonate [in units of CaCO 3 equivalent (CCE) per liter of substrate], whereby a strong mineral acid (HCl) was applied to a substrate sample and the evolved CO 2 gas was measured by liquid volume displacement. A ph titration method quantified the relationship between substrate-ph and milliequivalents of reacted base, a measure of total substrate alkalinity. These protocols were used to quantify substrate-ph and residual CCE in response to different carbonate lime rates in a peat substrate, and the effect of mineral acid drenches or ammonium fertilizer applied to different research or commercial media components and lime sources. Residual CCE increased as applied CaCO 3 concentration increased, particularly at ph above 7 because of limited CaCO 3 solubility. Increasing residual CCE was correlated with greater ph buffering, in both a greenhouse plant experiment using a 100% ammonium-n, acid-reaction fertilizer, and when substrates were drenched with HCl. Commercial substrates varied widely in residual CCE, ranging from 0.96 to 4.91 g CCE L -1 of substrate. Addition of acid through plant uptake of ammonium fertilizer or direct application of mineral acid reduced residual CCE over time. Residual limestone is an important substrate property which should be considered for ph management in greenhouse crop production. INTRODUCTION Liming materials differ in the rate at which substrate acidity is neutralized, 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, rather than substrate components. Although substrate components such as peat have a high CEC on a per weight basis (Lucas, 1982), their low bulk density results in a limited CEC per unit volume or container (Argo and Biernbaum, 1996). The most common liming materials used in greenhouse substrates are carbonatebased limestones. When carbonate-based limestones react with acid from proton (H + ) sources, such as acidic peat, then calcium (Ca 2+ ) and/or magnesium (Mg 2+ ), water (H 2 O), and carbon dioxide (CO 2 ) gas result: CaCO 3 (calcite) + 2H + Ca 2+ + H 2 O + CO 2 (gas) (1) CaMg(CO 3 ) 2 (dolomite) + 4H + Ca 2+ + Mg 2+ + 2H 2 O + 2CO 2 (gas) (2) The substrate concentration of residual lime [in units of calcium carbonate (CaCO 3 ) equivalents, CCE] on the left side of Equations 1 and 2 could be quantified by various methods, including gasometric and titration procedures. A gasometric procedure using a Chittick apparatus measures unreacted carbonate lime concentration through addition of a strong acid such as HCl, and subsequent measurement of released CO 2 gas through volume displacement (Huang et al., 2007a). In the titration method, a known quantity of HCl is added to the media and allowed to react with soil bases. After the Proc. IS on Growing Media 2007 Eds.: W.R. Carlile et al. Acta Hort. 819, ISHS 2009 249

reaction continues to completion, the unreacted HCl in the suspension solution is backtitrated using standardized NaOH, and CCE is calculated from the acid initially consumed by the alkalinity of the substrate. A series of experiments were conducted to evaluate the residual CCE and ph buffering in the container media. This article outlines the methods used, and describes (1) the effect of different rates of applied CaCO 3 on ph and residual CCE in a peat substrate, (2) ph buffering for substrates with and without residual limestone, in a greenhouse plant experiment where an acid-reaction fertilizer was applied, and (3) differences in ph buffering between commercial substrates. MATERIALS AND METHODS The Gasometric Method A gasometric system adapted from a Chittick device (Dreimanis, 1962) was described by Huang et al. (2007a) for measuring reaction rate of different carbonate limestone sources and particle size fractions. The gasometric system consisted of a gas measuring burette, acid dispensing burette, level burette, 1 liter decomposition flask and a magnetic stirrer. CCE was calculated based on volumetric measurement of CO 2 through displacement of a solution in the measuring burette using the Ideal Gas Law with known temperature and air pressure. A detailed procedure was developed for measuring carbonate based residual CCE in the substrate using the gasometric system (Huang et al., 2007b). The general procedure was to measure a 0.05 or 0.1 L substrate sample in a beaker, with the substrate packed to similar bulk density as would occur in an irrigated and drained container. These subsamples were placed into 1 L decomposition flasks. Deionized water was added to the substrate at 1.5 times the sample volume (i.e., 0.075 or 0.15 L for the substrate sample of 0.05 or 0.1 L, respectively). The flask was then attached to the gasometric system. The system was closed and 6M HCl was introduced into the decomposition flask at half the sample volume (0.025 or 0.05 L aliquot of 6M HCl for 0.05 or 0.1 L substrate samples, respectively). The sample in the flask was constantly stirred using a magnetic stirrer, and a heat isolation pad was placed between the top surface of the stirrer and the bottom of decomposition flask. The reaction time was 10 min. for reagent CaCO 3, and 30 min. (Dreimanis, 1962) for horticultural limestone. After reaction, the apparatus was left to stand for 2 min. for temperature and pressure within the apparatus to come to room conditions. The temperature and barometric pressure of air surrounding apparatus was then measured. Titration Method for Total Alkalinity Measurement A 50 ml substrate sample was measured (as described previously) and transferred into a 250 ml beaker. 20 ml of standardized 0.5N HCl was added to the sample and left overnight for complete reaction with substrate bases. The remaining unreacted HCl in the suspension solution was back-titrated using standardized 0.25N NaOH to ph 7.0. The residual CCE was calculated from the acid initially consumed by the carbonates. Experimental Details 1. CaCO 3 Incorporation into a Substrate. Reagent CaCO 3 was incorporated into a peat substrate at a rate of 3, 6, 9, 12 g CaCO 3 L -1 of substrate, which was maintained at container capacity. The volume of substrates for all experiments was generally measured using either a beaker (for small samples) or a soil measuring box (for large samples), with the substrates packed to similar bulk density as would occur in an irrigated and drained container. The research peat source used in all experiments was Canadian Sphagnum peat (Sun Gro Horticulture, Vancouver, Canada) with long fibers and little dust (Von Post scale 1-2; Puustjarvi and Robertson, 1975). After 14 days, residual CCE was measured using the gasometric method, and substrate-ph was measured using the saturated medium extract method (Warncke, 1995), with three replicates per lime rate. Reacted lime was calculated by subtracting applied CCE minus the residual CCE (g CCE L -1 of substrate). 250

2. Effect of Residual Lime on ph Buffering in a Greenhouse Crop. A 70% peat:30% perlite (by volume) substrate was amended with dolomitic hydrated lime (97% Ca(OH) 2 MgO, 92% of which passed through a 45-µm screen, National Lime and Stone, Findlay, Ohio, acid neutralizing value 161% CCE) at a rate of 2.8 kg m -3 to raise substrate-ph to 7.04. One week after substrate mixing, half of the substrate was further amended with 2.22 kg m -3 of a superfine dolomitic carbonate limestone (National Lime and Stone, Findlay, Ohio with a neutralizing value of 107% CCE). Super Elfin hybrid impatiens (Impatiens wallerana Hook. F.) seedling plugs were grown in a polycarbonate greenhouse for 6 weeks (average temperature 22.4±4.3 C, average daily light integral 10.9±3.0 mol m -2 d -1, mean ± standard deviation) in 10-cm-diameter pots. The substrate was either amended with the additional carbonate lime ( carbonate lime substrate ) or contained the hydrated lime only ( hydrated lime substrate ). Each pot was a replicate, and pots with the two substrates were randomly located on three benches (blocks). A highly acid reaction fertilizer (15N-1.7P-12.5K, with 100% of N as NH 4 -N, (Greencare, Ill.)) was applied at 100 mg L -1 N with each irrigation and zero leaching. Substrate-pH (saturated medium extract) was measured weekly, and residual CCE (gasometric and titration methods) was measured destructively every 2 weeks over a 6-week period. 3. Differences in ph Buffering between Commercial Substrates. Five commercial container substrates and one research substrate were selected that represented a wide range of residual CCE concentrations. The five commercial substrates were typical peatbased growing media produced by major media companies in the US, Canada, and Europe. The research substrate was 70% peat:30% perlite (by volume), and dolomitic hydrated lime (same lime source as experiment 2) was incorporated into the research substrate at 2.1 kg m -3. Initial substrate-ph (saturated medium extract) and residual CCE (gasometric method) were measured on 3 replicates for each substrate at the start of the experiments. ph buffering was measured in a 6-week greenhouse experiment with hybrid impatiens (Impatiens wallerana Hook. F.) and acid reaction fertilizer (15N-1.7P-12.5K) at 150 mg L -1 N, with a similar design to experiment 2 (Media 1 to 5 only were included in this trial). In a separate trial, 350 ml samples of each substrate were placed in open plastic bags at 25 C near container capacity (minimal evaporation occurred) for 7 days, and then received one dose of 0, 20, 40, 60, 80, or 100 meq of 0.5 N HCl L -1 of substrate. Substrate-pH was measured 7 days after the drench with four replicates. RESULTS AND DISCUSSION CaCO 3 Incorporation into a Substrate Within increasing applied concentrations of CaCO 3, substrate-ph showed a diminishing returns relationship (Fig. 1). This plateau in both ph and the concentration of reacted lime was expected above ph 7.0, because of reduced solubility of CaCO 3 at high ph. Substrate-pH ranged from 3.4 with no applied CaCO 3 to ph 7.2 following application of CaCO 3 at 6 g L -1. There was only a slight increase in ph (from ph 7.2 to 7.5) as applied CaCO 3 increased from 6 to 12 g L -1 of substrate. Application of additional lime beyond the plateau in ph level is expected to result in an increasing proportion of unreacted lime, which was measurable as residual CCE using the gasometric method (Fig. 1). The measured residual CCE was zero for CaCO 3 applied at 0 or 3 g L -1, and the residual CCE increased from 0.32 g L -1 up to 6.08 g L -1 following application of CaCO 3 from 6 to 12 g L -1. The calculated amount of reacted lime (correlated with ph) increased as CaCO 3 application rate rose from 3 to 6 g L -1, but increased only slightly between 6 and 12 g CaCO 3 L -1. The CCE of reacted lime was 5.68, 5.77 and 5.92 g L -1 for applied CaCO 3 at 6, 9, and 12 g L -1, respectively. The gasometric method could be used to identify residual CCE at different lime incorporation rates for various substrate and limestone combinations. This method may therefore have value for quality control in substrate formulation, and for development of substrates that have high buffering capacity. 251

Effect of Residual Lime on ph Buffering in a Greenhouse Crop Fertigation with an acid-reaction fertilizer applied to a greenhouse crop caused a decrease in substrate-ph over time (Fig. 2A). Substrate that contained carbonate plus hydrated lime ( carbonate lime substrate ) showed greater buffering to a drop in ph compared with the substrate with hydrated lime only ( hydrated lime substrate ). Over the 6-week growth period, the ph for carbonate lime and hydrated lime substrates dropped by 1.97 and 2.78 ph units, respectively. Using the gasometric method, residual CCE dropped from 2.5 to 1.2 g L -1 over time in the carbonate lime substrate (Fig. 2B). In the hydrated lime substrate, the gasometric method showed a consistently low level of residual CCE (0.2 to 0.4 g L -1 ). The initial residual CCE measured by the titration method was higher than the residual CCE measured with the gasometric method, and reduced over time in both substrates. At the end of the 6-week period, the residual CCE measured with titration was lower than from the gasometric method. Residual CCE measured with titration was higher for the carbonate lime substrate than the hydrated lime substrate throughout the experiment. The titration method measures alkalinity from all sources, including carbonate and bicarbonate, but also non-carbonate sources that would not be measured with the gasometric method, for example phosphates or substrate cation exchange capacity. At week 0, the higher residual CCE measured by the titration method may have been largely the result of cation exchange. Differences in ph Buffering between Commercial Substrates Initial ph values of five commercial substrates and one research substrate ranged from 5.79 to 6.45 one week after being moistened to container capacity. However, there was a wide range in residual CCE (Table 1), from 0.27 to 4.91 g L -1. The lowest residual CCE measured for a commercial substrate was 0.96 g L -1 (Media 2). The applied CCE from lime reported by companies ranged from 3.39 to 6.36 g L -1. Although the measured residual CCE was lower than the applied CCE, as expected, differences in media compaction during mixing by companies versus compaction during measurement with the gasometric method may cause errors in comparing applied versus residual CCE. In addition, Media 6 had the highest residual CCE, and this substrate contained vermiculite which is also a source of residual CCE. In a separate trial, we measured an average residual CCE from five horticultural vermiculite sources of 2.04±1.76 g L -1 (mean± standard deviation) using the gasometric method, compared with less than 0.2 g L -1 for coconut coir, perlite, and peat. Change in substrate-ph following different drench rates of HCl (Fig. 3A) showed that substrates with a higher amount of residual CCE (in the order of Media 1 to 6) tended to have greater buffering to ph change than substrates with less residual CCE. Similar trends in relative ph buffering between substrates occurred when impatiens were grown on these substrates for 6 weeks with 100% NH 4 -N acid fertilizer (Fig. 3B). Final residual CCE after 6 weeks equaled 0.30, 0.46, 0.97, 2.40, and 2.59 g L -1 for Media 1 to 5, respectively, showing a decline in residual CCE in all substrates except Media 1, and the same positive correlation of final residual CCE with final ph. These results showing improved ph buffering of substrates with high levels of residual CCE emphasize the importance of residual CCE in substrate formulation when developing growing media that are resistant to a downward trend in ph over time. ACKNOWLEDGEMENTS We thank the American Floral Endowment, and Young Plant Research Center partners including U.S. greenhouse firms and Blackmore Co., Ellegaard, Fafard, Greencare Fertilizers, Pindstrup, Premier Horticulture, Quality Analytical Laboratories, and Sun Gro Horticulture for financial support of this project. The use of trade names in this publication does not imply endorsement of the products named or criticism of similar ones not mentioned. 252

Literature Cited Argo, W.R. and Biernbaum, J.A. 1996. The effect of lime, irrigation-water source, and water-soluble fertilizer on the ph and macronutrient management of container rootmedia with impatiens. J. Amer. Soc. Hort. Sci. 121(3):442-452. Dreimanis, A. 1962. Quantitative gasometric determination of calcite and dolomite by using Chittick apparatus. J. Sedimentary Petrology 32:520-529. Huang, J.S., Fisher, P.R. and Argo, W.R. 2007a. A Protocol to quantify the reactivity of carbonate limestone for horticultural substrates. Comm. Soil Sci. Plant. Anal. 38:719-737. Huang, J.S., Fisher, P.R. and Argo, W.R. 2007b. A gasometric procedure to measure residual lime in container substrates. HortScience 42(7):1685-1689. Lucas, R.E. 1982. Organic soils (Histosols). Research Rpt. 435. Mich. Agr. Expt. Sta., East Lansing, Mich., USA. Puustjarvi, V. and Robertson, R.A. 1975. Physical and chemical properties. In: D.W. Robinson and J.G.D. Lamb (eds.), Peat in horticulture. Academic Press, London p.23-38. Warncke, D.D. 1995. Recommended test procedures for greenhouse growth media. p.76-83. In: Recommended soil testing procedures for the Northeastern United States, 2 nd Ed. Univ. of Delaware Agricultural Experiment Station, Bulletin #493, Dec. 1995. Tables Table 1. The residual CCE content (g CCE L -1 ) and substrate-ph for one research substrate (Media 1) and five commercial substrates (Media 2 to 6). Lime incorporation rate was reported by substrate manufacturers. Initial residual CCE content was measured using the gasometric method, and substrate-ph using the saturated medium extract method (3 replicates per substrate). Media Media components Lime Initial Initial ph no. incorporation rate residual CCE Mean ± 95% (g CCE L -1 ) (g CCE L -1 ) Mean ± 95% 1 Peat, perlite, hydrated dolomite lime 3.38 0.27 ± 0.04 6.10 ± 0.06 2 Peat, calcitic lime 3.39 0.96 ± 0.10 6.04 ± 0.07 3 Peat, calcitic and dolomitic lime 4.95 1.97 ± 0.31 6.21 ± 0.04 4 Peat, perlite, dolomitic lime 6.05 2.83 ± 0.13 5.79 ± 0.04 5 Peat,perlite, calcitic and 5.32 3.73 ± 0.35 6.11 ± 0.05 dolomitic lime 6 Peat, perlite, vermiculite, dolomitic lime 6.36 4.91 ± 0.02 6.45 ± 0.11 253

Figurese 15 12 ph Reacted Residual 8 7 Estimated CCE (g/l) 9 6 6 5 ph 3 4 0 0 3 6 9 12 Applied CCE (g/l) 3 Fig. 1. Substrate-pH response and residual CCE (gasometric system) in a peat substrate and the corresponding reacted CCE 14 d after CaCO 3 was incorporated into a peat substrate at a rate of 3, 6, 9, 12 g L -1 of substrate. Each sample represents the average of 3 replicates, and error bars represent 95% CIs. Reacted lime was calculated by subtraction of the applied CCE minus the estimated residual CCE (g L -1 of substrate). Substrate-pH 7.0 6.0 5.0 A Carbonate Lime Substrate Hydrated Lime Substrate Residual CCE (g.l -1 ) 5.0 4.0 3.0 2.0 1.0 B Carbonate Lime Substrate, Titration Carbonate Lime Substrate, Gasometric Hydrated Lime Substrate, Titration Hydrated Lime Substrate, Gasometric 4.0 0 1 2 3 4 5 6 Week 0.0 0 1 2 3 4 5 6 Week Fig. 2. Substrate-pH and residual CCE changes over time for impatiens plants grown in a 70% peat:30% perlite substrate. An acid reaction fertilizer (15N-1.7P-12.5K, 100% N in NH 4 -N form) was applied with each irrigation. (A) Average ph over time for the substrate containing carbonate and hydrated lime ( Carbonate Lime Substrate ), or containing hydrated lime only ( Hydrated Lime Substrate ). (B) The residual CCE over time for both substrates, using the gasometric or titration methods. Each symbol represents the average of 6 replicates, and error bars represent 95% CIs (for substrate-ph) or standard error (for residual CCE). 254

Substrate-pH Delta ph 0.0-1.0-2.0-3.0-4.0-5.0-6.0 7.0 6.0 5.0 4.0 3.0 Media 6 Media 5 Media 4 Media 3 Media 2 Media 1 0 20 40 60 80 100 Acid (HCl) applied (meq.l -1 ) Media 5 Media 4 Media 3 Media 2 Media 1 0 7 14 21 28 35 42 49 Day A B Fig. 3. Substrate-pH changes over time for one research substrate (Media 1) and five commercial substrates (Media 2 to 6, described in Table 1). (A) Effect of substrate drenches with HCl acid on substrate-ph. Titration curves were developed by plotting the meq of HCl applied versus average delta ph (ph of treatment minus the ph from the 0 HCl drench). (B) Substrate-pH changes over time when impatiens plants were grown with 100% NH 4 -N acid fertilizer applied at every irrigation for 6 weeks (Media 1 to 5 only). Error bars represent standard error with n=4. 255

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