Acta Hort. (ISHS) 819:249-256
Residual Lime and pH Buffering in Container Substrates
P.R. Fisher, Jinsheng Huang, W.E. Horner
and C.N. Johnson
Environmental Horticulture Dept.
University of Florida
P O Box 110670, Gainesville FL 32611-0670
USA
W.R. Argo
Blackmore Co.
10800 Blackmore Ave.
Belleville, MI 48111
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 CaCO3 equivalent (CCE)
per liter of substrate], whereby a strong mineral acid (HCl) was applied to a substrate
sample and the evolved CO2 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 CaCO3 concentration increased,
particularly at pH above 7 because of limited CaCO3 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 (Ca2+) and/or magnesium (Mg2+), water (H2O),
and carbon dioxide (CO2) gas result:
CaCO3 (calcite) + 2H+ ↔ Ca2+ + H2O + CO2 (gas)
(1)
+
2+
2+
CaMg(CO3)2 (dolomite) + 4H ↔ Ca + Mg + 2H2O + 2CO2 (gas)
(2)
The substrate concentration of residual lime [in units of calcium carbonate
(CaCO3) 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 CO2 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
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Acta Hort. (ISHS) 819:249-256
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 CaCO3 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 CO2 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 CaCO3, 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. CaCO3 Incorporation into a Substrate. Reagent CaCO3 was incorporated into a peat
substrate at a rate of 3, 6, 9, 12 g CaCO3·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).
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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 NH4-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
CaCO3 Incorporation into a Substrate
Within increasing applied concentrations of CaCO3, 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 CaCO3 at high
pH. Substrate-pH ranged from 3.4 with no applied CaCO3 to pH 7.2 following application
of CaCO3 at 6 g·L-1. There was only a slight increase in pH (from pH 7.2 to 7.5) as applied
CaCO3 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 CaCO3
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 CaCO3 from 6 to 12 g·L-1. The calculated amount of reacted lime
(correlated with pH) increased as CaCO3 application rate rose from 3 to 6 g·L-1, but
increased only slightly between 6 and 12 g CaCO3·L-1. The CCE of reacted lime was 5.68,
5.77 and 5.92 g·L-1 for applied CaCO3 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.
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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% NH4-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.
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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. 38:719737.
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.2338.
Warncke, D.D. 1995. Recommended test procedures for greenhouse growth media. p.7683. In: Recommended soil testing procedures for the Northeastern United States, 2nd
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
no.
1
2
3
4
5
6
Media components
Lime
incorporation rate
(g CCE·L-1)
Initial pH
Mean ± 95%
3.38
3.39
4.95
6.05
5.32
Initial
residual CCE
(g CCE·L-1)
Mean ± 95%
0.27 ± 0.04
0.96 ± 0.10
1.97 ± 0.31
2.83 ± 0.13
3.73 ± 0.35
Peat, perlite, hydrated dolomite lime
Peat, calcitic lime
Peat, calcitic and dolomitic lime
Peat, perlite, dolomitic lime
Peat,perlite, calcitic and
dolomitic lime
Peat, perlite, vermiculite,
dolomitic lime
6.36
4.91 ± 0.02
6.45 ± 0.11
6.10 ± 0.06
6.04 ± 0.07
6.21 ± 0.04
5.79 ± 0.04
6.11 ± 0.05
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Acta Hort. (ISHS) 819:249-256
Figurese
15
8
pH
Reacted
Residual
7
9
6
6
5
3
4
pH
Estimated CCE (g/L)
12
0
3
0
3
6
9
12
Applied CCE (g/L)
Fig. 1. Substrate-pH response and residual CCE (gasometric system) in a peat substrate
and the corresponding reacted CCE 14 d after CaCO3 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).
5.0
A
Residual CCE (g.L-1)
Substrate-pH
7.0
6.0
5.0
Carbonate Lime Substrate
Hydrated Lime Substrate
4.0
Carbonate Lime
Substrate, Titration
Carbonate Lime
Substrate, Gasometric
Hydrated Lime
Substrate, Titration
Hydrated Lime
Substrate, Gasometric
B
4.0
3.0
2.0
1.0
0.0
0
1
2
3
4
Week
5
6
0
1
2
3
Week
4
5
6
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 NH4-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).
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Acta Hort. (ISHS) 819:249-256
0.0
A
Delta pH
-1.0
-2.0
Media 6
Media 5
Media 4
Media 3
Media 2
Media 1
-3.0
-4.0
-5.0
-6.0
0
20
40
60
80
100
Acid (HCl) applied (meq.L-1)
Substrate-pH
7.0
B
6.0
5.0
Media 5
Media 4
Media 3
Media 2
Media 1
4.0
3.0
0
7
14
21
28
Day
35
42
49
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% NH4-N acid fertilizer applied at every
irrigation for 6 weeks (Media 1 to 5 only). Error bars represent standard error with
n=4.
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