CSIRO PUBLISHING www.publish.csiro.au/journals/ajsr
Australian Journal of Soil Research, 2010, 48, 201–207
Soil pH buffering capacity: a descriptive function and its application to some acidic tropical soils Paul N. Nelson A,B,C and Ninghu Su A,B A
School of Earth and Environmental Sciences, James Cook University, Cairns, Qld 4870, Australia. Department of Environment and Resource Management, Mareeba, Qld 4880, Australia. C Corresponding author. Email:
[email protected] B
Abstract. Calculation of soil acidification rates requires knowledge of pH buffering capacity (pHBC), which is measured using titration methods. The pHBC is often quoted as a single value for a particular soil, implying a linear relationship between pH and the amount of acid or alkali added. However, over its whole range, the relationship is sigmoid rather than linear, and in many soils pH is low or high enough to be outside of the linear range. In this work we fitted a simple sigmoid function to pH buffer curves of 8 tropical Australian soils obtained using one titration method and 58 Papua New Guinean (PNG) soils obtained using another titration method. The function described the curves well for all soils (adjusted r2 > 0.93 for all samples and >0.99 for 90% of samples), irrespective of the titration method, allowing pHBC to be calculated as a function of pH across the range of pH values established. Using the function, the contribution of variable charge to pHBC was calculated for the PNG soils; on average it was 93% at the pH buffer curves’ inflection point, which corresponds with the soil’s minimum pHBC. Factors other than variable charge became important at pH (1 : 5, 0.002 M CaCl2) values 6.0. The relationship between pHBC and soil organic matter content was closest at pH 6.0–6.5. Application of the sigmoid function could facilitate more accurate assessments of acidification risks, acidification rates, and potential management interventions, particularly as soils become increasingly acidic. Additional keywords: soil acidification, charge fingerprint, variable charge, organic matter, cation exchange capacity, Papua New Guinea, Andosol, sigmoid function.
Introduction Soil acidification is a major problem for agricultural sustainability throughout much of the world, and alkalisation is a problem in drier irrigated regions (Sumner and Noble 2003; Brady and Weil 2008). In Australia, the National Land and Water Resources Audit estimated that 12–24 Mha were extremely to highly acidic with pH values in 0.01 M CaCl2 4.8 (below the optimum for acid-sensitive agricultural plants) and that, in the absence of remedial lime application, the area would increase to 29–60 Mha within 10 years (NLWRA 2001). Soil pH change depends on initial pH, net inputs of acid or alkali, and the soil’s pH buffering capacity (pHBC). Soil pHBC is governed mostly by protonation/deprotonation of acidic groups on organic matter, oxides, and hydroxides; dissolution/precipitation of carbonates; complexation/decomplexation of Al by organic matter; and ion exchange; with dissolution of clay minerals and primary minerals also being significant at longer time scales or in soils with high contents of weatherable minerals (Bloom 2000). The relative significance of these processes varies with pH. Over the approximate pH range 4.0–6.5, there is a close relationship between soil pHBC and organic matter content, and to a lesser extent with clay content (e.g. Magdoff and Bartlett 1985; Aitken et al. 1990a; Helyar et al. 1990; Dolling et al. 1994; Noble et al. 1997). Those relationships have been exploited to estimate pHBC using pedotransfer functions based CSIRO 2010
on commonly measured parameters (e.g. Aitken et al. 1990a; Helyar et al. 1990). Soil pHBC, measured using titration techniques that produce a pH buffer curve, is used in the estimation of acidification risks and acidification rates (Helyar and Porter 1989; Ahern et al. 1993; Dolling and Porter 1994; Dolling et al. 1994; Dolling 1995; Porter et al. 1995; Moody and Aitken 1997; Lesturgez et al. 2006) and has also been used to determine lime requirement (McLean et al. 1966; Follett and Follett 1983). The titration technique is rather laborious, and for routine lime requirement tests it has been largely replaced by buffer solution methods (Aitken et al. 1990b), but it still remains an important technique for acidification studies. No standard titration method is in use, but various conditions have been assessed and recommendations made for the type and concentration of electrolyte and acid or alkali added, soil : solution ratio, and incubation conditions (Helyar and Porter 1989; Aitken et al. 1990b; Barrow and Cox 1990; Ridley et al. 1990; Aitken and Moody 1994; Oorts et al. 2004). The pH buffer curve is sigmoid in shape, and Helyar and Porter (1989) recommended that the titration curve should be characterised in any acidification study. However, a linear approximation has been shown to be adequate in the pH 4.5–6.5 range (1 : 2, 0.01 M CaCl2, Magdoff and Bartlett 1985), and most subsequent studies have used a linear approximation. 10.1071/SR09150
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For example, in a study of 40 acidic soils from Queensland, Aitken et al. (1990a) reported that the pH buffer curve was ‘essentially linear’ over the pH range 4.0–6.5. A logical extension of the linear approximation is a simplified 2-point titration for determining pHBC (Ridley et al. 1990; Noble et al. 2002). Although a linear approximation is valid over a certain range of pH, many soils have, or are approaching, the pH where pHBC starts to increase dramatically with decreasing pH. In these cases a linear approximation of the pH buffer curve can result in erroneous interpretations of management effects on acidification, and inaccurate predictions. For example, Lesturgez et al. (2006) showed that in acidic (pH 4) soils of Thailand, pHBC was effectively infinite, with no change in pH occurring despite considerable net acid addition rates. The importance of increasing values of pHBC at low pH has been recognised in modelling studies of pH change. Hochman et al. (1995, 1998) included pH and exchangeable Al in their estimates of pHBC (by multiple linear regression), along with effective cation exchange capacity and organic C content. Verburg et al. (2003) estimated pHBC as the sum of 3 terms. The first term represents the pHBC of the mineral phase, which remains constant during the simulation. The second term represents the pHBC of the organic matter and could change in response to changes in soil organic matter content. The third term varies with the concentration of Al3+ in soil solution, taking into account the increase in pHBC at low pH. An important point to keep in mind is that, although additions of acid have less and less of an effect on pH as soils become more acidic, acidification (loss of acidneutralising capacity) is still occurring. Loss of acid-neutralising capacity due to dissolution of minerals at low pH can be considered permanent, unlike acidification processes in the pH range 4.5–6.0. Given the complexity of modelling acidification, including calculation of pHBC at changing values of pH, and the amount of experimental data required to do it mechanistically, an alternative approach may be useful—to measure the soil’s pH buffer curve using appropriate conditions and then to use the empirical curve in models. The aims of this work were (a) to determine if a simple sigmoid function adequately described empirical pH buffer curves over the whole pH range for a variety of acidic tropical soils, and (b) to determine relationships between pHBC and soil properties over a range of pH values.
Methods Two sets of soil samples were analysed. An Australian set comprised 4 highly weathered soils from north Queensland, each sampled at 0–0.1 and 0.7–0.8 m depth (Table 1). Pingin is a Ferrosol (Murtha et al. 1996), Inlet is a Hydrosol (Murtha et al. 1996), Nicotine is a Tenosol (Enderlin et al. 1997), and Arriga is a Vertosol (Enderlin et al. 1997). A Papua New Guinean (PNG) soil set comprised 10 Andosols, mostly quite young—8 from West New Britain Province (Kumbango, Dami, Malilimi, Bilomi, Bialla, Balaha, and Navo oil palm plantations and Kumbango forest) and 2 from Oro Province (Ambogo and Mamba oil palm plantations), each sampled at a range of depths to ~1 m (Table 1). In the West New Britain soils, sampling was
P. N. Nelson and N. Su
Table 1. Properties of soils examined Clay content pH1 : 5A (%)
Soil
Pingin 0–0.10 m Pingin 0.70–0.80 m Inlet 0–0.10 m Inlet 0.70–0.80 m Arriga 0–0.10 m Arriga 0.70–0.80 m Nicotine 0–0.10 m Nicotine 0.70–0.80 m
Australian 37.8 49.7 13.9 43.9 40.1 44.1 10.5 19.7
soils 4.27 4.07 4.10 4.47 4.47 5.30 4.77 4.17
CECTB (mmolc/kg)
Organic CC (g/kg)
n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
41.7 24.9 22.6 12.2 25.1 21.5 8.8 6.9
Papua New Guinean soils in Figs 1,2,3,5 Kumbango forest 8.0 6.40 215 0–0.08 m Kumbango forest 1.3 6.48 52 0.85–1.07 m Navarai 0–0.13 m 5.4 5.02 125 Navarai 0.79–1.00 m 15.9 4.09 75 Malilimi 0–0.16 m 12.3 5.78 125 Malilimi 0.77–0.97 m 2.2 5.90 10 Mean Minimum Maximum
Whole Papua New Guinean soil dataset 6.8 5.78 66 0.4 4.65 4 31.5 6.75 280
67.7 1.5 44.5 2.0 49.3 1.0 16.6 0.3 67.7
A
In 0.04 M KCl for Australian soils and 0.002 M CaCl2 for PNG soils. At soil pH. C By wet oxidation for PNG soils and from loss on ignition divided by 2 for Australian soils. B
carried out according to visible horizons, which in many cases corresponded to discrete tephra layers. The pH buffer curves were determined in different ways for the 2 sample sets. For the Australian soil set, titration curves were made using NaOH and HCl additions in a background of 0.04 M KCl and an equilibration time of 1 h. For the PNG soil set, the laboratory analyses reported here were carried out and reported by Gillman and Gillman (2001). Samples were kept field-moist until analysis. The pH buffer curves were determined concurrently with measurements of charge characteristics or ‘charge fingerprints’. The charge fingerprint measurements involved initial saturation of the soils with Ca using 0.1 M CaCl2, followed by washing to a concentration of 0.002 M CaCl2, pH adjustment of separate samples to values ranging from 4 to 7 using measured amounts of HCl or Ca(OH)2, and finally, measurements of total cation exchange capacity (CECT, occupied by Ca2+ + Al3+), CEC occupied by Ca2+ (CECB), anion exchange capacity (occupied by Cl–), and pH (Gillman and Sumpter 1986). The shape of pH buffer curves can be approximated by the sigmoid function, Eqn 1: pH ¼ pHmin þ
a A Amid b 1þe
ð1Þ
in which pHmin is the minimum value of pH reached, pHmin + a is the maximum value of pH reached, A is the amount of acid (negative) or alkali (positive) added, Amid is the A value of the curve’s inflection point, and b defines the shape of the
Soil pH buffering capacity
Australian Journal of Soil Research
curve. At A = Amid, pH = pHmin + a/2 = pHmid. The function is symmetrical in shape around the inflection point. As pH of a soil suspension is highly unlikely to reach values 13, the constraints of pHmin >1 and pHmin + a 0.99 for 61 of the samples and >0.93 for all 66 samples (Table 2). In all cases r2 values for the sigmoid function were greater than for a linear function over the measured range. Soil pHBC decreased with depth in most samples, indicating the importance of organic matter. In the Australian soils there was a strong linear relationship between pHBC and organic C content (r2 = 0.71). An exception was the Pingin soil, in which pHBC was greater at depth than at the surface (Fig. 2, Table 2). In the PNG soils the relationship between pHBCmin and organic C content was not as strong as for the Australian soils (linear r2 = 0.54). Most of the PNG soils had a similar ratio of pHBCmin : organic C to the Australian soils, but for some of them the ratio was much higher. In many of the soils, pHBC varied considerably with pH (Fig. 2). In almost all of the PNG soils, pHBC increased markedly at pH 0.7 at pH 6.5–7), indicating that the higher organic matter contents in the surface horizon were also associated with higher variability in the acid/base behaviour of the organic matter. The contribution of organic matter to pHBC was due not only to its variable charge, as shown by 2 lines of evidence. First, at any particular pH value (in the range 4–7), organic C content was more closely
Soil pH buffering capacity
Australian Journal of Soil Research
300
205
6 Kumbango forest 0–8 cm
Kumbango forest 0–8 cm
Kumbango forest 85–107 cm
250
Kumbango forest 85–107 cm
5
Navarai 0–13 cm
Navarai 0–13 cm
Ratio of pHBC to ΔCEC
CECB (mmolc/kg)
Navarai 79–100 cm Malilimi 0–16 cm
200
Malilimi 77–97 cm
150
100
50
Navarai 79–100 cm
4
Malilimi 0–16 cm Malilimi 77–97 cm
3
2
1
0 3
4
5
6
7
8
0 4.0
pH
4.5
5.0
5.5
6.0
6.5
7.0
pH
Fig. 3. Charge fingerprints of selected Papua New Guinean soils.
Fig. 5. Ratio of pH buffering capacity (pHBC at pHmid) to variable charge (DCEC, at pHmid) for selected Papua New Guinean soils, as a function of pH. Points do not correspond to measured values; they are shown only for the purpose of distinguishing between lines.
60 y = 1.074x + 1.590 r 2 = 0.885
pHBC (mmol/kg.pH unit))
50
Discussion 40
30
20
10
0 0
10
20
30
40
50
60
ΔCEC (mmolc/(kg.pH unit)) Fig. 4. Relationship between pH buffering capacity (pHBC at pHmid) and variable charge (DCEC at pHmid) for all the Papua New Guinean soils.
related to pHBC (r2 0.25–0.61 depending on pH) than to DCEC (r2 0.19–0.38). Second, the ratio of pHBC to DCEC (at pH values >5.5) was higher for topsoils than for the corresponding subsoils (Fig. 5). The difference between acidification rates predicted using the sigmoid function described here and a linear function was assessed for the PNG soils. For the linear approximation, the most linear part of the curve was used (pHBC at pHmid). The difference between predictions made using the 2 methods increased as the ‘target’ pH decreased. So on average over all the PNG soils, the amount of acid required to reduce soil pH from its existing value to values of 5.0, 4.5, or 4.0 was 6.8, 13.6 or 32.5% higher, respectively, using the sigmoid model than using the linear model.
The relationship between pHBC and pH differed between the Australian and PNG soils and between soils within each set. Soil pHBC increased markedly at pH values
