Acid–base reactions between an acidic soil and plant residues

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Geoderma 123 (2004) 219–232

Acid–base reactions between an acidic soil and plant residues

G.M. Sakalaa,b, D.L. Rowella,*, C.J. Pilbeama,c

aDepartment of Soil Science, The University of Reading, Whiteknights, P.O. Box 233, Reading, Berkshire RG6 6DW, UKbMount Makula Research Station, Private Bag 7, Chilanga, Zambia

cCranfield University School of Management, Cranfield, Bedford, AL MK43 0, USA

Received 8 August 2002; received in revised form 22 December 2003; accepted 3 February 2004

Available online 12 March 2004

Abstract

The elemental composition of residues of maize (Zea mays), sorghum (S. bicolor), groundnuts (Arachis hypogea), soya

beans (Glycine max), leucaena (L. leucocephala), gliricidia (G. sepium), and sesbania (S. sesban) was determined as a basis for

examining their alkalinity when incorporated into an acidic Zambian Ferralsol. Potential (ash) alkalinity, available alkalinity by

titration to pH 4 and soluble alkalinity (16 h water extract titrated to pH 4) were measured. Potential alkalinity ranged from 373

(maize) to 1336 (groundnuts) mmol kg� 1 and was equivalent to the excess of their cation charge over inorganic anion charge.

Available alkalinity was about half the potential alkalinity. Cations associated with organic anions are the source of alkalinity.

About two thirds of the available alkalinity is soluble. Residue buffer curves were determined by titration with H2SO4 to pH 4.

Soil buffer capacity measured by addition of NaOH was 12.9 mmol kg� 1 pH� 1. Soil and residue (10 g:0.25 g) were shaken in

solution for 24 h and suspension pH values measured. Soil pH increased from 4.3 to between 4.6 (maize) and 5.2 (soyabean)

and the amounts of acidity neutralized (calculated from the rise in pH and the soil buffer capacity) were between 3.9 and 11.5

mmol kg� 1, respectively. The apparent base contributions by the residues (calculated from the buffer curves and the fall in pH)

ranged between 105 and 350 mmol kg� 1 of residue, equivalent to 2.6 and 8.8 mmol kg� 1 of soil, respectively. Therefore, in

contact with soil acidity, more alkalinity becomes available than when in contact with H2SO4 solution. Available alkalinity (to

pH 4) would be more than adequate to supply that which reacts with soil but soluble alkalinity would not. It was concluded that

soil Al is able to displace cations associated with organic anions in the residues which are not displaced by H+, or that residue

decomposition may have begun in the soil suspension releasing some of the non-available alkalinity. Soil and four of the

residues were incubated for 100 days and changes in pH, NH4+ and NO3

� concentrations measured. An acidity budget equated

neutralized soil acidity with residue alkalinity and base or acid produced by N transformations. Most of the potential alkalinity

of soyabean and leucaena had reacted after 14 days, but this only occurred after 100 days for gliricidia, and for maize only the

available alkalinity reacted. For gliricidia and leucaena, residue alkalinity was primarily used to react with acidity produced by

nitrification. Thus, the ability of residues to ameliorate acidity depends not only on their available and potential alkalinity but

also on their potential to release mineral N.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Plant residues; Plant composition; Plant alkalinity; Soil acidity; Buffering; Amelioration; Zambia

0016-7061/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.geoderma.2004.02.002

* Corresponding author. Tel.: +44-118-9316557; fax: +44-118-9316660.

E-mail address: d.l.rowell@reading.ac.uk (D.L. Rowell).

G.M. Sakala et al. / Geoderma 123 (2004) 219–232220

1. Introduction

The use of plant residues as ameliorants of soil

acidity for the highly weathered soils of the sub-

humid tropics offers a viable alternative to commer-

cial lime (Bessho and Bell, 1992; Wong et al., 1995,

2000; Tang et al., 1999). The amelioration is possible

because of the alkalinity that the residues contain,

which is larger in legumes than non-leguminous

plants (Pierre and Banwart, 1973; Noble et al.,

1996; Larsen, 1998). The inherent liming potential

of plant residues is attributable to the organic ligands

which serve to balance charge and regulate pH within

plants in response to unbalanced cation and anion

uptake and to nitrate reduction in the leaves (Kraffc-

zyk et al., 1984; Touraine et al., 1990; Imas et al.,

1997).

Increase in soil pH following the addition of plant

residues to an acid soil has been demonstrated in

incubation experiments (Hoyt and Turner, 1975; Bes-

sho and Bell, 1992; Pocknee and Sumner, 1997). The

mechanisms responsible for the pH increases include

ligand exchange (Parfitt et al., 1977), nitrogen miner-

alization (Hoyt and Turner, 1975; Pocknee and

Sumner, 1997) and decomposition of base cation-

containing organic compounds (Pocknee and Sumner,

1997). Wong et al. (1998) found that in stable humi-

fied materials (composted organic materials, peat and

farmyard manure), about 60% of the base cations

were not involved in proton exchange when titrated

down to pH 4, and concluded that base cations

occurring as neutral inorganic salts and as structural

constituents were not involved as sources of alkalinity.

In a subsequent paper (Wong et al., 2000), young tree

prunings were incubated with two tropical soils.

Relationships were found between the total base

cation contents of the prunings and the alkalinity

reacting with the soils, although no account was taken

of the inorganic anion content of the prunings and the

authors comment that nitrogen mineralization and

nitrification resulting from the addition of prunings

to the soil may have influenced the results.

Most of the data reported in the literature regarding

pH changes of soils amended with plant residues are

measured after long periods of incubation and are

interpreted in the context of potential alkalinity (Noble

et al., 1996; Pocknee and Sumner, 1997; Wong et al.,

1998, 2000). This total fails to distinguish between

that which is immediately available and that which

becomes available over long periods of time due to

decomposition of the residue. It is the former which

determines the immediate ameliorating effect of plant

residues on soil acidity and thus the likely success of a

young plant sown soon after residues have been

ploughed into the soil.

A 14-day incubation of organic materials with

three tropical soils by Wong et al. (1998) was used

to show that the final pH obtained could be approx-

imately predicted from a knowledge of the initial pH

values of the soil and organic material and their buffer

characteristics, if the buffer capacity of the soil was

measured by incubation with lime for 14 days and that

of the organic material was measured by titration with

acid down to pH 4: the time taken for the titration was

not recorded. Using tree prunings, Wong et al. (2000)

found that the pH of an ultisol increased over a period

of up to 98 day showing that there was a release of

initially non-available alkalinity. In an oxisol, howev-

er, pH increased during 14 days but then decreased

probably as a result of mineralization and nitrification

of the nitrogen compounds in the prunings. Thus, not

all of the alkalinity in a residue is active from the

onset of its addition to the soil. We postulate that four

fractions of residue alkalinity can be distinguished,

namely: (i) potential (or ash) alkalinity, (ii) Available

alkalinity, (iii) non-available (or reserve) alkalinity,

and (iv) soluble alkalinity. Ash or potential alkalinity

may be defined for plant material as the sum of

cations minus the sum of inorganic anions (Pierre

and Banwart, 1973). The excess positive charge is

balanced by organic anions which estimate the liming

potential of a residue. The organic anions may be

divided into those that readily ionize (available alka-

linity) and those that do not readily ionize (non-

available alkalinity). The soluble alkalinity is a frac-

tion of the available alkalinity.

The experiments reported here include measure-

ments of the various forms of plant residue alkalinity,

the buffering properties of the residues and their

interaction with the soil buffer system during incuba-

tion. The objectives are (a) to allow conclusions to be

drawn regarding the suitability of the analytical meth-

ods for predicting the ability of residues to ameliorate

soil acidity in the short and long-term, (b) to quantify

the role of nitrogen transformations through determi-

nation of proton budgets based on measurements of

G.M. Sakala et al. / Geoderma 123 (2004) 219–232 221

changes in NH4+ and NO3

� concentrations in the

incubated soils, and (c) to establish the relative

importance of alkalinity production from base cations

and acidity production by nitrogen transformations for

seven plant residues.

2. Materials and methods

2.1. Soil

The soil used in the experiment was an acid soil

(Rhodic Ferralsol) from northern Zambia. The mean

annual rainfall and temperature of the area are 1247

mm and 19.7 jC, respectively. The soil was sampled

from the top 20 cm from cleared land that had been

fallow for at least 5 years with no known fertilizer

history. Particle size analysis was determined by the

hydrometer method (Carter, 1993). Soil pH was

determined in 10 mM CaCl2 at a soil/solution ratio

of 1:2.5. Exchangeable base cations were determined

as described by Thomas (1982). Exchangeable alu-

minium and hydrogen were determined by the NaOH/

NaF method (Rowell, 1994).

Soil buffer capacity was determined after a 24-

h shaking with 10 mM NaOH. NaOH solutions (0, 1,

2, 3, 4 and 5 ml) were added in duplicate to 100-ml

plastic bottles containing 10 g of soil. An amount of

10 mM CaCl2 was added to give a final volume of 25

ml in all cases. The bottles were tightly closed and

placed on an end-over-end shaker for 24 h. The pH

was then measured.

2.2. Plant material

Seven plant residues from Zambia were character-

ised (Sakala, 1998). The residues included three

leguminous tree materials commonly found in agro-

forestry systems in the tropics and four crop residues.

The trees were leucaena (L. leucocephala), gliricidia

(G. sepium) and sesbania (S. sesban). Crop materials

included both leguminous and non-leguminous resi-

dues, namely, soyabeans (Glycine max.), groundnuts

(Arachis hypogea), maize (Zea mays) and sorghum (S.

bicolor). Crop materials included leaves and stover or

stem taken at physiological maturity while the tree

materials were prunings of young leaves and twigs.

All the residues were dried at 80 jC for 72 h and

ground to pass a 2-mm sieve for use in the incubation

experiment. A finer fraction ( < 0.1 mm) was prepared

by grinding in a Terra mill for use in chemical analysis

where < 1 g of material was required.

2.2.1. Total composition

Total base cations (Ca2 +, Mg2 +, K+ and Na+) were

determined in nitric acid digests. Ground plant mate-

rial (0.5 g) of each residue was weighed in duplicate

into 100-ml Kjeldahl digestion tubes. Ten milliliters of

concentrated nitric acid was added. Each tube was

closed and the samples were left to stand for 16 to 18

h. The tubes were then heated to 60 jC in a digestion

block. After 3 h, the temperature was increased to 110

jC and digestion continued for a further 6 h. The

tubes were then allowed to cool. The digests were

transferred to pre-washed filter papers and filtered into

100-ml volumetric flasks. The Ca2 + and Mg2 + con-

tents of the extracts were determined by atomic

absorption spectrometry and K+ and Na+ by flame

emission spectrometry.

Total N was determined by mass spectrometry after

combustion of residues at 1000 jC, removing water

vapour and CO2 and reducing the oxides of nitrogen

to N2. Total C was determined by dry combustion at

1000 jC using a LECO SC-444 gas analyser. Total

soluble phenolics were measured colorimetrically at

725 nm using the Folin-Ciocalteau reagent after

ultrasonic extraction in 70% aqueous acetone (Single-

ton and Rossi, 1965). Sodium carbonate was used for

colour development and gallic acid was used to

prepare standards. Hence, the values given are gallic

acid equivalents. Acid detergent fibre (ADF) and acid

detergent lignin were determined by the method of

Van Soest and Wine (1968).

2.2.2. Soluble components

Ten grams of plant material was weighed in dupli-

cate into 150-ml plastic bottles. One hundred milliliters

of de-ionized water was added, and the bottles were

shaken for 16 h. The suspensions were filtered and the

washings stored at 4 jC and analysed within 24 h for

base cations, P and Cl and for soluble alkalinity

(below). Sulphate was determined by the barium per-

chlorate-thorin method of Henriksen and Bergmann-

Paulsen (1974). NH4+ and NO3

+ were determined by

automated spectrophotometric methods. NO3� was re-

duced to NO2� using a copper–cadmium reductor

G.M. Sakala et al. / Geoderma 123 (2004) 219–232222

column followed by the formation of a diazo com-

pound. NH4+ was reacted with phenol in the presence of

sodium hypochlorite and sodium nitroprusside. The

solids remaining on the filter paper were washed with

de-ionized water, dried at 80 jC and weighed.

2.2.3. Forms of alkalinity and their determination

2.2.3.1. Potential (ash) alkalinity. Potential alkalin-

ity was determined by the method of Jarvis and

Robson (1983). Sub-samples of 0.5 g were heated

slowly to 400 jC in a muffle furnace and then held at

500 jC for 1 h. The ash was treated with 5 ml of 1 M

HCl followed by addition of three drops of phenol-

phthalein indicator. The excess HCl was then back-

titrated against 0.25 M NaOH.

2.2.3.2. Available alkalinity. Available alkalinity

was determined by titrating 0.25 g of plant material

in 25 ml of 10 mM CaCl2 from its natural pH to pH 4

with 5 mM H2SO4 in an autotitrator (MTS 800 Multi-

titration System). The autotitrator was set to give a 90-

s delay time for equilibration of the sample before

automatically recording the total acid added and the

resulting pH (about 1 h in total.) The suspension was

allowed a 15-min equilibration time prior to the

titration. A blank titration was carried out on 25 ml

of 10 mM CaCl2. This method is based on that used

by Wong et al. (1998) for stable humified materials.

When used for plant residues, the measurements are

less stable and depend on the total titration time.

Preliminary experiments showed that after 1 h further

changes were very small.

2.2.3.3. Non-available or reserve alkalinity. The

difference between the potential (or ash) alkalinity

and available alkalinity gave an estimate of the non-

available (or reserve) alkalinity of the residues.

2.2.3.4. Soluble alkalinity. Two milliliters of residue

washings, with 23 ml of 10 mM CaCl2, was titrated

with 5 mM H2SO4 down to pH 4 in the autotitrator.

2.2.4. Plant residue buffer curves

Buffer curves of the residues were measured in the

following way: 0.25 g of each of the seven residues

was placed in a plastic bottle with a screw cap. To

each of the bottles, 10 mM CaCl2 was added followed

by 5 mM H2SO4 which was added in increments from

zero up to an amount which brought the pH down to

4. The total volume of all the treatments was adjusted

to 25 ml by 10 mM CaCl2. The treatments were

duplicated. The bottles were closed and placed on

an end-over-end shaker for 24 h. Buffer curves for the

various plant materials were determined by plotting

pH obtained at that time against the acid addition

rates. Because of the similarity in the methods used

for the buffer curves and available alkalinity, the

buffer curves represent the release of available alka-

linity as pH is reduced to 4.

2.3. The interaction of the soil and plant buffer

systems

2.3.1. The 24-h interaction

The main objective of this investigation was to

quantify the short-term (24 h) pH changes attributable

to the available alkalinity of plant residues added to an

acid soil and to estimate the proportion of the non-

available alkalinity converted to the available form.

Ten grams of soil was weighed and transferred to

labelled 100-ml plastic bottles with screw caps. Each

of the residues of 0.25 g was weighed and added to

the bottles. A control treatment without any residue

amendment was included. Twenty five millileters of

10 mM CaCl2 was added to all the treatments. The

bottles were tightly stoppered and placed on an end

over end shaker. The pH was measured after 24 h.

Each treatment was duplicated.

2.3.2. The 100-day incubation

The residues used were leucaena, gliricidia, soya-

bean and maize. To 300 g of soil (four replicates) was

added 9 g of residue, followed by thorough mixing

and the addition of 45 ml of H2O. A control without

residue was included. They were incubated at 30 jCin polythene bags loosely folded and occasionally

opened and shaken to ensure adequate aeration. Water

was added to maintain the moisture content.

Samples were extracted from all treatments before

the first addition of water to give data at zero

incubation time, and then at 14-day intervals. Mois-

ture content was determined. pH was measured in 10

mM CaCl2 at a 1:2.5 soil/solution ratio. Twenty grams

of soil was extracted with 100 ml of 1 M KCl for 2 h,

filtered and analysed for Ca2 +, Mg2 +, Al3 +, H+, NH4+

Table 2

Cation and anion balance in the plant residues, and potential and

available alkalinity, mmolc kg� 1

G.M. Sakala et al. / Geoderma 123 (2004) 219–232 223

and NO3�. Potassium and Na+ were extracted from 6 g

of soil shaken with 30 ml of 1 M ammonium acetate

buffered at pH 7.

AanionsPotential

alkalinitycAvailable

alkalinityd

Maize 727 365 362 373 181

Sorghum 936 512 424 365 268

Groundnuts 1618 303 1315 1336 392

Soyabeans 1686 466 1220 1264 577

Leucaena 1164 562 602 794 415

Gliricidia 1524 513 1011 1024 427

Sesbania 1372 502 870 987 432

All values are means of duplicates.a Values for Acations are Ca+ +Mg2 + +K+ +Na+.b Values for Aanions are Cl� +H2 PO4

� + SO42�.

c Ash alkalinity.d Titratable alkalinity (to pH 4.0).

3. Results and discussion

3.1. Soil analysis

The properties of the soil are as follows: pH, 4.24;

exchangeable cations cmolc kg� 1, 0.39 Ca2 +, 0.16

Mg 2 +, 0.27 K+, 0.06 Na+, 0.04 NH4+, 0.55 Al3 +, 0.14

H+; ECEC 1.60 cmolc kg� 1; 34% Al saturation; 56%

base saturation; 0.07% N; 0.86% organic C; 51%

sand, 8% silt, 41% clay; buffer capacity 12.9 mmol

OH� kg� 1 pH� 1. For the pH range 4.2–6.5, the

buffer curve is linear.

3.2. Plant residues

The contents of cations and anions in the plant

residues are shown in Table 1. In terms of charge

balance, Ca2 +, Mg2 + and K+ are the major cations

and SO42� the major inorganic anion. The cereals

(maize and sorghum) have lower Ca2 + contents than

the legumes. The grain legumes (groundnuts and

soyabeans) contained more Mg 2 + than cereals or tree

legumes. Contents of monovalent cations were com-

parable across species. Except for gliricidia, Cl�

contents were lower in legumes than cereals. The

largest SO42 � contents were in the tree legumes.

Phosphate content did not vary significantly between

plant types. Based on these values, Table 2 shows the

Table 1

Cation and inorganic anion content of the plant materials

Ca2 + Mg2 + K+ Na+ C

mmolc kg� 1 (%)

Maize 239 (0.48) 189 (0.23) 298 (1.17) 1.1 ( < 0.01)

Sorghum 251 (0.50) 207 (0.25) 477 (1.86) 1.1 ( < 0.01) 2

Groundnuts 767 (1.54) 458 (0.56) 392 (1.53) 1.1 ( < 0.01)

Soyabeans 879 (1.76) 444 (0.54) 360 (1.41) 2.9 ( < 0.01)

Leucaena 547 (1.15) 238 (0.29) 374 (1.46) 4.7 ( < 0.01)

Gliricidia 790 (1.58) 278 (0.34) 456 (1.78) 0.2 ( < 0.01)

Sesbania 845 (1.69) 188 (0.23) 336 (1.31) 2.9 ( < 0.01) 6

All values are means of duplicates.

L: lignin.

SP: soluble phenolics.

cation–anion balance and the potential (ash) alkalin-

ities which illustrate the expected equivalence of

potential alkalinity and the cation excess. Because

the Aanion concentration does not vary much between

the residues, the amounts of cations control the

alkalinity. The small potential alkalinity values of

maize and sorghum are associated with small contents

of both cations and anions and the large values of

groundnuts and soya beans reflect their ability to

accumulate large amounts of cations.

Available (titratable) alkalinity is on average

about half the potential alkalinity indicating the

limited ability of the cations to dissociate from the

organic anions, and maybe the lack of accessibility

within the residue particles. It should be noted that

available alkalinity is defined by the method used

l� H2PO4�(P) SO4

2�(S) C N L SP

%

74 (0.26) 32 (0.10) 259 (0.41) 47 1.10 5.38 0.14

12 (0.75) 50 (0.15) 250 (0.40) 46 1.34 7.56 0.15

18 (0.07) 39 (0.12) 245 (0.39) 41 1.86 8.80 0.17

18 (0.07) 102 (0.32) 345 (0.55) 46 1.66 9.42 0.17

10 (0.04) 53 (0.16) 499 (0.80) 51 4.26 7.85 6.56

97 (0.34) 66 (0.20) 350 (0.56) 49 3.20 7.38 1.70

.7 (0.02) 48 (0.15) 447 (0.71) 50 3.79 4.36 2.65

G.M. Sakala et al. / Geoderma 123 (2004) 219–232224

for its measurement. It seems likely that available

alkalinity will be related to cation excess in the

residues. Since the concentrations of anions were

similar for all species (Table 2), the total uptake of

cations was the dominant difference between species

in their net cation uptake: the correlation coefficient

between available alkalinity and Acations is 0.78

(Fig. 1a). Of the cations, only Ca2 + concentration is

correlated with available alkalinity. Of the anions,

only H2PO4� is correlated (Fig. 1b). This is a

positive correlation even though increases in anion

concentration should decrease the cation excess.

This anomaly arises because the H2PO4� concentra-

tions are too small to have much affect on Aanions

Fig. 1. The relationship between available alkalinity and plant residue conce

Ca 0.75, Mg 0.24, K not correlated, H2PO4 0.59, SO4 0.09, Cl 0.16 (neg

(note the difference in the axes between Fig. 1a

and b).

The cation and anion contents of the soluble

fraction of the plant residues are given in Tables 3

and 4. On average, about one third of the mass of the

residues is soluble, and about one tenth of the soluble

matter is the mass of inorganic ions. The remaining

nine tenths must be organic. Based on the data in

Tables 1 and 3, the amount of each soluble ion

expressed as an average percentage of its total in the

residues is as follows: Ca2 + 37, Mg2 + 70, K+ 82, Na+

79, Cl� 100, H2PO4� 74 and SO4

2� 16, with ions

being released most easily from maize. There is an

excess of cations over anions (Table 4) indicating the

ntration of (a) cations and (b) phosphate. Correlation coefficients are

ative).

Table 3

Cation and anion composition of the soluble fraction of the residues, mmolc kg� 1. Values in parentheses are % amounts

Ca2 + Mg2 + K+ Na+ NH4+ Cl� H2PO4

�(P) SO42�(S)

Maize 144 (0.29) 174 (0.21) 290 (1.13) 1.1 (0.00) 0 (0) 74.0 (0.26) 30.4 (0.09) 44.0 (0.07)

Sorghum 65.9 (0.13) 156 (0.19) 400 (1.56) 1.0 (0.00) 0 (0) 212 (0.75) 37.2 (0.11) 22.3 (0.04)

Groundnuts 251 (0.50) 280 (0.34) 304 (1.19) 0.7 (0.00) 0 (0) 18.4 (0.07) 30.6 (0.09) 50.6 (0.08)

Soyabeans 329 (0.66) 315 (0.38) 290 (1.13) 1.6 (0.00) 0 (0) 18.6 (0.07) 63.0 (0.20) 180 (0.29)

Leucaena 169 (0.34) 134 (0.16) 282 (1.10) 2.7 (0.01) 70.6 (0.10) 10.0 (0.04) 23.2 (0.07) 30.8 (0.05)

Gliricidia 233 (0.47) 194 (0.24) 378 (1.47) 0.2 (0.00) 27.1 (0.04) 96.6 (0.34) 57.7 (0.18) 10.7 (0.02)

Sesbania 370 (0.74) 121 (0.15) 263 (1.03) 2.4 (0.01) 32.7 (0.05) 6.7 (0.02) 38.0 (0.12) 20.0 (0.03)

All values are means of duplicates.

NO3� was measured but values were very small ( < 0.1 mmolc kg

� 1).

G.M. Sakala et al. / Geoderma 123 (2004) 219–232 225

presence of organic anions. The amounts of soluble

(titratable) alkalinity are equivalent to about 40% of

the cation excess in solution, again indicating the

limited ability of the organically complexed cations

to dissociate when titrated down to pH 4. About two

thirds of the available alkalinity (Table 2) is soluble

(Table 4).

3.3. Residue buffer curves

The buffer curves are shown in Fig. 2. The change

of slope below pH 4, apparently reflecting increased

buffering, is due to the fact that the amount of acid

remaining after reaction becomes an increasingly

large part of the value on the axis. Above pH 4, the

values for added acid can be taken as equal to the

amount of reacted acid. Sorghum and maize were the

Table 4

Cation and inorganic anion balance in the soluble fraction of the

plant residues, and soluble alkalinity, mmolc kg� 1

Soluble

fraction %,

m/m

Acationsa Aanionsb Acations�Aanions

Soluble

alkalinityc

Maize 29 609 148 461 151

Sorghum 22 623 272 351 211

Groundnuts 28 836 100 736 218

Soyabeans 26 936 262 674 402

Leucaena 25 588(659) 64 524(595) 239

Gliricidia 38 805(832) 165 640(667) 297

Sesbania 35 756(789) 65 691(724) 251

All values are means of duplicates.a Values for Acations are Ca2 + +Mg2 + +K+ +Na+. The values

in parentheses include NH4+.

b Values for Aanions are Cl� +H2PO4� + SO4

2�.c Titratable alkalinity (to pH 4.0).

least buffered residues. The other residues all had

similar buffer capacities although their ability to react

with acid varied because of their different initial pH

values. For example, soyabeans with an initial pH

value of 6.25 was a better source of alkalinity than

gliricidia (pH 5.35). The amounts of alkalinity react-

ing with acid down to pH 4 (shaken for 24 h) shown

in Fig. 2 agree well with the measurements of avail-

able alkalinity (titrated for 1 h) indicating that, for

these periods, time of reaction is not important. The

buffer capacities of the residues, calculated as an

average over the pH range from the residue pH down

to 4.0, range from 117 mmol kg� 1 pH� 1 for maize to

352 mmol kg� 1 pH� 1 for gliricicidia.

3.4. Interaction of the soil and residue buffer system

3.4.1. The 24-h interaction

When the soil and residues were shaken together,

soil acidity reacted with residue alkalinity to give a

new pH value for the mixture. Table 5 shows these

values and the amounts of soil acidity that were

neutralized based on the soil’s buffer capacity and

the increase in soil pH from 4.29 to the new value.

Soyabean had the greatest effect on soil acidity,

raising the soil pH by nearly one unit and neutralizing

11.5 mmol kg� 1 of soil.

Using the residue buffer curves (Fig. 2) and the

measured soil-residue pH values, the amounts of base

contributed by the residues have been calculated and

are shown in Table 5. These values are based on the

assumption that the residues would react with soil

acidity in the same way that they react with H2SO4

solution. This appears not to be the case, as the

calculated base contribution by the residues is insuf-

Fig. 2. Buffer curves for the various crop and tree residues.

G.M. Sakala et al. / Geoderma 123 (2004) 219–232226

ficient to account for the neutralization of the soil

acidity (except for leucaena). The table gives the base

from other sources. Overall about two thirds of the

neutralization of soil acidity is accounted for by the

base contributions calculated from the buffer curves.

It is possible that decomposition and release of non-

available alkalinity was stimulated as a result of

contact of residues with soil (Pocknee and Sumner,

1997). It is also possible that the remaining one third

is the result of the presence of acidity in the soil

associated with Al ions. Al forms more stable com-

plexes with organic anions than Ca does (Schnitzer

and Skinner, 1963) and so it is likely that some of the

Table 5

Interaction of residues with the Zambian soil

Initial Final Soil acidity neutralize

residue

pHa

soil-residue

pHa mmol kg� 1 soil (mm

Maize 5.50 4.59 3.87(155)

Sorghum 6.08 4.71 5.42(217)

Groundnuts 5.98 4.90 7.87(315)

Soyabeans 6.22 5.18 11.48(459)

Leucaena 6.06 4.87 7.48(299)

Gliricidia 5.32 4.76 6.06(242)

Sesbania 5.68 4.96 8.64(346)

a The control was soil alone. Its initial pH was 4.24 and after incubatb Calculated from the rise in pH from 4.29 to the values given in Colc Obtained from Fig. 2 using the pH values in Column 2 (0.25 g residd The difference between Columns 3 and 4.

non-available alkalinity in the residues is made avail-

able by reactions of the following type:

ðRCOOÞ2Caþ AlOH2þ ¼ ðRCOOÞ2AlOHþ Ca2þ

Removal of AlOH2 + from the solution would

cause disequilibrium in the Al system, which would

readjust as follows:

AlðOHÞþ2 ¼ AlOH2þ þ OH�

Table 2 shows that there is more than enough

potential alkalinity to supply the total amounts of base

reacting with the soil (bracketed values in Table 5), and

db Base contributedc Base from other sourcesd

ol kg� 1 residue)

2.41(96) 62 1.46(59)

4.10(164) 76 1.32(53)

4.90(196) 62 2.97(119)

7.77(311) 68 3.71(148)

7.14(286) 95 0.34(13)

4.55(182) 75 1.51(60)

5.18(207) 60 3.46(139)

ion was 4.29.

umn 2 and a soil buffer capacity of 12.9 mmol kg� 1 pH� 1.

ue: 10 g soil).

G.M. Sakala et al. / Geoderma 123 (2004) 219–232 227

the amounts of available alkalinity (by titration down

to pH 4) would also be adequate. Thus, the ‘‘base from

other sources’’ may be produced by reactions of Al

with organic anions which, in the absence of soil Al,

provided alkalinity in reaction with H2SO4 at lower pH

values than those shown in Table 5. Soluble alkalinity

Fig. 3. Changes in the soil during incubation with plant residues, (a) pH, (

days for maize and at 28 days for soyabeans) represent zero values which

(Table 4) appears to be a major component of the

alkalinity reacting with the soil.

3.4.2. The 100-day incubation

The changes in pH, NH4+ and NO3

- in the moist soil

incubated with plant residues are shown in Fig. 3. The

b) ammonium N, and (c) nitrate N. Note: The missing data (14–70

cannot be plotted on a logarithmic chart.

G.M. Sakala et al. / Geoderma 123 (2004) 219–232228

pH in all treatments rose during the first 2 weeks and

then fell back again especially for the tree legumes so

that after 100 days, the pH values were a little higher

than the zero time values except for leucaena. The

increases in pH over the first 40 days were to some

extent in line with increases in NH4+ concentrations,

with increases in NO3- aligning with the subsequent

decreases in pH. Thus, the proton transfers associated

with ammonification and nitrification appear to have a

major role in causing the measured pH changes. The

pH values at zero time are slightly higher but of the

same order as those after 24 h shown in Table 5. The

residue application in the 24-h experiment was 2.5%,

whereas in this incubation, it was 3% with only 30

min in suspension before the measurement of pH.

The amounts of N added in the plant material

were 330, 498, 960 and 1278 Ag N g-1 soil for

maize, soyabean, gliricidia and leucaena, respective-

ly. After 100 days, the amounts of N mineralized

were 0%, 18%, 63%, and 18% of that added,

respectively. Maize caused immobilization. With on-

ly four plant materials, it was not expected that clear

relationships between release of mineral N and

composition would be seen. The relationship be-

tween the fraction of the plant N mineralized and

C/N was y = 0.56� 0.13x, r = 0.15, for L/N there was

no relationship and for (L + SP)/N, it was y = 0.83�0.14x, r = 0.23, where L= lignin content and SP= sol-

uble polyphenol content. Rather better relationships

were found when the amount of mineral N released

was used instead of the fraction mineralized in the

above three ratios: y =570� 14x, r = 0.29; y = 600�100x, r = 0.26; y = 923� 161x, r= 0.59, respectively.

These latter relationships reflect both the concentra-

tion of N in the plant material and the ease with

which it is released.

The components of the acid–base reactions are

shown in Fig. 4. They were calculated as follows.

(a) Base reacted with the soil. The difference between

the pH values of the control in Fig. 3, and the

values at each time of sampling were determined

for each residue treatment. These changes in soil

pH were multiplied by the buffer capacity of 12.9

mmol kg� 1 pH� 1 which was assumed to apply

for all the times of measurement.

(b) Contribution of base from the available alkalinity

in the residues. The pH values of the residues are

shown in Table 5. When incubated with the soil

the values changed to those shown in Fig. 3.

Assuming that the buffering properties of the

residues shown in Fig. 2 apply, amounts of acidity

reacting with the residues were calculated as mmol

kg� 1 residue. These were converted to amounts of

base reacting with the soil (30 g residue + 1 kg

soil). These amounts are not likely to represent the

total contribution because they are based on the

buffering properties measured with H2SO4 over

about 1 h and so represent the fractions of the

available alkalinity which would theoretically

react simply as a result of the above pH changes.

Any contribution in excess of these calculated

amounts must be from the pool of non-available

alkalinity. It should be noted that the partitioning

of available and non-available alkalinity is directly

dependent on the method used to measure the

buffering properties of the residues.

(c) Contribution of base from N transformations. The

data for NH4+ and NO3

� in Fig. 3 were used. At

time zero, it was assumed that no transformations

had occurred and that the concentrations were

those resulting from soil mineral N plus the

soluble NH4+ and NO3

� from the residues. Thus, at

each time, the change in NH4 concentration in the

treated soil was found by difference with the zero

time value. Similarly, the change was found in

the control soil and was subtracted from that in

the treated soil to give a release of NH4 from the

residue. In doing this, it was assumed that the

ammonification of soil–N was unaffected by the

residue. For NO3�, a similar procedure was used.

The acid–base changes were calculated by

assuming that each mol of NH4+ released is

accompanied by 1 mol of OH� and each mol of

NO3� released by 1 mol of H+.

(d) Total base contributions from the residues other

than through N transformations. These values =

base reacting with soil�base contribution through

N transformations, (a)–(c) above, and are shown

in Table 6. The residue alkalinity values are those

from Tables 2 and 4 expressed as mmol per 30 g

of residue to make them comparable to the base

values in mmol per kg of soil.

For maize (initial pH 5.5, Table 5), the calculated

contribution of base from the available alkalinity as

Fig. 4. The amount of acidity neutralized in the soil compared to the base contribution from available alkalinity and base produced from N

transformations for various residues.

G.M. Sakala et al. / Geoderma 123 (2004) 219–232 229

defined and measured above is small (Fig. 4), because

the change in pH of the residue is small (Fig. 3).

Because of the small amount of N present in the

maize, transformations make little or no contribution

to the budget. However, base is released from the

residue to react with the soil (Table 6) so that the soil

pH values are raised to close to that of the maize alone

(5.5, Table 5). This base may be from available

alkalinity, made available by the presence of soil Al,

or from non-available alkalinity as decomposition

progresses.

For soyabean (initial pH 6.2), the contribution of

base from the available alkalinity is of more impor-

tance, and NO3 production makes a significant con-

tribution of acidity after 56 days which reacts partly to

reduce the soil pH and partly with residue alkalinity.

Thus, because proton consuming and producing reac-

tions from N transformations are considered separate-

Table 6

The amounts of base reacting with the soil, and the amounts

contributed from the residues other than through N transformations

compared to the alkalinity in the residues

Time, days 0 14 28 42 56 70 84 100

mmol OH� kg� 1 soil

Maize: Alkalinity, mmol OH� per 30 g: potential 23.8, available

12.5, soluble 7.2

Base reactinga 5.3 14.1 12.5 13.8 11.2 10.5 11.2 10.1

Base from residueb 5.3 14.1 12.6 13.8 11.2 10.5 10.9 9.9

Soyabean: Alkalinity, mmol OH� per 30 g: potential 37.9,

available 17.3, soluble 12.1

Base reacting 17.3 24.5 24.5 27.1 25.4 19.7 20.5 19.9

Base from residue 17.3 24.5 24.2 25.1 25.8 24.9 25.4 26.8

Leucaena: Alkalinity, mmol OH� 1 per 30 g: potential 23.8,

available 12.5, soluble 7.2

Base reacting 12.1 18.1 20.9 22.2 13.3 3.6 3.6 3.2

Base from residue 12.1 20.2 17.8 18.6 19.6 18.5 18.9 20.7

Gliricidia: Alkalinity, mmol OH� 1 per 30 g: potential 30.7,

available 12.8, soluble 8.9

Base reacting 9.3 33.8 39.9 37.2 17.7 10.5 5.6 11.1

Base from residue 9.3 5.1 4.8 7.7 6.0 18.3 19.2 24.5

a Base reacting is that calculated under (a) in the text.b Base from residue is that calculated under (d) in the text.

G.M. Sakala et al. / Geoderma 123 (2004) 219–232230

ly from residue alkalinity (organic anion reactions),

the contribution of base from residue alkalinity

becomes numerically larger than the net amount of

base reacting with the soil. For this residue, non-

available alkalinity was released during the first 14

days with little release thereafter.

For leucaena (initial pH 6.1), there is a further

increase in the amount of base contributed from the

available alkalinity, and a larger contribution of acid-

ity from mineralization showing its effect after 42

days. After 70-day incubation, the pH of the soil was

similar to its untreated value so that overall, only a

small amount of base had reacted with the soil.

However, base from the residue had reacted with the

acidity produced by nitrification to the extent that

most of the potential alkalinity had reacted. Thus,

leucaena had only a minor benefit as a liming material

after 70 days, because of its large N content. In the

field, uptake of N and loss by leaching would add

further components to the acidity budget.

For gliricidia (initial pH 5.3), the production of

NH4+ during the first 14 days results in the production

of a large amount of base. Soil pH rises from 4.2 to a

peak of 7.3, requiring 39.9 mmol OH� kg� 1. It is not

clear what the buffer reaction with the residue would

have been since we have no data (Fig. 2) for an

increase in pH. Using a fitted buffer equation for

gliricidia in Fig. 2 ( y = 10� 6x2� 0.0035x + 5.30,

r2 = 0.98), values for the amount of base reacting with

the residue were obtained and are plotted in Fig. 4.

However, examination of the general shape of the

curves in Fig. 2 suggests that base may not react to

any extent with the residue even though the pH rises.

Regardless of this uncertainty, it is clear that the base

produced by ammonification was not quite adequate

to supply that required by the soil, and in Table 6

values are given for the amounts of base which were

released from the residue. These are relatively small

up to 56 days, but indicate that base was released

during decomposition even though the pH was rising.

After 56 days, NO3� production began to dominate,

with more base being released by the residue to react

with the associated acidity. Eventually, a large fraction

of the potential alkalinity had reacted.

4. Conclusions

For the plant materials studied here, potential

alkalinity is simply given by Acations�Ainorganicanions showing that cations associated with organic

anions are its source. The available alkalinity is

composed of about two thirds which is soluble and

one third insoluble. The soluble fraction of the avail-

able alkalinity is that part of the soluble cation–

organic anion component which dissociates when

pH is reduced to 4, and the insoluble fraction of the

available alkalinity must be on or in the residue and

again able to dissociate. The available alkalinity

appears to be related to the amount of Ca +Mg and

H2PO4 in the plant material. About half of the

potential alkalinity is available indicating that only

half of the cation–organic anion component is able to

dissociate or is accessible. More alkalinity reacts with

soil acidity over 24 h than with H2SO4 indicating that

the standard method for the measurement of available

alkalinity is not entirely satisfactory, and the extra

reacting alkalinity may reflect an initiation of decom-

position caused by the presence of soil organisms.

Most of the initially non-available alkalinity in

soyabean and leucaena becomes available and reacts

G.M. Sakala et al. / Geoderma 123 (2004) 219–232 231

within 14 days, but for gliricidia and maize up to 100

days of incubation is required because reaction is

delayed until nitrification and acid production begins.

The residues influence soil acidity depending on their

buffering characteristics (as indicated by short-term

reactions with H2SO4 and longer term reactions due to

decomposition), their N content and N transforma-

tions. In a field situation, the amelioration of soil

acidity will also depend on the fate of the mineralized

N. Uptake of NO3� may cause a release of OH�

depending on the cation–anion balance as nutrients

enter roots. Leaching of nitrate will leave in the soil

the acidity released by mineralization and nitrification.

Denitrification will cause a release of OH� so neu-

tralizing acidity produced during nitrification (Reuss

and Johnson, 1986).

The range of potential alkalinities found here

(373–1336 mmol kg� 1) is within the range reported

by Noble et al. (1996) and Larsen (1998). This is an

indication of the long-term potential effects of resi-

dues on soil acidity. In the short-term, the available

alkalinity is the fraction involved in reaction, acting as

a buffer resulting from the presence of a range of

reactive groups with different pKa values (Raven and

Smith, 1976; Perrin and Dempsey, 1974). The primary

buffers are phosphates, carbonates and organic acids.

The buffer capacities of the residues (117–352 mmol

kg� 1 pH� 1) are an order of magnitude larger than the

soil’s buffer capacity (12.9 mmol kg� 1 pH� 1), which

is an indication of the effectiveness of small amounts

of residue to neutralize soil acidity. The amounts of

residue added in this study are larger than those

normally applied under field conditions except per-

haps in organic farming systems. However, residues

are not uniformly mixed into soil and so even normal

application rates may have an important effect in the

volumes of soil where the residues occur. The increase

in soil pH would be occurring in the regions where

nutrients are being released from the residue, thus

allowing a crop to benefit from the interaction be-

tween the amelioration of the local soil environment

and the increased amounts of available nutrients. The

measurement of soluble alkalinity is a useful index of

the immediate reaction of residues with moist soil.

The value of the method used to measure available

alkalinity has been shown to be limited. This appears

to be due to the added effect of the soil compared to

H2SO4 probably as a result of initiation of decompo-

sition during the first few weeks. Thus, the measure-

ment of available alkalinity is only of limited value as

an index of the short-term effects of residues on soil

acidity. Only for maize is there agreement between

available alkalinity and the measured amount of base

reacting with the soil. The measurement of potential

alkalinity is less ambiguous, but again as an index of

base reacting with the soil, it has limited value,

depending on the extent to which the non-available

alkalinity is released by decomposition. Production of

acidity and alkalinity associated with nitrogen miner-

alization, subsequent transformations and leaching

losses are not accounted for in the measurement of

potential alkalinity, limiting further its value as an

index of potential changes in soil acidity.

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