Hindawi
Applied and Environmental Soil Science
Volume 2019, Article ID 5794869, 9 pages
https://doi.org/10.1155/2019/5794869
Review Article
The Role of Soil pH in Plant Nutrition and Soil Remediation
Dora Neina
Department of Soil Science, P.O. Box LG 245, School of Agriculture, College of Basic and Applied Science, University of Ghana,
Legon-Accra, Ghana
Correspondence should be addressed to Dora Neina; dneina@gmail.com
Received 26 August 2019; Accepted 5 October 2019; Published 3 November 2019
Academic Editor: Marco Trevisan
Copyright © 2019 Dora Neina.(is is an open access article distributed under the Creative Commons Attribution License, which
permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
In the natural environment, soil pH has an enormous influence on soil biogeochemical processes. Soil pH is, therefore, described
as the “master soil variable” that influences myriads of soil biological, chemical, and physical properties and processes that affect
plant growth and biomass yield. (is paper discusses how soil pH affects processes that are interlinked with the biological,
geological, and chemical aspects of the soil environment as well as how these processes, through anthropogenic interventions,
induce changes in soil pH. Unlike traditional discussions on the various causes of soil pH, particularly soil acidification, this paper
focuses on relationships and effects as far as soil biogeochemistry is concerned. Firstly, the effects of soil pH on substance
availability, mobility, and soil biological processes are discussed followed by the biogenic regulation of soil pH. It is concluded that
soil pH can broadly be applied in two broad areas, i.e., nutrient cycling and plant nutrition and soil remediation (bioremediation
and physicochemical remediation).
1. Introduction greatest impact on the components of human well-being in
terms of safety, the basic material for a good life, health, and
To many, soil pH is only essential for the chemistry and good social relations.
fertility of soils. However, the recognition of soil functions In the natural environment, the pH of the soil has an
beyond plant nutrient supply and the role soil as a medium enormous influence on soil biogeochemical processes. Soil
of plant growth required the study of the soil and its pH is, therefore, described as the “master soil variable” that
properties in light of broader ecosystem functions through a influences myriads of soil biological, chemical, and physical
multidisciplinary approach. (is allows scientists to view properties and processes that affect plant growth and bio-
processes from landscape to regional and global levels. One mass yield [6, 7]. Soil pH is compared to the temperature of a
process that denotes the multidisciplinary approach to soil patient during medical diagnoses because it readily gives a
science is soil biogeochemistry, which studies bio- hint of the soil condition and the expected direction of many
geochemical processes. (e ecosystem functions of soil, to soil processes (lecture statement, Emeritus Prof. Eric Van
some extent, have a strong relationship with soil bio- Ranst, Ghent University). For instance, soil pH is controlled
geochemical processes, which are linkages between bi- by the leaching of basic cations such as Ca, Mg, K, and Na far
ological, chemical, and geological processes [1]. (e soil is beyond their release from weathered minerals, leaving H+
the critical element of life support systems because it delivers and Al3+ ions to dominant exchangeable cations; the dis-
several ecosystem goods and services such as carbon storage, solution of CO2 in soil water producing carbonic acid, which
water regulation, soil fertility, and food production, which dissociates and releases H+ ions; humic residues from the
have effects on human well-being [2–4]. (ese ecosystem humification of soil organic matter, which produces high-
goods and services are broadly categorized as supporting, density carboxyl and phenolic groups that dissociate to
provisioning, regulating, and cultural services [5]. According release H+ ions; nitrification of NH + to NO − produces H+4 3
to the Millennium Ecosystem Assessment [5], the pro- ions; removal of N in plant and animal products; and inputs
visioning and regulating functions are said to have the from acid rain and N uptake by plants [8]. On the other
2 Applied and Environmental Soil Science
hand, pH controls the biology of the soil as well as biological Biodegradation
processes. Consequently, there is a bidirectional relationship of organic
between soil pH and biogeochemical processes in terrestrial pollutants 
ecosystems, particularly in the soil. In this sense, the soil pH Mineralisation Rhizosphere
influences many biogeochemical processes, whereas some of organic processes
biogeochemical processes, in turn, influence soil pH, to some matter 
extent, as summarised in Figure 1. Organic
For many decades, intensive research has revealed that Ammonia amendments volatilisation 
soil pH influences many biogeochemical processes. Recent Soil
advances in research have made intriguing revelations about Precipitation: pH Dissolution:
the important role of soil pH in many soil processes. (is organic matter, organic matter,heavy metals
important soil property controls the interaction of xeno- heavy metals 
biotics within the three phases of soil as well as their fate, Soil enzyme Nitrification
translocation, and transformation. Soil pH, therefore, de- activities and
termines the fate of substances in the soil environment. (is denitrification
has implications for nutrient recycling and availability for Microbial
crop production, distribution of harmful substances in the ecophysiologicalindicators 
environment, and their removal or translocation. (is
functional role of soil pH in soil biogeochemistry has been Figure 1: Some biogeochemical processes and their relations with
exploited for the remediation of contaminated soils and the soil pH.
control of pollutant translocation and transformation in the
environment. Unfortunately, in many studies, soil pH is and 53%, respectively. In contrast, he found that 50% of Cd
often measured casually as a norm without careful con- and Zn sorbed onto humic acids between pH 4.8–4.9 [13].
sideration for its role in soil. (is paper seeks to explore the (e fate of readily available trace elements depends on both
importance of pH as an indicator of soil biogeochemical the properties of their ionic species formed in the soil so-
processes in environmental research by discussing the lution and that of the chemical system of soil apart from soil
biogeochemical processes that are influenced by soil pH, the pH itself [14]. Research has established that with increasing
biogeochemical processes that also control soil pH, and the soil pH, the solubility of most trace elements will decrease,
relevance of the relationship for future research, planning, leading to low concentrations in soil solution [14]. Any
and development. increase or decrease in soil pH produces distinct effects on
metal solubility. (is may probably depend on the ionic
2. Biogeochemical Processes Influenced by species of the metals and the direction of pH change. Rengel
Soil pH [15] observed that the solubility of divalent metals decreases a
hundred-fold while trivalent ones experience a decrease of up
2.1. Substance Translocation. Simultaneously, in accordance to a thousand-fold. In contrast, Förster [10] found that a
with biochemical changes, physicochemical processes, in- decrease in soil pH by one unit resulted in a ten-fold increase
cluding dissolution, precipitation, adsorption, dilution, in metal solubility. In an experiment, he observed that at pH
volatilization, and others, influence leachate quality [9]. 7, only about 1mg Zn·L− 1 of the 1200mg·kg− 1 total Zn
content was present in soil solution. At pH 6, the concen-
tration reached 100mg Zn·L− 1 while at pH 5, 40mg Zn·L− 1
2.1.1. Trace Element Mobility. Soil pH controls the solubility, was present. Aside from adsorption, trace element concen-
mobility, and bioavailability of trace elements, which de- trations at high soil pH may also be caused by precipitation
termine their translocation in plants [10]. (is is largely with carbonates, chlorides, hydroxides, phosphate, and sul-
dependent on the partition of the elements between solid and phates [11, 16]. Apatite and lime applied to soils produced the
liquid soil phases through precipitation-dissolution reactions highest effect on pH and simultaneously decreased the
[10, 11] as a result of pH-dependent charges in mineral and concentrations of available, leachable, and bioaccessible Cu
organic soil fractions. For instance, negative charges domi- and Cd [16].
nate in high pH whereas positive charges prevail in low pH
values [12]. Additionally, the quantity of dissolved organic
carbon, which also influences the availability of trace ele- 2.1.2. Mobility of Soil Organic Fractions. Soil organic matter
ments, is controlled by soil pH. At low pH, trace elements are exists in different fractions ranging from simple molecules
usually soluble due to high desorption and low adsorption. At such as amino acids, monomeric sugars, etc. to polymeric
intermediate pH, the trend of trace element adsorption in- molecules such as cellulose, protein, lignin, etc. (ese occur
creases from almost no adsorption to almost complete ad- together with undecomposed and partly decomposed plant
sorption within a narrow pH range called the pH-adsorption and microbial residues [17]. (e solubility and mobility of
edge [13]. From this point onwards, the elements are the fractions differ during and after decomposition and
completely adsorbed [13]. For instance, Bradl [13] found that could lead to the leaching of dissolved organic carbon and
at pH 5.3, the adsorption of Cd, Cu, and Zn onto a sediment nitrogen in some soils. Dissolved organic carbon is defined
composite consisting of Al-, Fe-, and Si-oxides was 60%, 62%, as the size of organic carbon that passes through a 0.45mm
Applied and Environmental Soil Science 3
diameter filter [18]. Soil pH increases the solubility of soil 2.2.2. Soil Enzyme Activities. Extracellular enzymes are
organic matter by increasing the dissociation of acid produced by soil microorganisms for the biogeochemical
functional groups [19] and reduces the bonds between the cycling of nutrients [33]. Soil pH is essential for the proper
organic constituents and clays [20]. (us, the content of functioning of enzyme activity in the soil [34, 35], and may
dissolved organic matter increases with soil pH and con- indirectly regulate enzymes through its effect on the mi-
sequently mineralizable C and N [20]. (is explains the crobes that produce them [36]. However, there are myriad of
strong effects of alkaline soil pH conditions on the leaching enzymes in biological systems which assist in the trans-
of dissolved organic carbon and dissolved organic nitrogen formation of various substances. Besides, enzymes are of
observed in many soils containing substantial amounts of different origins and with differing degrees of stabilization
organic matter [19, 21].(e same observation has beenmade on solid surfaces. (us, the pH at which they reach their
on the dissolved organic carbon concentration in peatland optimum activity (pH optima) is likely to differ [33]. It is
soils [22]. (e pH-dependence of dissolved organic carbon striking that enzymes that act on the same substrates could
concentration gets more pronounced beyond pH 6 [23]. vary considerably in their pH optima. (is is evident in
Within the pH condition in a specific soil system, the phosphorus enzymes, which have both acid and alkaline
solubility of organic matter is strongly influenced by the type windows of functioning in the range of pH 3–5.5 and pH
of base and is particularly greater in the presence of 8.5–11.5 [33]. In a study on the optimum pH for specific
monovalent cations than with multivalent ones [23]. enzyme activity in soils from seven moist tropical forests in
According to Andersson and Nilsson [24] and Andersson Central Panama, Turner [33] classified enzymes into three
et al. [19], soil pH controls the solubility of organic matter in groups depending on their pH optima as found in the soils.
two major ways: (i) its influence on the charge density of the (ese were: (a) enzymes with acidic optima that appeared
humic compounds, and (ii) either the stimulation or re- consistent among soils, (b) enzymes with acidic pH optima
pression of microbial activity. (e former is found to be that varied among the soils, and (c) enzymes with optima in
more pronounced than the latter [19]. both acid and alkaline soil pH. Stursova and Walker [37]
found that organophosphorus hydrolase has optimal activity
at higher pH. For instance, glycosidases have an optimal pH
2.2. Soil Biological Processes range between 4 and 6 compared to proteolytic and oxidative
enzymes whose optima was between 7 and 9 [35, 36, 38].
2.2.1. Microbial Ecophysiological Indicators. Ecophysiology is Shifts in microbial community composition could poten-
an interlinkage between cell-physiological functioning un- tially influence enzyme production if different microbial
der the influence of environmental factors [25]. It is esti- groups require lower nutrient concentrations to construct
mated using the metabolic quotient (qCO2) as an index [25] biomass, or have enzymes which differ in affinity for nu-
to show the efficiency of organic substrate utilization by soil trients [39].
microbes in specific conditions [26]. A decrease in microbial
community respiration makes C available for more biomass
production, which yields higher biomass per unit [27]. (e 2.2.3. Biodegradation. Soil microorganisms are described as
metabolic quotient is, therefore, described as a cell-physi- ecosystem engineers involved in the transformation of sub-
ological entity that reflects changes in environmental con- stances in the soil. One of such transformations is bio-
ditions [25]. (is implies that any change in environmental degradation, a process through which microbes remediate
conditions towards the adverse state will be indicated by the contaminated soils by transforming toxic substances and
index [25]. (is is controlled by soil pH [28]. Soil pH as a xenobiotics into least or more toxic forms. Biodegradation is
driving force for microbial ecophysical indices stems from the chemical dissolution of organic and inorganic pollutants
its influence on the microbial community together with the by microorganisms or biological agents [34, 40]. Like many
maintenance demands of the community [28] and was soil biological processes, soil pH influences biodegradation
among the predictors of the metabolic quotient [29, 30]. (e through its effect on microbial activity, microbial community
metabolic quotient was found to be two-and-a-half times and diversity, enzymes that aid in the degradation processes as
higher in low pH soils compared to neutral pH soils [28]. well as the properties of the substances to be degraded. Soil
(is has been associated with the divergence of the internal pHwas themost important soil property in the degradation of
cell pH (usually kept around 6.0) from the surrounding pH atrazine [41]. Generally, alkaline or slightly acid soil pH
conditions, which increases the maintenance requirements enhances biodegradation, while acidic environments pose
and reduces total microbial biomass produced [25]. limitations to biodegradation [34, 37, 42]. Usually, pH values
It is observed from the literature that soil pH conditions between 6.5 and 8.0 are considered optimum for oil degra-
required for microbial activity range from 5.5–8.8 dation [43]. Within this range, specific enzymes function
[26, 31, 32]. (us, soil respiration often increases with soil within a particular pH spectrum. For instance, the pesticide
pH to an optimum level [26]. (is also correlates with fenamiphos degraded in two United Kingdom soils with high
microbial biomass C and N contents, which are often higher pH (>7.7) and two Australian soils with pH ranging from pH
above pH 7 [26]. In low pH conditions, fungal respiration is 6.7 to 6.8. (e biodegradation process rather slowed down in
usually higher than bacterial respiration and the vice versa three acidic United Kingdom soils (pH 4.7 to 6.7) in 90 days
[25] because fungi are more adapted to acidic soil conditions after inoculation [42]. Xu [44] found some strains of bacteria
than bacteria. isolated from petroleum-contaminated soil in northern China
4 Applied and Environmental Soil Science
being able to degrade over 70% of petroleum at pH 7 and 9. In denitrification rate where the ratio of N2/N2O increased
a degradation experiment involving polycyclic aromatic hy- exponentially with an increase in soil pH. (is is because
drocarbons (PAHs), half of the PAHs degraded at pH 7.5 low pH prevents the assembly of functional nitrous oxide
within seven days representing the highest amount degraded reductase, the enzyme reducing N2O to N2 in de-
[34]. (is was associated with the highest bacterial pop- nitrification [15, 20] and this mostly depends on the natural
ulations [34]. Furthermore, Houot et al. [41] found increased soil pH [49]. However, the soil pH at which the highest
degradation of atrazine in French and Canadian soils, which activity of nitrous oxide reductase occurred was around pH
occurred with increased soil pH. (ey observed maximum 7.3. (is occurred in soils amended with potassium hy-
soil respiration in atrazine-contaminated soils at soil pH droxide (KOH) [51]. (is suggests the inhibition of de-
values higher than 6.5 compared to those with soil pH value nitrification at high pH, particularly up to pH 9 [50].
less than 6.0 where metabolites rather accumulated. Furthermore, maximum denitrification of between 68%
and 85% occurred in a sandy and a loamy soil with pH 5.2
2.2.4. Mineralization of Organic Matter. Organic matter and 5.9, respectively [52]. (e optimum pH for long-term
mineralization is often expressed as carbon (C), nitrogen potential denitrification was between 6.6 and 8.3. Addi-
(N), phosphorus (P), and sulphur (S) mineralization tionally, the short-term denitrifying enzyme activity
through microbial action. Soil pH controls mineralization in depended on the natural soil pH [49]. (e effect of soil pH
soils because of its direct effect on the microbial population on denitrification is partly due to pH controls over the
and their activities. (is also has implications for the denitrifying microbial populations. (e population size of
functions of extracellular enzymes that aid in the microbial the resident nitrate-reducing bacterial population in-
transformation of organic substrates. Additionally, at a creased dramatically when the pH of the acid soil was
higher soil pH, the mineralizable fractions of C and N in- increased [53].
crease because the bond between organic constituents and
clays is broken [20]. In a study on the mineralization of C 2.2.6. Ammonia Volatilization. (e volatilization of am-
and N in different upland soils of the subtropics treated with monia is a phenomenon that occurs naturally in all soils [54]
different organic materials, Khalil et al. [45] found that soil and has been attributed to the dissociation of NH + to NH3
pH and C/N ratio were responsible for 61% of the de- 4and H+ shown in equation (1) [55]
composition rate, with corresponding increases in CO2 ef- + +
fluxes, net N mineralization, and net nitrification in alkaline NH4 ⟷NH3 + H (1)
than in acid soils. Similar results had earlier on been ob- (e dissociation approaches equilibrium through the
tained by Curtin et al. [20]. acidification of the medium. (e rate of acidification de-
pends on the initial and final concentrations of ammonium
2.2.5. Nitrification and Denitrification. Nitrification and as well as on the buffering capacity of the medium [55].
denitrification are important nitrogen transformation pro- When solution pH increases above 7, H+ is consumed in the
cesses of environmental concern. Like many of the bio- reaction. (us, the dissociation of ammonium to ammonia
geochemical processes, the processes, to a large extent, are in equation (1) will favour ammonia volatilization. In neutral
controlled by soil pH. Nitrification involves the microbial and acid soils, NH −4 containing fertilizers are less subject to
conversion of ammonium to nitrate. It generally increases NH3 loss than urea and urea-containing fertilizers [54].
with increasing soil pH but reaches an optimum pH [45–47]. However, the degree will also depend on the specific fer-
In a four-year study, Kyveryga et al. [47] observed that soil tilizer and its effect on soil pH. In a study involving ammonia
pH range of 6 to 8 strongly influenced the nitrification rates volatilization from an alkaline salt-affected soil cultivated
of fertilizer N. Generally, the nitrification rate decreases at with rice, Li et al. [56] found that ammonia volatilization
lower soil pH values. In some soils, nitrification and nitri- increased rapidly with pH and peaked at pH 8.6. Ammonia
fication potential substantially decrease or are negligible volatilization is strongly correlated with pH and calcium
below a pH value of 4.2. However, nitrification may still carbonate, which suggested that the soil pH was a key factor
occur even below pH 4.14, suggesting that ammonia-oxi- in ammonia volatilization because calcium carbonate in-
dizing and nitrifier communities might remain active at low creases soil pH which in turn controls the concentration of
soil pH [48]. ammonia and ammonium in soil solution [57].
Denitrification is the microbiological process in which
oxidized N species such as nitrate (NO −3 ) and nitrite 3. Biogenic Regulation of Soil pH
(NO −2 ) are reduced to gaseous nitric oxide (NO), nitrous
oxide (N2O), and molecular nitrogen (N2) under limited Soil biological processes from living organisms and bio-
oxygen conditions [49]. Soil pH affects denitrification rate, chemical transformations of the remains of dead organisms
potential denitrification, and the ratio between the two induce changes in soil pH. (is can either occur through the
main products of denitrification (N2O and N2). (e ratio direct effect of biochemical processes occurring in the living
has an inverse relation with soil pH [49]. At pH values organisms in the soil system, mostly through rhizosphere
below 7, N2O was the main denitrification product whereas processes or through the direct and indirect effects of applied
N2 prevailed at pH values above 8 [49]. Sun et al. [50] organic residues, whether in unburnt, burnt, or charred
discovered that soil pH was the best predictor of forms as well as their decomposition.
Applied and Environmental Soil Science 5
3.1. Rhizosphere Processes. (e rhizosphere is the volume of plants [67]. (is was revealed in an experiment on apple trees
soil in the neighbourhood of roots that is influenced by root (Malus pumila Miller), buckwheat (Fagopyrum esculentum
andmicrobial activities [58–60] Hiltner 1904 cited by [60]. It Moench), corn (Zeamays L.), cowpeas (Vigna unguiculata (L)
is a longitudinal and radial gradient [61], ranging from 0 to Walp.), kaffir lime (Citrus hystrixDC.), lettuce (Lactuca sativa
2.0mm from the root mat [62, 63]. In this small soil volume, L.), pine trees (Pinus sp. L.), and wheat (Triticum aestivum L.),
roots take up water and nutrients, undergo root elongation where Metzger [66] found maximum concentrations of
and expansion, release exudates, respire, and thus have HCO −3 in the rhizosphere during the blooming and fruiting
higher microbial activity [59, 63]. (rough some of these stages (Figure 2), which was 10–29% higher compared to the
biological processes, plant roots have the ability to induce bulk soil. (e concentrations of HCO −3 in the rhizosphere of
pH changes in the rhizosphere either by releasing protons the plants was in the order, lettuce� buckwheat> pine>
(H+) or hydroxyl ions (OH− ) to maintain ion balance apple> kaffir> cowpeas> corn>wheat. (ese values were
[58, 64], depending on the nutritional status of the plants much lower than those obtained in the rhizosphere of soybean
[65]. (erefore, rhizosphere pH could increase or decrease (Glycine max (L.) Merr.) [64]. Furthermore, Turpault et al.
depending on the prevailing process and types of ions [59] found that 93% of NO3-N was taken up by a Douglas fir
released. (Pseudotsuga menziesii (Mirb.) Franco) stands during
Plant root-induced soil pH change in the rhizosphere is April–September compared to 83% uptake during the Oc-
controlled by specific processes and factors such as (i) ion tober-March period.(is likely increased rhizosphere pH and
uptake coupled with the release of inorganic ions that implies that during periods of low nitrate uptake, soil pH
maintain electroneutrality, (ii) the excretion of organic acid may decrease due to buffering or due to a response to the
anions, (iii) root exudation and respiration, (iv) redox- uptake of NH −4 .
coupled processes, (v) microbial production of acids after
the assimilation of released root carbon, and (vi) plant
genotype [58, 59]. Surprisingly, roots have a greater ten- 3.2. Raw and Combusted Organic Materials. When unburnt
dency to raise the pH of the rhizosphere rather than lower it organic materials or raw plant residues are applied to the
[65, 66]. (e dominant mechanism responsible for pH soil, the pH increases to a peak and decrease afterwards. For
changes in the rhizosphere is plant uptake of nutrients in the instance, Forján et al. [68] found initial increases in soil pH
form of cations and anions [58, 59, 65], primarily due to when they applied a mixture of sludge from a bleach plant,
plant uptake of the two major forms of inorganic nitrogen urban solid waste and mine wastes, and a mixture of sludge
(NH +4 and NO −3 ), which is usually taken up in large from a purification plant, wood chips, and remnants from
quantities [59]. Nitrogen is taken up by plants in three major agri-food industries to the soil. Furthermore, the addition of
forms: ammonium (NH +4 ), nitrate (NO −3 ), and molecular young Kikuyu (Pennisetum clandestinum L.) shoots also
nitrogen (N2) [59], although amino acids can also be taken increased soil pH by up to one pH unit [69]. (e major
up [58]. (e uptake of each of the three forms of nitrogen causes of this pH change is due to the (i) release of excess
accompanies the release of corresponding ions to maintain residue alkalinity attributed to the basic cations such as Ca,
electroneutrality in the rhizosphere. When nitrate domi- K, Mg, and Na [70]; (ii) decarboxylation of organic anions
nates in soil or when its uptake dominates, plants must that occurs during C mineralisation, causing the con-
release bicarbonate (HCO −3 ) or hydroxyl ions (OH
− ) to sumption of protons and release of OH− [71, 72]; (iii)
maintain electrical neutrality across the soil-root interface ammonification of the residue N; (iv) nitrification of min-
resulting in rhizosphere pH increase [58, 59, 64]. In contrast, eralised residue N; and (v) association/dissociation of or-
protons are released by plants in response to NH +4 uptake, ganic compounds [70]. (ese processes are determined by
causing a decrease in rhizosphere pH [58, 62]. It has been the quantity applied and the prevailing soil and environ-
revealed that 15, 6, and 0%, respectively, of the NH −4 N from mental conditions [70]. According to Xu et al. [70]; direct
the total N present in the soil is required to decrease rhi- chemical reactions and oxidation of the organic anions
zosphere pH decrease by 1.2 units, maintain it, or increase it during residue decomposition are the main mechanisms
by 0.4 pH unit [62]. involved in organic anion-induced soil pH increase. Ad-
(e extent of effects of the processes and factors con- ditionally, organic anions and other negatively charged
trolling rhizosphere pH change depends on plant species and chemical functional groups present in organic matter can
growth stages [65]. For instance, in a study on rhizosphere undergo association reactions with H+ ions [71, 73].
acidification interactions, Faget et al. [67] found differences (e increase in soil pH after residue application also
between rhizosphere acidification in maize (Zea mays L.) and depends on the type of residue (either from monocots or
beans (Phaseolus vulgaris L.). Maize initially acidified the dicots), which is related to the amount of alkalinity present,
rhizosphere and gradually alkalized it over time while beans residue quality (C/N ratio), the rate of residue application
showed opposite effects. (ey found an interaction effect of and decomposition, the initial pH, and buffering capacity of
the two plant species on the rhizosphere pH change whereby the soil [70, 71]. Different residues have different chemical
the degree of acidification or alkalization was weaker when and biochemical compositions, which determine the pro-
roots grew within the same neighbourhood than when the cesses responsible for soil pH change.(is was detected in an
roots were not growing near each other. However, the rhi- incubation experiment involving three soils and five dif-
zosphere pH changes with time as a result of variable uptake ferent residue types where soil pH increased according to
of nitrogen ions, plant species, and their growth stages of the lucerne> chickpea>medic>high-N wheat> low-N wheat [70].
6 Applied and Environmental Soil Science
350 in the more acidic Podzol. (is peaked at 14 days after
application and declined afterwards. However, in a field
300 study on the same soils [74], the application of chickpea
residue increased soil pH by 1.3 units in both soils and
250 reached a maximum at 3 months, whereas canola residue
200 increased pH by 0.82 and 1.02 units in the Podzol and
Cambisol, respectively, and reached a maximum pH at 9
150 months.
Similar to unburnt organic materials, burnt or charred
100 plant residues contain a larger amount of alkalinity due to
the volatilization of organic constituents under thermal
50 conditions leading to the concentration of alkaline con-
stituents. (e actual alkalinity depends on the type of bio-
0 mass involved, their origin, and burnt temperature. Burnt
and charred forms of organic materials include biochar and
ash. Biochar is a solid consistent product pyrolysis, while ash
is a loose powdery material obtained by combustion.(e pH
Plants of biochar produced at 500–600°C was 6.4–9.3 and showed a
Rhizosphere strong relationship with the total alkalinity (i.e., organic and
Bulk soil inorganic alkalinities) [75]. (e inorganic alkalinity in-
Figure 2: (e compositions of bicarbonate as found in the rhi- creased with increasing pyrolysis temperature and with
zosphere and bulk soil of some plants grown in a greenhouse. Error increasing divalent cation contents [75] because the organic
bars are± one standard deviation (n� 2 to 34). Lettuce and pine constituents volatilize during pyrolysis. (is alkalinity of
had only data one data point each and could not be presented with biochar neutralizes acidity and increases soil pH depending
error bars (data from [66]). on the amount of alkalinity and soil buffering capacity [76].
Biomass ash contains substantial alkalinity, which is often
Furthermore, in a 59-day laboratory incubation [71] and field expressed as percent calcium carbonate equivalence (%
experiments [74], it was found that the magnitude of soil pH CCE). It ranges from 17–95% [77, 78]. Similarly to biochar,
increase following residue amendment was in the order the combustion temperature has effects on the alkalinity of
chickpea> canola>wheat [71, 74]. (ey observed that 40–62% biomass aside the biomass type and source. Recently, Neina
of soluble alkalinity in canola and chickpea residues were re- et al. (submitted) found that ash from charcoal had higher
sponsible for the pH increases. It is obvious from these, and CCE, pH, and K contents than firewood ash. Depending on
many other studies [69], that the residues of dicots, particularly the alkalify and buffering capacity of the soil receiving the
legumes, have high alkalinity and produce larger effects on soil biomass ash, soil pH increase can be high or low. For in-
pH change than monocots. (e pH increase after residue ad- stance, in two Ghanaian Acrisols, biomass ash applied at
dition often reaches a peak and declines thereafter as a result of 2.5 g·kg− 1 soil increased soil pH by about 1 unit after 12
nitrification. Residues with low carbon-nitrogen (C/N) ratios are weeks of laboratory incubation [79]. (is pH change is
often associated with sharp pH decline after a certain period and mostly short-lived due to other biogeochemical processes.
the extent varies with soil type and soil buffering capacity
[70, 71, 74], whereas those with high C/N ratios produce smaller 4. Conclusions
pH increase, or none at all [70].
(e initial pH and buffering capacity of soils receiving (e content of this paper highlights the role of soil pH as a
plant residues have a profound role in the extent of pH master soil variable that has a bidirectional relationship with
change after application. For instance, three soil types of soil biogeochemical processes. Although not all bio-
different initial soil pH, namely, Wodjil sandy loam with geochemical processes were discussed in this paper, those
pH(CaCl2) 3.87, Bodallin sandy loam soil with pH 4.54, and discussed have substantial influences on soil health, nutrient
Lancelin sandy soil with pH 5.06, were incubated with availability, pollution, and potential hazards of pollutants as
residues of chickpea, lucerne, medic, high-Nwheat, and low- well as their fate in the food chain. (e mobility of un-
N wheat. (ereafter, the pH increased by about 3.3 units wholesome substances through the hydrological cycle can-
with lucerne in the Wodjil soil (3.87), 1.6 with chickpea, 1.5 not be overlooked here because of the intimate relationship
with medic, and 0.5 with high-N wheat, and no increase with between soil and water. (us, an understanding of this can
low-N wheat. (e pH increased and peaked at 42 days of form a basis and a guide to decisions and choices of soil
incubation for Bodallin andWodjil sandy loams followed by management, remediation, rehabilitation, and the mainte-
a decline whereas, in the Lancelin sandy soil, the pH peaked nance of soil quality. (e observed soil pH-biogeochemistry
at day 14 before declining [70]. In another incubation study relationships provide insight for future applications for
[71], a Podzol with an initial pH of 4.5 and a Cambisol with increased yields for specific crops through nutrient recycling
an initial pH of 6.2 were amended with residues of canola, and availability, which enhances crop growth. (e transient
chickpea, and wheat. For all the residues, the pH increase in rhizosphere soil pH could also be used to enhance the
the moderately acidic Cambisol was up to sixfold larger than availability of certain nutrients in certain soil conditions
Bicarbonate content (mg·kg–1 soil)
Apple
Buckwheat
Corn
Cowpeas
Kaffir
Lettuce
Pine
Wheat
Applied and Environmental Soil Science 7
[80]. More importantly, soil pH could be useful for soil [14] A. Kabata-Pendias, Trace Elements in Soils and Plants, CRC
pollution control through the distribution and removal of Press, Boca Raton, FL, USA, 2011.
harmful substances from systems. For instance, the min- [15] Z. Rengel, “Genotypic differences in micronutrient use effi-
eralization and degradation processes such as those of C and ciency in crops,” Communications in Soil Science and Plant
N mineralisation and the degradation of pesticide occur Analysis, vol. 32, no. 7-8, pp. 1163–1186, 2001.
between pH 6.5 and 8, while the maximum degradation of [16] H. Cui, Y. Fan, G. Fang, H. Zhang, B. Su, and J. Zhou,
petroleum and PAHs occur between pH 7 and 9. (ese, as “Leachability, availability and bioaccessibility of Cu and Cd ina contaminated soil treated with apatite, lime and charcoal: a
well as pH maxima for various microbial enzymes, could be five-year field experiment,” Ecotoxicology and Environmental
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Conflicts of Interest describing oxidation processes,” Water Research, vol. 34,
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(e author declares that there are no conflicts of interest [19] S. Andersson, S. I. Nilsson, and P. Saetre, “Leaching of dis-
regarding the publication of this article. solved organic carbon (DOC) and dissolved organic nitrogen
(DON) in mor humus as affected by temperature and pH,”
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