Soil Acidity: Development, Impacts, and Management
Nguyen Hue
Professor of Environmental and Soil Chemistry
University of Hawaii, Honolulu, Hawaii 96822, USA
Abstract:
Soil acidity is a serious problem worldwide. Its causes can be both natural and anthropogenic. Natural
processes involve a) leaching losses of base cations such as calcium (Ca
2+
), magnesium (Mg
2+
), potassium
(K
+
), and replacing with proton (H
+
) and aluminum (Al
3+
) on the surface of soil particles wherever rainfall is
substantial; b) weathering of rock and soil minerals; c) hydrolysis of Al
3+
; d) differential uptake: more
cations than anions are absorbed by plants; and e) oxidation of soil organic matter and sulfide minerals.
Human-induced processes include a) the release of SO
2
and NO
x
gases into the atmosphere by fossil fuel
consumption that forms acid rain, and b) the excessive use of ammonium (NH
4
+
) containing fertilizers. Soil
acidity reduces crop production, forest health, and aquatic lives. The main culprits are the toxicities of Al
and/or manganese (Mn), and the deficiency of Ca, and to a lesser extent of Mg, phosphorus (P) and
molybdenum (Mo). Aluminum toxicity usually damages the root system first whereas Mn toxicity
adversely affects above-ground plant parts. Calcium deficiency impairs cell growth and integrity causing
poor crop production and quality. To manage soil acidity, liming with OH
-
producing materials (e.g.,
CaCO
3
, CaMg(CO
3
)
2
, or CaSiO
3
) is traditionally employed; alternatively, materials such as gypsum, animal
and green manures, or biochar, if available, could be applied for ‘short-term’ amelioration. Selecting and
growing acidity-tolerant plants is also a viable strategy in dealing with acid soils that occupy nearly 30% of
the ice-free land area of the world.
1. Introduction
Soil acidity is a term describing the unique properties of soils with a pH value (1:1 in water) below
7.0, the mid-point of the pH scale (0 – 14). By definition, pH is the negative logarithm of the hydrogen ion
or proton (H
+
) activity in the soil solution. The lower is the pH, the more acidic the soil. In fact, acid soils
are classified into many levels from extremely acid to neutral and slightly alkaline based on their pH
values (Table 1).
Table 1. Different levels of acidity of a soil (adapted from Havlin et al. 2017)
Descriptive acidity levels
pH range
Extremely acid
< 4.5
Very strongly acid
4.5 – 5.0
Strongly acid
5.1 – 5.5
Moderately acid
5.6 – 6.0
Slightly acid
6.1 – 6.5
Neutral
6.6 – 7.3
Slightly alkaline
7.4 – 7.8
About 30% of the global ice-free land is acid (Figure 1). And nearly 75% of the acid soils also overlay
acid subsoils (Havlin et al. 2017). Most acid soils occur in the Americas (1780 million ha), Africa (880
million ha), and Asia (690 million ha) (Sumner and Noble 2003).
Figure 1. Major acid soil regions in the world.
Source: https://nelson.wisc.edu/sage/data-and-models/atlas/maps/soilph/atl_soilph.jpg
(accessed 3/9/2021).
Acid soils are a serious constraint to food production and have adverse ecological impacts from crop
failure to forest decline (Bolan et al. 2005; Sanchez 2019). Figure 2 illustrates this point for a highly
weathered acid soil in South Africa where no crop can grow if the soil (pH 3.84) was not amended.
Figure 2. Crop response to lime on an acid soil in KwaZulu-Natal, South Africa.
Source: https://commons.wikimedia.org/wiki/File:Crops_in_acid_soil_demo_2017_05_09_6748i.jpg
(accessed 4/17/2021).
2. Development of Soil Acidity
Production of H
+
ions acidifies soils, and that process can occur naturally or anthropogenically.
However, these two pathways are often interrelated and may not be clearly distinguishable (e.g., effects
of SO
2
from volcano activity vs. from coal burning on the formation of acid rain).
2.1 Naturally Occurring Acid Soils
Acid soils are common in humid, tropical regions. Wherever rainfall is substantial (and often exceeds
evapotranspiration), soil acidification takes place. That is because rain is naturally acidic ( pH ~ 5.6) mainly
because of atmospheric CO
2
dissolution as shown below:
CO
2
(gas) + H
2
O (liquid) → H
2
CO
3
(aqueous) ↔ HCO
3
-
+ H
+
R.1
(R. stands for reaction)
The H
+
ions (protons and sometimes written as H
3
O
+
when in water) gradually displace other
positively charged ions, which are held on the soil surface (called exchangeable cations) such as Ca
2+
, Mg
2+
, and K
+
. These cations are termed base cations and are essential for plant growth. The H
+
ions become a
part of the soil’s solid, while an equivalent amount of base cations is released into the soil solution and is
subject to loss by leaching (Figure 3). Proton-saturated soils are not stable and will be further weathered
(transformed) to more stable minerals, eventually to oxides and hydroxides of Al, iron (Fe), Mn and
titanium (Ti) (Robarge 2008; Strawn et al. 2020). As an example, the transformation of smectite to
kaolinite, and finally to gibbsite is chemically shown below (Sposito 1989).
Figure 3. Leaching of exchangeable cations (e.g., Ca
2+
ion) by H
+
from acidity generating sources (adapted
from Weil and Brady 2017).
Al
0.3
[Si
7.5
Al
0.5
]Al
3.6
Mg
0.4
O
20
(OH)
4
+ 0.8H
+
+ 8.2H
2
O ↔
(smectite)
1.1[Si
4
Al
4
O
10
(OH)
8
] + 3.1Si(OH)
4
+ 0.4Mg
2+
R.2
(kaolinite)
and Si
4
Al
4
O
10
(OH)
8
+ 10H
2
O ↔ 2Al
2
(OH)
6
+ 4Si(OH)
4
R.3
(kaolinite) (gibbsite)
In fact, under acidic conditions, minerals such as kaolinite or even gibbsite can be dissolved to
produce soluble Al
3+
(Robarge 2008; Hue 2008).
Si
4
Al
4
O
10
(OH)
8
+ 12H
+
↔ 4Al
3+
+ 4Si(OH)
4
+ 2H
2
O R.4
(kaolinite) (soluble Al)
and
Al
2
(OH)
6
+ 6H
+
↔ 2Al
3+
+ 6H
2
O R.5
Soluble Al
3+
, having small crystal radius (0.5 A
0
) and high charge (+3), forms a six-fold coordination
(octahedral configuration) with six surrounding water molecules and undergoes further hydrolysis
(splitting water molecules) as shown below for the first four reactions (McBride 1994; Robarge 2008).
Al(H
2
O)
6
3+
+ H
2
O ↔ Al(OH)(H
2
O)
5
2+
+ H
3
O
+
K
1
= 10
-4.97
R.6
K is equilibrium constant)
Al(OH)(H
2
O)
5
2+
+ H
2
O ↔ Al(OH)
2
(H
2
O)
4
+
+ H
3
O
+
K
2
= 10
-4.93
R.7
Al(OH)
2
(H
2
O)
4
+
+ H
2
O ↔ Al(OH)
3
(H
2
O)
3
0
+ H
3
O
+
K
3
= 10
-5.7
R.8
Al(OH)3(H
2
O)
3
0
+ H
2
O ↔ Al(OH)
4
(H
2
O)
2
-
+ H
3
O
+
K
4
= 10
-7.4
R.9
Soil acidity, thus, intensifies by these hydrolytic Al species along with H
3
O
+
(proton in water).
Another source of protons is the oxidation of soil organic matter (SOM). SOM is formed from
microbial decomposition of forest litter, dead plant and animal tissues present in soils. Chemical structure
of SOM is complex but contains many acid functional groups, such as carboxylic, phenolic, ketonic
(Stevenson 1982, see Figure 4). Given the K values of these functional groups, particularly carboxylic
group (R-COOH) range from 10
-1
to 10
-7
, SOM can deprotonate and release protons along with the
corresponding conjugated organic anions which can complex metals, especially Al.
R-COOH ↔ R-COO
-
+ H
+
K = 10
-1
– 10
-7
R.10
Figure 4. A proposed chemical structure of humic acid (a component of SOM) (adapted from Stevenson
1982).
Differential uptake of cations and anions by plant roots may also contribute to soil acidity. For each
positive charge taken in as a cation, a root must maintain charge balance by absorbing an equivalent
anion or by exuding a positive charge as a different cation (electrical neutrality must be maintained). In
some plants, particularly legumes, more cations (e.g., K
+
, NH
4
+
, Ca
2+
and Mg
2+
) are absorbed than anions
(e.g., NO
3
-
, SO
4
2-
, H
2
PO
4
-
). Thus, such plants usually exude H
+
ions into the soil solution resulting in lower
soil pH (Figure 5).
Figure 5. Possible differential uptake of cations and anions by roots (adapted from Weil and Brady 2017).
Oxidation of elemental sulfur (S) and S-containing minerals forms sulfuric acid and releases large
quantities of protons. Coastal wetland areas in Southeast Asia (e.g., Indonesia, Malaysia, Thailand,
Vietnam), coastal Australia, Northern Europe (e.g., The Netherlands), West Africa, and the Southern
United States (e.g., Florida, Georgia, Louisiana, the Carolinas) commonly contain soils formed from
sediments having considerable quantities of sulfide minerals, such as pyrite (FeS
2
) and monosulfides
(Andriesse and van Mensvoort 2017). Sulfides begin to oxidize once they are exposed to an aerobic
environment. Such oxidizing environment can occur by natural events (e.g., oceanic retreat or tectonic
uplift) or by human activities, such as dredging or draining land for agriculture, forestry or other
developments. The principal reactions involved are (Weil and Brady 2017):
FeS
2
+ 3 ½O
2
+ H
2
O ↔ FeSO
4
+ H
2
SO
4
R.11
(pyrite) (ferrous sulfate)
FeSO
4
+ ½O
2
+ 1 ½H
2
O ↔ FeOOH + H
2
SO
4
R.12
(Iron (ferric) oxyhydroxide or goethite mineral)
The resulting large quantities of H
2
SO
4
lower soil pH values to below 3.5, sometimes even as low as
2.0. These S-oxidizing reactions can occur chemically, but will proceed much faster with the help of some
microbes, such as Thiobacillus ferrooxidans.
2.2 Anthropogenic Sources of Acidity
Combustion of fossil fuels and the smelting of S-containing metal ores emit enormous quantities of
nitrogen (N) and S-containing gases into the atmosphere (Figure 6). More specifically, much of the world’s
coal used for energy contains approximately 2% S, half of which is FeS
2
, and the remainder is organic
(Blake 2005). Coal burning produces SO
2
as follows.
4FeS
2
+ 11O
2
↔ 2Fe
2
O
3
+ 8SO
2
R.13
Nitric oxide (NO) and nitrogen dioxide (NO
2
) –collectively called NO
x
—enter the atmosphere mainly
from the burning of fossil fuels in motor vehicles and stationary furnaces. The formation of NO from N
2
and O
2
occurs at high temperatures.
N
2
+ O
2
↔ 2NO, and NO + ½O
2
↔ NO
2
R. 14
Figure 6. Release of SO
2
and NO
x
gases by fossil fuel burning activities (adapted from Weil and Brady
2017).
Once NO
x
has been formed, rapid cooling of exhaust gases prevents further reaction and traps the
oxides in the atmosphere (NO is also formed naturally in the atmosphere through reaction of O
2
and N
2
caused by lightning.) In the presence of water vapor and O
2
, NO
2
is oxidized to HNO
3
as follows.
2NO
2
+ ½O
2
+ H
2
O ↔ 2HNO
3
R.15
A combination of H
2
SO
4
and HNO
3
in the atmosphere will form acid rain, a popular term which
includes all forms of acidified precipitation: rain, snow, fog, and dry deposition. The pH of acid rain
commonly is between 4.0 and 4.5, and may be as low as 2.0 (normal, clean rainwater has a pH ~ 5.6 due
to dissolved CO
2
). The serious impacts of acid rain fall on downwind areas from major industrial centers,
weakly buffered lakes and streams, as well as forest (Blake 2005; Vance 2017).
Under intensive agronomic crop production, the use of ammoniacal fertilizers has considerably
acidified the soils (Cao et al. 2019), even with anhydrous ammonia (NH
3
). The principal reactions are:
NH
3
+ H
2
O ↔ NH
4
+
+ OH
-
R.16
Reaction R.16 will temporarily (2 – 4 weeks) raises soil pH.
NH
4
+
+ 2O
2
↔ NO
3
-
+ H
2
O + 2H
+
(nitrification process) R.17
Net reaction (R.16 + R.17) yields
NH
3
+ 2O
2
↔ NO
3
-
+ H
2
O + H
+
R.18
Thus, eventually one mole of N added as NH
3
will produce one mole of H
+
as shown in R.18
The application of the common urea fertilizer is also undergone similar reactions after being
hydrolyzed with the help of urease enzyme produced by soil microbes.
NH
2
-CO-NH
2
+ H
2
O ↔ 2NH
3
+ CO
2
R.19
(urea)
Elemental S added either by man or by volcanic eruption (in 2008, the Kilauea volcano in Hawaii, USA,
which had been erupting continuously since 1983, released over 1000 tons/day of SO
2
gas) is also oxidized
to produce strong H
2
SO
4
acid.
S + O
2
↔ SO
2
; SO
2
+ ½O
2
+ H
2
O ↔ 2H
+
+ SO
4
2-
R.20
Table 2 shows the theoretical quantity of acidity produced per unit of N or S fertilizer applied (Havlin
et al. 2017).
Table 2. Common N and S fertilizers, their chemical reactions, and their potential acidity production.
Fertilizer Source
Soil Reaction
Mole H
+
/mole N or S
Anhydrous ammonia
NH
3
+2O
2
→ NO
3
-
+ H
2
O + H
+
1
Urea
(NH
2
)
2
CO + 4O
2
→ 2NO
3
-
+ H
2
O + CO
2
+ 2H
+
1
Ammonium nitrate
NH
4
NO
3
+ 2O
2
→ 2NO
3
-
+ H
2
O + 2H
+
1
Ammonium sulfate
(NH
4
)
2
SO
4
+ 4O
2
→ 2NO
3
-
+ H
2
O + SO
4
2-
+ 4H
+
2
Monoammonium
phosphate
NH
4
H
2
PO
4
+ O
2
→ NO
3
-
+ H
2
PO
4
-
+ H
2
O + 2H
+
2
Elemental S
S + 1 ½ O
2
+ H
2
O → SO
4
2-
+ 2H
+
2
Ammonium thiosulfate
(NH
4
)
2
S
2
O
3
+ 6O
2
→ 2SO
4
2-
+ 2NO
3
-
+ H
2
O + 6H
+
1.5
3. Impacts of Soil Acidity
3.1 Aluminum Toxicity
The most common and severely harmful effect of soil acidity is Al toxicity to plants, microbial
community, and the environment (Weil and Brady 2017; Patra et al. 2021). In acid, weathered soils of the
tropics, Al in soil solution is often controlled by the solubility of gibbsite mineral (Al
2
(OH)
6
but often
written as Al(OH)
3
). Thus, Al activity (or effective concentration) as a function of pH can be predicted by
the following dissolution reaction of gibbsite and its equilibrium constant (K).
Al(OH)
3
+ 3H
+
↔ Al
3+
+ 3H
2
O K = 10
8.04
R.21
(gibbsite)
or (Al
3+
) = 10
8.04
(H+)
3
R.22
R.22 predicts that for each unit pH drop, Al
3+
activity would increase by 1,000 fold. In other word, in
order to keep (Al
3+
) at sub-micromolar levels, soil pH must be maintained above 5.0. This is because
trivalent Al
3+
is the most toxic Al form to plants and animals, and Al
3+
activity as low as 1 – 10 μM in soil
solution would damage many crops (Kamprath 1984; Parker 2005; Miyasaka et al. 2007; Hue 2011;
Blamey et al. 2015).
Figure 7. Distribution of Aluminum (Al) hydrolytic species as a function of pH.
Determination of Al
3+
in soil solution is
not an easy task because of its many
hydrolytic species having variable degrees of
toxicity as shown in Figure 7 (and derived
from R.6 – R.9). Al
3+
can also form complexes
with other soil-solution ions, such as fluoride
(F
-
), SO
4
2-
, H
2
PO
4
-
, and organic anions (e.g.,
citrate, malate, oxalate; Hue et al. 1986). It is
simpler to measure exchangeable Al (as
extracted with a neutral salt such as 1M KCl)
and Al saturation percentage (ratio of
exchangeable Al to CEC
*
100). There is a
strong positive correlation between soluble
Al
3+
, soil pH and exchangeable Al (Kamprath
and Smyth 2005; Smyth 2012; Sanchez 2019).
Figure 8 from the work on an Oxisol in Puerto
Rico as cited by Sanchez (2019) shows that an
Al saturation percentage range of 40-60%
would be toxic (yield drops by half) to most
crops.
Figure 8. Crop yields as a function of soil Al saturation %
(adapted from Sanchez 2019).
Aluminum toxicity usually
damages the root system first,
while the tops may look normal or
may appear drought stress, P or
Ca deficiency. Aluminum-affected
roots tend to be shortened and
swollen, having a stubby
appearance (Figure 9). A high level
of Al impairs root elongation and
decreases nutrient uptake; it
interferes with cell division at the
root apex, increases the rigidity of
the cell wall by crosslinking of
pectins which usually carry
negative charge, and reduces DNA
replication because of increased
rigidity of the double helix (Gupta
et al. 2013; Eekhout et al. 2017;
Bojorquez-Quintal et al. 2017).
Figure 9. Aluminum effect on roots. Sesbania seedlings grown in an
Ultisol (non-amended pH 4.2, right; and limed pH 5.5, left) of Hawaii.
3.2 Manganese Toxicity
Some soils in the tropics, particularly those of the Oxisol order, can contain high levels of Mn. For
example, the Wahiawa series, Oxisol order, in Hawaii has 1.2 – 1.6% total Mn mostly as MnO
2
(Hue et al.
2001). For comparison, background levels of total Mn in world’s soils average about 0.05% (500 mg/kg dry
weight) (WHO 2004). Under acidic conditions and with the supply of electron (e-) from SOM, MnO
2
will
dissolve into soluble Mn
2+
according to the reaction:
MnO
2
+ 4H
+
+ 2e
-
↔ Mn
2+
+ 2H
2
O R.23
Equilibrium constant of R.23 can be expressed as:
K = (Mn
2+
)/{(H
+
)
4
*(e
-
)
2
} R.24
If we assume that the system is poised, meaning log (H
+
) + log(e
-
) constant, which is often the case in
soils (Lindsay 1979), then R.24 becomes
Log(Mn
2+
) = constant -2pH (Hue and Mai 2002) R.25
R.25 would predict that for every pH unit decrease, (Mn
2+
) activity (and concentration) would
increase by 100 fold. In reality, however, because soil solution may contain other inorganic and organic
ions/molecules that can complex Mn
2+
and keep more Mn
2+
in solution regardless of pH, Mn
2+
only
increases about 10 fold for each pH unit drop as shown in Figure 10.
Figure 10. Manganese (Mn
2+
) concentration in the saturated paste extract of an Oxisol of Hawaii as a
function of soil pH (adapted from Hue and Mai 2002).
Hue and Mai (2002) also reported that a Mn concentration of 36 μM (or 2 mg/L) of Mn in the
saturated paste caused toxicity in watermelon (Citrullus lanatus cv. Crimson Sweet) grown on the
Wahiawa Oxisol; and the corresponding soil pH was 5.7.
Unlike Al, Mn toxicity first shows up in plant tops. The symptoms vary among plant species, but often
specific for a given species. For example, stunted, crinkled and chlorotic leaves are the Mn toxicity
symptoms in soybean (Glycine max) (Figure 11A). In watermelon, Mn toxicity first appears as dark brown
spots on leaves (Figure 11B); then the leaf margins dry up (necrosis), and finally the entire leaf dies out
and falls off just a few days after flowering (Hue et al. 1998). Also, unlike Al, the leaf tissue content of Mn
usually correlates with Mn toxicity, which begins at around 200 mg/kg in sensitive plants to over 5,000
mg/kg in tolerant ones. Figure 12 illustrates leaf Mn levels and yield of bean (Phaseolus vulgaris) and
cabbage (Brassica sp.) as a function of soil pH (Weil and Brady 2017).
Figure 11. Manganese toxicity symptoms in soybean (Glycine max) (A), and in watermelon (Citrullus
lanatus) (B).
Figure 12. Manganese concentration in plant tissue and relative crop yield as a function of soil pH.
(adapted from Weil and Brady 2017)
Manganese toxicity in plants is partially alleviated by high levels of tissue Ca, so the Mn/Ca ratio is
often used to diagnose Mn toxicity in addition to the absolute Mn concentration in leaf (Hue et al 1998;
WHO 2004). High Mn, on the other hand, may reduce the uptake of iron (Fe); Mn toxicity is often
accompanies by Fe deficiency symptoms (Mengle and Kirkby 1979; Silva et al. 2006; Eaton 2015).
At low levels, Mn is an essential nutrient because it is a co-factor of many enzymes. Decarboxylases
and dehydrogenases of the Tri-Carboxylic Cycle (TCA) are activated by Mn (Eaton 2015). At high levels,
however, Mn can cause oxidative stress by over-production of reactive oxygen species and increased
peroxidase activity (Horigushi and Fukumoto 1987; Martinez-Finley et al. 2013).
3.3 Hydrogen ion (H
+
) Toxicity
At pH levels below 4.0 – 4.5, H
+
ions themselves are of sufficient concentration to be toxic to some
plants, mainly by damaging the root membranes (Adams 1984; Weil and Brady 2017). Such low pH, even
in the absence of high Al or Mn, has been found to kill certain soil bacteria, such as Rhizobium bacteria
which are more sensitive to low pH than their host in the nitrogen-fixation symbiosis. The nitrifying
bacteria responsible for the conversion of NH
4
+
to NO
3
-
perform best at soil pH > 5.5 (Sanchez 2019).
Low pH (pH ~ 3 - 4) of acid rain can damage buildings, sculptures, and monuments that are
constructed using weatherable materials like limestone, marble, bronze, and galvanized steel (National
Sci. Tech. Council 2005). Agricultural soils are less impacted by acid rain (and H
+
) because of their
relatively higher buffering capacity than those of forests and aquatic environments (Vance 2017). In the
US, many important forest areas, such as the Adirondacks of New York and the Green Mountains of
Vermont have experienced sustained decreases in tree growth in the late 1900s (National Acid Precip.
Assess. Task force report 1992). Because of acid rain, base cations (e.g., Ca, Mg) in forest soils would be
leached and more Al becomes soluble. Along with NO
3
-
and SO
4
2-
, these cations end up in water bodies
and adversely affect aquatic lives. In general, when water pH of streams and lakes drops below 5.0, many
fish are affected, even die. Influx of H
+
and/or Al
3+
into fish gills stimulates excessive efflux of Na
+
that can
cause mortality (Bush 1997).
3.4 Calcium Deficiency
Although Al toxicity is often considered the central problem of soil acidity, Ca deficiency also occurs
very often, especially in acid weathered soils in the tropics (Sanchez 2019). For example, many acid soils in
Hawaii are Oxisols characterized by high proportion of Fe and Al oxides, and variable charges (Uehara and
Gillman 1981; Fox et al. 1991). These soils have very low base cations, especially Ca. In fact, Ca deficiency
is more common than Al toxicity in many acid soils of Hawaii (Hue 2008; 2011). As an example, the Kapaa
series (Oxisol) on the Kauai island has only 0.7 cmol
c
/kg Ca as extracted by 1 M ammonium acetate pH
7.0. This value is far below the recommended exchangeable Ca level of 7.5 cmol
c
/kg for optimal growth of
most crops (Yost and Uchida 2000).
Since Ca is fairly immobile inside the plant, its deficiency symptoms appear first in meristematic
tissues such as root tips, growing points of upper plant parts and storage tissues (White and Broadley
2003; White 2015). In corn (Zea mays) and taro (Colocasia esculenta), Ca deficient plants are stunted;
young leaves are unable to fully unfurl, then the leaf tips or margins soon die; in tomato (Lycopersicon
esculentum), blossom end rot occurs in immature fruit when Ca is deficient (Figure 13). In peanut (Arachis
hypogaea), Ca deficiency adversely affect its below-ground fruit development, and reduced pod yield
(Adams 1984; Smyth 2012). Abbas et al. (2018) reported that gypsum was required for one of the highest
pod yields of peanut grown in a field in Berhampur, India by Mohapatra and Dixit in 2010. They concluded
that Ca was essential to the pegging and pod forming stages of peanut.
Calcium is required for cell elongation and cell division. Its deficiency impairs cell membrane
permeability, causing leakage; leaf senescence and abscission are also affected by low Ca (Mengel and
Kirkby 1979; White and Broadley 2003).
4. Management of Soil Acidity
Soil acidity can be managed by either amending the problem soils with materials that generates OH
-
(liming materials) or growing plants that tolerate acidity. A combination of the two strategies would be
desirable, wherever possible.
4.1. Amending Acid Soils with Liming Materials
To decrease soil acidity (and raise soil pH), the soil is usually amended with alkaline materials (lime)
that provide conjugated bases of weak acids. These bases are anions, such as CO
3
2-
, OH
-
, and silicate
(SiO
3
2-
), that can react with H
+
and Al
3+
ions to form water or precipitates in a series of steps as follows.
a) Lime is dissolved (slowly) by moisture in the soil to produce hydroxide ions (OH
-
) and Ca
2+
CaCO
3
+ H
2
O (moisture in soil) → Ca
2+
+ 2OH
-
+ CO
2
(gas)
b) Newly produced Ca
2+
will exchange with Al
3+
and H
+
on the surface of acid soils.
c) Lime-produced OH
-
will react with H
+
to form H
2
O and with Al
3+
to form solid Al(OH)
3
:
OH
-
+ H
+
→ H
2
O
and 3OH
-
+ Al
3+
→ Al(OH)
3
(solid)
Figure 13. Symptoms of Ca deficiency in some common crops: (a) cracking in tomato (Lycopersicon
esculentum); (b) tipburn in lettuce (Lactuca sativa); (c) damaged tip in celery (Apium graveolens); (d)
blossom end-rot in immature tomato fruit, (e ) bitter pit in apples (Malus sp.); (f) necrotic leaf edge in taro
(Colocasia esculenta). Images (A-E) are adapted from White and Broadley 2003; and (F) from Hue 2008.
Thus, liming eliminates toxic Al
3+
and H
+
through the reactions with OH
-
. Excess OH
-
from the
dissolved lime will raise the soil pH, which is the most recognizable effect of liming. Another benefit of
liming is the supply of Ca
2+
(if CaCO
3
is used) as well as Mg
2+
(if dolomite [CaMg(CO
3
)
2
is used] or even K
+
(if
wood ash [K
2
O, KOH, CaO, MgO] is used.
Silicates can be used as liming materials that do not contain carbon and therefore do not release CO
2
into the atmosphere when they react with acid soils. The most commonly used silicates are calcium
silicate, a by-product of steel making. Calcium silicate reacts with an acid soil as follows.
Biochar is a solid material obtained from the thermochemical conversion (i.e., heating or pyrolysis) of
biomass (e.g., discarded wood, crop residue, manure, biosolids, etc.) in an oxygen limited environment
(IBI 2012). Depending on the feedstock and the treatment process, most biochars have high surface area
and contain many reactive surface functional acid groups, such as carboxylic and phenolic, that can
complex Al, Mn, and Ca. The ash portion of biochar composes mostly of K
2
O, CaO, CaCO
3
, MgO, resulting
in its alkaline pH (Hue 2020; Masud et al. 2020). Biochar application rates often are many tons
(commonly 5-20 tons/ha) per hectare on average. Thus, biochar can be used as a liming material that
effectively neutralizes all exchangeable Al in acid soils. An example of biochar use as a liming material on
an acid Ultisol of Hawaii is shown in Figure 14).
Figure 14. Exchangeable Al of an Hawaii’s acid Ultisol as a function of biochar’s acid neutralizing capacity
(adapted from Berek and Hue 2016).
Commonly used liming materials and their relative neutralizing values are given in Table 3. The
neutralizing value, or calcium carbonate equivalent (CCE), is defined as the amount of acid a given
quantity of the lime will neutralize when it is totally dissolved. The relative neutralizing value is calculated
as a percentage of the neutralizing power of pure CaCO
3
, which is given a value of 100.
Table 3. Common liming materials, their chemical names and formulas, and relative neutralizing values
(modified version of Weil and Brady 2017).
Liming material
Chemical name & formula
Relative neutralizing value
Calcitic limestone
Calcium carbonate, CaCO
3
100
Quick lime
Calcium oxide, CaO
150-175
Hydrated lime
Calcium hydroxide, Ca(OH)
2
120-135
Dolomitic lime
Calcium-magnesium carbonate,
CaMg(CO
3
)
2
95-108
Basic slag
Calcium silicate, CaSiO
3
70-90
Wood ashes
Mixture of oxides, CaO, MgO,
K
2
O, KOH
40-80
Biochar
Burned biomass, black carbon
5-30
Because most liming materials dissolve slowly, they should be finely ground to increase their reactive
surface for effective reactions with soil acidity components. Lime fineness is measured by using sieves
with different mesh sizes. The standard mesh size numbers indicate the number of wires per inch. Thus,
higher mesh size numbers signify smaller holes, which limit passage to finer particles. Note that 20-30
mesh lime is not as effective in raising
soil pH as the finer lime (Figure 15).
Also, it seems that lime particles of 50-
100 mesh size would be adequately
effective in neutralizing soil acidity.
Finer sizes (<100 mesh) would waste
money (and harder to spread), whereas
coarser grades may not react quickly
enough. Furthermore, the full effect of
liming might not be realized until
several months after application.
In brief, the capacity to neutralize
soil acidity depends on both the CCE
and the particle size of the liming
materials. Sometimes the two factors
are combined and called the Effective
Calcium Carbonate Equivalent (ECCE).
Figure 15. Soil pH changes in time as affected by different particle sizes of a liming material.
4.2 Lime Requirements of Acid Soils
4.2.1 Titration Curves with Commercially Available CaCO
3
Materials
The amount of lime required to raise soil pH from
the initial value to a desired value can be accurately and
specifically determined by this method as follows.
Various quantities of a commercially available lime
source (e.g., 0, 0.25, 0.50, 1.0, 2.0, 4.0, and 8.0 g) are
thoroughly mixed with 100 g acid soil. The mixture is
then moistened to the field-water-holding capacity.
Subsequently, the treated moist soil samples are air-
dried gradually for a week or two, re-moistened, and
dried again, so that the lime has had enough time to
react with the soil acidity. At the end of the second
incubation/equilibration period, soil pH (e.g., 20 g of
the treated soil in 20 ml of water) is measured with a
pH meter. An example of lime titration curves for an
Oxisol from Hawaii using pure CaCO
3
and a local lime
source is shown in Figure 16.
4.2.2 Buffer pH Methods for Lime Requirement
A simpler and less time consuming approach (often being used by soil testing laboratories) to
estimating lime requirements is to equilibrate a soil sample with a multi-component solution that has a
known initial pH value and is buffered against changes by acidity. This implies that the greater the acidity,
the more the solution’s buffering is overcome. Thus, the pH drop in the buffer solution equilibrated with a
soil is proportional to the amount of base (i.e., lime) that would be needed to raise the pH of that soil.
Empirical equations (derived from a database of several hundred soil samples) will estimate the quantity
of lime required (in ton/ha) based on two factors: (i) buffer-solution pH drop and (ii) desired final pH of
the tested soil. For example, if pH of the buffer solution drops 0.20 unit, and the target soil pH is 6.5, then
the regression equation (used by this buffer method) may recommend 3 tons of lime per hectare. A
popular buffer solution developed in Alabama in the 1960s (Adams and Evans 1962; Hue and Evans 1986)
and its recent modified version (where the toxic p-nitrophenol was replaced with KH
2
PO
4
; Huluka 2005)
for low CEC soils of the Southeast region of the US could be well suited for acid weathered Oxisols and
Ultisols of the tropics.
Figure 16. Lime titration curves of a Hawaiian Oxisol
using pure CaCO
3
and a local lime source.
4.2.3 Lime Requirement Based on Exchangeable Al and Al Saturation Percentage
This method assumes that Al is the principal factor controlling soil acidity, so lime quantity must be
provided to neutralize either all exchangeable Al or to decrease Al saturation percentage to a much lower
and non-toxic level. However, precautions should be taken, because lime not only reacts with
exchangeable acidity (exchangeable Al + exchangeable H) but also reacts with non-exchangeable acidity
that includes Al bound to SOM and H
+
of carboxylic and phenolic functional groups of SOM, and with OH
of Fe and Al oxyhydroxides. Thus, lime requirements based on exchangeable Al should be increased by a
factor of 1.5 to 3.0 in practice (Sanchez 2019).
4.2.4 Management of Acidity in Subsoil and in No-till Condition
Where subsoil acidity is a problem or where either no lime is available or the plowing/tilling is not
feasible, then approaches different from traditional liming practices should be explored.
Given the fact that gypsum (CaSO
4
.2H
2
O) is much more soluble than lime (CaCO
3
), gypsum has been
found to be effective in alleviating subsoil acidity without markedly changing soil pH (Sumner 1993). More
specifically, by applying gypsum to the top soil, acid subsoil showed an increase in exchangeable Ca, a
decrease in exchangeable Al, and as a result, a marked increase in root growth (Sumner 1993). Contrary
to lime whose OH
-
ions are consumed by Al
3+
and H
+
of the acid surface soil, preventing Ca
2+
from moving
downward, SO
4
2-
of the dissolved gypsum can accompany Ca
2+
cations in leaching. Once the Ca
2+
and SO
4
2-
ions move down to the subsoil, Ca
2+
can replace Al
3+
ions from the exchange site, and the released Al
3+
can
react with SO
4
2-
to form Al-SO
4
solids (e.g., basaluminte mineral) or soluble, but non-toxic AlSO
4
+
ion pair
(Hue et al. 1985; Kinraide 1997). Furthermore, SO
4
2-
can replace terminal OH of Fe and Al oxyhydroxides,
releasing some OH
-
and raising soil pH and precipitating Al (Hue et al. 1985).
and 3Ca(OH)
2
+ 2Al
3+
→ 2Al(OH)
3
+ 3Ca
2+
Figure 17 shows considerable reductions of
exchangeable Al saturation in subsoil of an acid
Ultisol by surface applications of gypsum or chicken
manure (Hue and Licudine 1999).
In fact, application of organic materials (e.g., crop
residues, animal wastes) not only can increase SOM but
also ameliorates the detrimental effects of soil
acidity as shown in Figure 17 and Table 4. Such
acidity ameliorating effects of organic materials are
convincingly explained by Weil and Brady (2017) as
quoted below.
“1.
High molecular
w
e
i
g
h
t
organic
matter
can bind
tightly with
a
l
u
m
i
nu
m
i
on
s
a
n
d
p
r
e
v
e
n
t
them
f
r
o
m
reaching toxic
c
o
n
c
e
n
t
r
a
t
i
on
s
i
n
the
soil
s
o
l
u
t
i
on
.
Figure 17. Effect of Gypsum, lime, and chicken manure
applied to the surface of an acid Ultisol of Hawaii on Al
saturation percentage at different soil depths (Hue and
Licudine 1999). Adapted from Weil and Brady 2017.
2.
L
o
w
-
m
o
l
ec
u
l
a
r
-
w
e
i
g
h
t
organic
acids
produced
by
microbial
d
e
c
o
m
p
o
s
i
t
i
on
or
r
oo
t
exudation
can
f
o
r
m
s
o
l
u
b
l
e
c
o
m
p
l
e
x
e
s
w
i
t
h
a
l
u
m
i
nu
m
i
o
n
s
t
h
a
t
a
r
e
n
on
t
o
x
i
c
t
o
p
l
a
n
t
s
and
microbes
.
3.
Many
organic
a
m
e
nd
me
n
t
s
c
o
n
t
a
i
n
s
u
b
s
t
a
n
t
i
a
l
a
m
ou
n
t
s
of
c
a
l
c
i
u
m
held
i
n
o
r
g
a
n
i
c
c
o
m
p
l
e
x
e
s
c a n
l
ea
c
h
quite readily
down the
s
o
il
profile.
T
h
er
e
f
o
r
e
,
if
s
u
c
h
a
m
e
n
d
m
e
n
t
s
as
l
e
g
u
m
e
residues, animal
manure,
or
s
e
w
a
g
e
s
l
ud
g
e
a
r
e
high
i
n
Ca, they
ca
n
e
f
f
e
c
t
i
v
e
l
y
c
o
m
b
a
t
a
l
u
m
i
nu
m
t
o
x
i
c
i
t
y
and raise Ca and
pH
l
e
v
e
l
s
,
not
o
n
l
y
i
n
t
h
e
s
u
r
f
a
c
e
s
o
i
l
where
they
a
r
e
i
n
c
o
r
po
r
a
t
e
d
,
but
also
quite
deep
i
n
t
o
the
s
ub
s
o
i
l.”
Table 4. Effects of crop residue –fresh or burned—on soil acidity and soil Al (adapted
from Weil and Brady 2017).
4.3 Growing Acid-tolerant Plants
When lime is not available because of high cost or poor transportation, it is better to solve soil
acidity problems by growing acid tolerant plant species than by trying to amend the soil. Due to their
relatively high tolerance to Al and low requirement for Ca, some crops such as pineapple (Ananas
comosus), sugarcane (Saccharum officinarum), cassava (Manihot esculenta) can grow well in acid
weathered soils whereas crops such as corn and soybean would perform poorly or even die. It is well
known that the acidity tolerance varies among plant species, but such tolerance also varies widely
among cultivars within a given species. Dr. Charles Foy of the USDA was one of the leading scientists
who screened many wheat (Triticum aestivum) varieties for their tolerance to Al, and to a lesser
extent Mn (Foy and Brown 1964; Foy 1974; Johnson et al. 1997; Kamprath and Foy 1985). Some Al
tolerant genes, such as ALMT1 (a malate transporter) in wheat, and Alt
SB
(a citrate transporter) in
sorghum (Sorghum bicolor) have been identified as cited by Sanchez (2019). In corn (Zea mays),
ZmAT6 gene has been shown to confer Al tolerance by scavenging reactive oxygen species (Du et al.
2020). Since plant breeding technologies have been grown rapidly in the past few decades, with
breakthrough research in genetics and genomics, it is no doubt that many acidity tolerant crops will
soon be developed (Deka 2021).
Two main strategies have been suggested for Al tolerance in plants: (i) Minimizing Al uptake by
exclusion or avoidance, and (ii) Detoxifying absorbed Al by chelation and vacuole containment. The
chelation of Al, and to a lesser extent Mn, by reactive organic acids (mainly citric and malic, and
perhaps oxalic) either as root exudates or as cell metabolites is believed to be the main mechanism
for acidity tolerance in many plant species (Kochian 2001; Liao et al. 2006; Gupta et al. 2013;
Bojorquez-Quintal
et al.
(2017). In fact, some plant
species such as tea (Camellia
sinensis) and hydrangea
(Hydrangea macrophylla)
need high levels of Al for
better growth and quality.
Hydrangeas are known to
change blossom (sepals) color
from pink when grown in low-
Al soils to bright blue (due to
the biding of Al with the
flower anthocyanin pigment
called delphinidin-3-
glucoside) in the presence of
high Al (Figure 18).
Figure 18. Hydrangea (Hydrangea macrophylla) petals show red in low Al and blue
in high Al conditions. (adapted from Weil and Brady 2017)
In case of tea, Sun et al. (2020) reported that root growth was stimulated in the presence of Al:
better growth in 200 and 1,000 μM Al solutions than in the treatments of 100 μM Al or no Al
solutions. Furthermore, the length of new roots in 1,000 μM Al was twice that of new roots in the
200 μM Al treatment (Figure 19). Tea shoots can contain as much as 3% Al, perhaps as Al-oxalate
complexes (Morita et al 2011).
Figure 19. Tea (Camellia sinensis) root responses to Al concentrations (A) and time after Al
treatment (B) (adapted from Sun et al. 2020).
As with Al tolerance, there are differences in Mn tolerance among plant species and varieties
within a species (Kamprath and Foy 1985; Foy et al. 1988). Macadamia (Macadamia integrifolia)
leaves can contain as much as 1% Mn (dry weight basis) without any apparent toxicity symptoms
(Warner and Fox 1972). Proteoid (cluster) roots apparently play a significant role in Mn
accumulation in macadamia (Rengel 2000). Manganese tolerance seems to be controlled by many
genes (Tang et al. 2021). In any case, in the tropics where most soils are acidic and highly
weathered, there exist many tolerant species and cultivars that can provide a viable alternative to
the management of soil acidity (Sanchez 2019).
5. Concluding Remarks
Acid soils occupy nearly 30% of ice-free area, and over 50% arable land of the world. Soil acidity
adversely affects crop production, forest growth, and aquatic lives. Soils become acidic through
natural processes of weathering, especially in areas of high rainfall because base cations (e.g., Ca
2+
,
Mg
2+
, K
+
) are easily leached and are replaced with H
+
and Al
3+
. Aluminum toxicity damages the root
system first, while Mn toxicity appears predominantly in plant tops. Calcium deficiency places havoc
on growing points such as root tips and meristems. Liming with common sources such as CaCO
3
,
CaMg(CO
3
)
2
or CaSiO
3
can effectively raise soil pH and alleviate Al and/or Mn toxicities and Ca
and/or Mg deficiencies. However, in some cases where lime is not available or is too costly,
alternative management options need be sought, so do for subsoil acidity or no-till situations.
Alternative strategies may include utilizing gypsum, organic manures (e.g., crop residue, animal
waste) or a combination of those along with growing acidity-tolerant crops. With deep
understanding and proper management in dealing with soil acidity, it is our hope that we can
steadily increase food production to feed our ever expanding population and to preserve/improve
our environment for a better future in this planet.
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e
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h
n
o
l
ogy
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National
A
c
i
d
P
r
e
c
i
p
i
t
a
t
i
on
A
s
s
e
s
s
n
1
e
n
t
P
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Outline of Chapter
Soil Acidity: Development, Impacts, and Management
Nguyen Hue
Email: nvhue@hawaii.edu
Professor of Environmental and Soil Chemistry
University of Hawaii, Honolulu, Hawaii 96822, USA
Abstract
1. Introduction
2. Development of Soil Acidity
2.1 Naturally Occurring Acid Soils
2.2 Anthropogenic Sources of Acidity
3. Impacts of Soil Acidity
3.1 Aluminum Toxicity
3.2 Manganese Toxicity
3.3 Hydrogen ion (H
+
) Toxicity
3.4 Calcium Deficiency
4. Management of Soil Acidity
4.1 Amending Acid Soils with Liming Materials
4.2 Lime Requirements of Acid Soils
4.2.1 Titration Curves with Commercially Available CaCO
3
Materials
4.2.2 Buffer pH Methods for Lime Requirement
4.2.3 Lime Requirement Based on Exchangeable Al and Al Saturation Percentage
4.2.4 Management of Acidity in Subsoil and in No-till Condition
4.3 Growing Acid-tolerant Plants
5. Concluding Remarks
References
