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
Anhydrous ammonia
NH
3
+2O
2
→ NO
3
-
+ H
2
O + H
+
Urea
(NH
2
)
2
CO + 4O
2
→ 2NO
3
-
+ H
2
O + CO
2
+ 2H
+
Ammonium nitrate
NH
4
NO
3
+ 2O
2
→ 2NO
3
-
+ H
2
O + 2H
+
Ammonium sulfate
(NH
4
)
2
SO
4
+ 4O
2
→ 2NO
3
-
+ H
2
O + SO
4
2-
+ 4H
+
Monoammonium
phosphate
NH
4
H
2
PO
4
+ O
2
→ NO
3
-
+ H
2
PO
4
-
+ H
2
O + 2H
+
Elemental S
S + 1 ½ O
2
+ H
2
O → SO
4
2-
+ 2H
+
Ammonium thiosulfate
(NH
4
)
2
S
2
O
3
+ 6O
2
→ 2SO
4
2-
+ 2NO
3
-
+ H
2
O + 6H
+
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).