Soil Acidity: Development, Impacts, and Management

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 (Ca²⁺), magnesium (Mg²⁺), and potassium (K⁺) and replacing with proton (H⁺) and aluminum (Al³⁺) on the surface of soil particles wherever rainfall is substantial; (b) weathering of rock and soil minerals; (c) hydrolysis of Al³⁺; (d) differential uptake, i.e., 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 SO₂ and NO_x gases into the atmosphere by fossil fuel consumption that forms acid rain and (b) the excessive use of ammonium (NH₄⁺)-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., CaCO₃, CaMg(CO₃)₂, or CaSiO₃) 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 are also a viable strategy in dealing with acid soils that occupy nearly 30% of the ice-free land area of the world.

5.1 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 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 5.1). About 30% of the global ice-free land is acid (Fig. 5.1). 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). 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 5.2 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 (Fig. 5.2).

Table 5.1 Different levels of acidity of a soil (adapted from Havlin et al. 2017)

Fig. 5.1

Major acid soil regions in the world (accessed 9 March 2021). Source: https://nelson.wisc.edu/sage/data-and-models/atlas/maps/soilph/atl_soilph.jpg

Fig. 5.2

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 17 April 2021)

5.2 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 SO₂ from volcano activity vs. from coal burning on the formation of acid rain).

5.2.1 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 CO₂ dissolution as shown below:

$$ {\mathrm{CO}}_2\left(\mathrm{gas}\right)+{\mathrm{H}}_2\mathrm{O}\ \left(\mathrm{liquid}\right)\to {\mathrm{H}}_2{\mathrm{CO}}_3\left(\mathrm{aqueous}\right)\leftrightarrow {{\mathrm{H}\mathrm{CO}}_3}^{-}+{\mathrm{H}}^{+}\left(\mathrm{stands}\ \mathrm{for}\ \mathrm{reaction}\right) $$

(5.1)

The H⁺ ions (protons and sometimes written as H₃O⁺ when in water) gradually displace other positively charged ions, which are held on the soil surface (called exchangeable cations) such as Ca²⁺, 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 number of base cations is released into the soil solution and is subject to loss by leaching (Fig. 5.3). 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).

Fig. 5.3

Leaching of exchangeable cations (e.g., Ca²⁺ ion) by H⁺ from acidity generating sources (adapted from Weil and Brady 2017)

$$ {\displaystyle \begin{array}{l}{\mathrm{Al}}_{0.3}\left[{\mathrm{Si}}_{7.5}{\mathrm{Al}}_{0.5}\right]{\mathrm{Al}}_{3.6}{\mathrm{Mg}}_{0.4}{\mathrm{O}}_{20}{\left(\mathrm{OH}\right)}_4+0.8{\mathrm{H}}^{+}+8.2{\mathrm{H}}_2\mathrm{O}\\ {}\left(\mathrm{smectite}\right)\ \\ {}\kern1em \leftrightarrow 1.1\left[{\mathrm{Si}}_4{\mathrm{Al}}_4{\mathrm{O}}_{10}{\left(\mathrm{OH}\right)}_8\right]+3.1\mathrm{Si}{\left(\mathrm{OH}\right)}_4+0.4{\mathrm{Mg}}^{2+}\\ {}\kern4em \left(\mathrm{kaolinite}\right)\ \end{array}} $$

(5.2)

$$ {\displaystyle \begin{array}{l}{\mathrm{Si}}_4{\mathrm{Al}}_4{\mathrm{O}}_{10}{\left(\mathrm{OH}\right)}_8+10{\mathrm{H}}_2\mathrm{O}\leftrightarrow 2{\mathrm{Al}}_2{\left(\mathrm{OH}\right)}_6+4\mathrm{Si}{\left(\mathrm{OH}\right)}_4\\ {}\kern1em \left(\mathrm{kaolinite}\right)\kern8em \left(\mathrm{gibbsite}\right)\end{array}} $$

(5.3)

In fact, under acidic conditions, minerals such as kaolinite or even gibbsite can be dissolved to produce soluble Al³⁺ (Robarge 2008; Hue 2008).

$$ {\displaystyle \begin{array}{l}{\mathrm{Si}}_4{\mathrm{Al}}_4{\mathrm{O}}_{10}{\left(\mathrm{OH}\right)}_8+12{\mathrm{H}}^{+}\leftrightarrow 4{\mathrm{Al}}^{3+}+4\mathrm{Si}{\left(\mathrm{OH}\right)}_4+2{\mathrm{H}}_2\mathrm{O}\\ {}\left(\mathrm{kaolinite}\right)\kern8em \left(\mathrm{soluble}\ \mathrm{Al}\right)\end{array}} $$

(5.4)

and

$$ {\mathrm{Al}}_2{\left(\mathrm{OH}\right)}_6+6{\mathrm{H}}^{+}\leftrightarrow 2{\mathrm{Al}}^{3+}+6{\mathrm{H}}_2\mathrm{O} $$

(5.5)

Soluble Al³⁺, having small crystal radius (0.5 A⁰) and high charge (+3), forms a sixfold 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).

$$ {\displaystyle \begin{array}{l}\mathrm{Al}{{\left({\mathrm{H}}_2\mathrm{O}\right)}_6}^{3+}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow \mathrm{Al}\left(\mathrm{OH}\right){{\left({\mathrm{H}}_2\mathrm{O}\right)}_5}^{2+}+{\mathrm{H}}_3{\mathrm{O}}^{+}\kern1em {K}_1=1{0}^{-4.97}\\ {}\kern20em K\;\mathrm{is}\ \mathrm{equilibrium}\ \mathrm{constant}\Big)\end{array}} $$

(5.6)

$$ \mathrm{Al}\left(\mathrm{OH}\right){{\left({\mathrm{H}}_2\mathrm{O}\right)}_5}^{2+}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow \mathrm{Al}{\left(\mathrm{OH}\right)}_2{{\left({\mathrm{H}}_2\mathrm{O}\right)}_4}^{+}+{\mathrm{H}}_3{\mathrm{O}}^{+}\kern1em {K}_2=1{0}^{-4.93} $$

(5.7)

$$ \mathrm{Al}{\left(\mathrm{OH}\right)}_2{{\left({\mathrm{H}}_2\mathrm{O}\right)}_4}^{+}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow \mathrm{Al}{\left(\mathrm{OH}\right)}_3{{\left({\mathrm{H}}_2\mathrm{O}\right)}_3}^0+{\mathrm{H}}_3{\mathrm{O}}^{+}\kern1em {K}_3=1{0}^{-5.7} $$

(5.8)

$$ \mathrm{Al}\left(\mathrm{OH}\right)3{{\left({\mathrm{H}}_2\mathrm{O}\right)}_3}^0+{\mathrm{H}}_2\mathrm{O}\leftrightarrow \mathrm{Al}{\left(\mathrm{OH}\right)}_4{{\left({\mathrm{H}}_2\mathrm{O}\right)}_2}^{-}+{\mathrm{H}}_3{\mathrm{O}}^{+}\kern1em {K}_4=1{0}^{-7.4} $$

(5.9)

Soil acidity, thus, intensifies by these hydrolytic Al species along with H₃O⁺ (proton in water).

Another source of protons is the oxidation of soil organic matter (SOM). SOM is formed from microbial decomposition of forest litter and dead plant and animal tissues present in soils. Chemical structure of SOM is complex but contains many acid functional groups, such as carboxylic, phenolic, and ketonic (Stevenson 1982; see Fig. 5.4). 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.

Fig. 5.4

A proposed chemical structure of humic acid (a component of SOM) (adapted from Stevenson 1982)

$$ \mathrm{R}\hbox{-} \mathrm{COOH}\leftrightarrow \mathrm{R}\hbox{-} {\mathrm{COO}}^{-}+{\mathrm{H}}^{+}\kern1em K=1{0}^{-1}\hbox{--} 1{0}^{-7} $$

(5.10)

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⁺, NH₄⁺, Ca²⁺, and Mg²⁺) are absorbed than anions (e.g., NO₃⁻, SO₄²⁻, H₂PO₄⁻). Thus, such plants usually exude H⁺ ions into the soil solution resulting in lower soil pH (Fig. 5.5).

Fig. 5.5

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 (FeS₂) 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):

$$ {\displaystyle \begin{array}{l}{\mathrm{FeS}}_2+\mathrm{3}\mathrm{\frac{1}{2}}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{FeS}\mathrm{O}}_4+{\mathrm{H}}_2{\mathrm{SO}}_4\\ {}\left(\mathrm{pyrite}\right)\kern7em \left(\mathrm{ferrous}\ \mathrm{sulfate}\right)\end{array}} $$

(5.11)

$$ {\displaystyle \begin{array}{l}{\mathrm{FeSO}}_4+\frac{1}{2}{\mathrm{O}}_2+\mathrm{1}\mathrm{\frac{1}{2}}{\mathrm{H}}_2\mathrm{O}\leftrightarrow \kern0.75em \mathrm{FeOOH}+{\mathrm{H}}_2{\mathrm{SO}}_4\\ {}\kern11em \left(\mathrm{iron}\ \left(\mathrm{ferric}\right)\ \mathrm{oxyhydroxide}\ \mathrm{or}\ \mathrm{goethite}\ \mathrm{mineral}\right)\end{array}} $$

(5.12)

The resulting large quantities of H₂SO₄ 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.

5.2.2 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 (Fig. 5.6). More specifically, much of the world’s coal used for energy contains approximately 2% S, half of which is FeS₂ and the remainder is organic (Blake 2005). Coal burning produces SO₂ as follows:

$$ 4{\mathrm{Fe}\mathrm{S}}_2+11{\mathrm{O}}_2\leftrightarrow 2{\mathrm{Fe}}_2{\mathrm{O}}_3+8{\mathrm{SO}}_2 $$

(5.13)

Fig. 5.6

Release of SO₂ and NO_x gases by fossil fuel burning activities (adapted from Weil and Brady 2017)

Nitric oxide (NO) and nitrogen dioxide (NO₂)—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 N₂ and O₂ occurs at high temperatures.

$$ {\mathrm{N}}_2+{\mathrm{O}}_2\leftrightarrow 2\mathrm{NO},\mathrm{and}\ \mathrm{NO}+\frac{1}{2}{\mathrm{O}}_2\leftrightarrow {\mathrm{N}\mathrm{O}}_2 $$

(5.14)

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 O₂ and N₂ caused by lightning). In the presence of water vapor and O₂, NO₂ is oxidized to HNO₃ as follows:

$$ 2{\mathrm{NO}}_2+\frac{1}{2}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\leftrightarrow 2{\mathrm{H}\mathrm{NO}}_3 $$

(5.15)

A combination of H₂SO₄ and HNO₃ 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 CO₂). 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 (NH₃). The principal reactions are:

$$ {\mathrm{NH}}_3+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {{\mathrm{NH}}_4}^{+}+{\mathrm{OH}}^{-} $$

(5.16)

Reaction (5.16) will temporarily (2–4 weeks) raise the soil pH.

$$ {{\mathrm{NH}}_4}^{+}+2{\mathrm{O}}_2\leftrightarrow {{\mathrm{NO}}_3}^{-}+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{H}}^{+}\left(\mathrm{nitrification}\ \mathrm{process}\right) $$

(5.17)

Net reaction ((5.16) + (5.17)) yields

$$ {\mathrm{NH}}_3+2{\mathrm{O}}_2\leftrightarrow {{\mathrm{NO}}_3}^{-}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{H}}^{+} $$

(5.18)

Thus, eventually one mole of N added as NH₃ will produce one mole of H⁺ as shown in (5.18).

The application of the common urea fertilizer has also undergone similar reactions after being hydrolyzed with the help of urease enzyme produced by soil microbes.

$$ {\displaystyle \begin{array}{l}{\mathrm{NH}}_2\hbox{-} \mathrm{CO}\hbox{-} {\mathrm{NH}}_2+{\mathrm{H}}_2\mathrm{O}\leftrightarrow 2{\mathrm{NH}}_3+{\mathrm{CO}}_2\\ {}\left(\mathrm{urea}\right)\end{array}} $$

(5.19)

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 SO₂ gas) is also oxidized to produce strong H₂SO₄ acid.

$$ \mathrm{S}+{\mathrm{O}}_2\leftrightarrow {\mathrm{SO}}_2;{\mathrm{SO}}_2+\frac{1}{2}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\leftrightarrow 2{\mathrm{H}}^{+}+{{\mathrm{SO}}_4}^{2-} $$

(5.20)

Table 5.2 shows the theoretical quantity of acidity produced per unit of N or S fertilizer applied (Havlin et al. 2017).

Table 5.2 Common N and S fertilizers, their chemical reactions, and their potential acidity production

5.3 Impacts of Soil Acidity

5.3.1 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 (Al₂(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).

$$ {\displaystyle \begin{array}{l}\mathrm{Al}{\left(\mathrm{OH}\right)}_3+3{\mathrm{H}}^{+}\leftrightarrow {\mathrm{Al}}^{3+}+3{\mathrm{H}}_2\mathrm{O}\kern2em K=1{0}^{8.04}\\ {}\left(\mathrm{gibbsite}\right)\end{array}} $$

(5.21)

or

$$ \left({\mathrm{Al}}^{3+}\right)=1{0}^{8.04}{\left({\mathrm{H}}^{+}\right)}^3 $$

(5.22)

Reaction (5.22) predicts that for each unit pH drop, Al³⁺ activity would increase by 1000-fold. In other words, in order to keep (Al³⁺) at sub-micromolar levels, soil pH must be maintained above 5.0. This is because trivalent Al³⁺ is the most toxic Al form to plants and animals, and Al³⁺ activity as low as 1–10 μM in soil solution would damage many crops (Kamprath 1984; Kinraide et al. 2005; Parker 2005; Miyasaka et al. 2007; Hue 2011; Blamey et al. 2015).

Determination of Al³⁺ in soil solution is not an easy task because of its many hydrolytic species having variable degrees of toxicity as shown in Fig. 5.7 (and derived from Reactions (5.6)–(5.9)). Al³⁺ can also form complexes with other soil solution ions, such as fluoride (F⁻), SO₄²⁻, H₂PO₄⁻, 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 Al³⁺, soil pH, and exchangeable Al (Kamprath and Smyth 2005; Smyth 2012; Sanchez 2019). Figure 5.8 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.

Fig. 5.7

Distribution of Aluminum (Al) hydrolytic species as a function of pH

Fig. 5.8

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 present drought stress and P or Ca deficiency. Aluminum-affected roots tend to be shortened and swollen, having a stubby appearance (Fig. 5.9). 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).

Fig. 5.9

Aluminum effect on roots. Sesbania seedlings grown in an Ultisol (non-amended pH 4.2, right; and limed pH 5.5, left) of Hawaii

5.3.2 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 MnO₂ (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, MnO₂ will dissolve into soluble Mn²⁺ according to the reaction:

$$ {\mathrm{Mn}\mathrm{O}}_2+4{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\leftrightarrow {\mathrm{Mn}}^{2+}+2{\mathrm{H}}_2\mathrm{O} $$

(5.23)

Equilibrium constant of (5.23) can be expressed as:

$$ K=\left({\mathrm{Mn}}^{2+}\right)/\left\{{\left({\mathrm{H}}^{+}\right)}^4\ast {\left({\mathrm{e}}^{-}\right)}^2\right\} $$

(5.24)

If we assume that the system is poised, meaning log (H⁺) + log(e⁻) constant, which is often the case in soils (Lindsay 1979), then (5.24) becomes (Hue and Mai 2002)

$$ \mathrm{Log}\left({\mathrm{Mn}}^{2+}\right)=\mathrm{constant}-2\mathrm{pH} $$

(5.25)

Reaction (5.25) would predict that for every pH unit decrease, (Mn²⁺) 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 Mn²⁺ and keep more Mn²⁺ in solution regardless of pH, Mn²⁺ only increases about 10-fold for each pH unit drop as shown in Fig. 5.10.

Fig. 5.10

Manganese (Mn²⁺) 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) 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) (Fig. 5.11a). In watermelon, Mn toxicity first appears as dark brown spots on leaves (Fig. 5.11b); 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 5000 mg/kg in tolerant ones. Figure 5.12 illustrates leaf Mn levels and yield of bean (Phaseolus vulgaris) and cabbage (Brassica sp.) as a function of soil pH (Weil and Brady 2017).

Fig. 5.11

Manganese toxicity symptoms in soybean (Glycine max) (a), and in watermelon (Citrullus lanatus) (b)

Fig. 5.12

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 accompanied 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).

5.3.3 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 NH₄⁺ to NO₃⁻ 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 Science and Technology 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 United States, 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 Precipitation Assessment Program 1992). Because of acid rain, base cations (e.g., Ca, Mg) in forest soils would be leached, and more Al becomes soluble. Along with NO₃⁻ and SO₄²⁻, 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 and even die. Influx of H⁺ and/or Al³⁺ into fish gills stimulates excessive efflux of Na⁺ that can cause mortality (Bush 1997).

5.3.4 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 1M 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, and then the leaf tips or margins soon die; in tomato (Lycopersicon esculentum), blossom end rot occurs in immature fruit when Ca is deficient (Fig. 5.13). 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 Morita et al. (2011). They concluded that Ca was essential to the pegging and pod forming stages of peanut.

Fig. 5.13

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)

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 (Mengle and Kirkby 1979; White and Broadley 2003).

5.4 Management of Soil Acidity

Soil acidity can be managed by either amending the problem soils with materials that generate OH⁻ (liming materials) or growing plants that tolerate acidity. A combination of the two strategies would be desirable, wherever possible.

5.4.1 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 CO₃²⁻, OH⁻, and silicate (SiO₃²⁻), that can react with H⁺ and Al³⁺ 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 Ca²⁺

$$ {\mathrm{Ca}\mathrm{CO}}_3+{\mathrm{H}}_2\mathrm{O}\ \left(\mathrm{moisture}\ \mathrm{in}\ \mathrm{soil}\right)\to {\mathrm{Ca}}^{2+}+2{\mathrm{OH}}^{-}+{\mathrm{CO}}_2\left(\mathrm{gas}\right). $$

• b. Newly produced Ca²⁺ will exchange with Al³⁺ and H⁺ on the surface of acid soils.

• c. Lime-produced OH⁻ will react with H⁺ to form H₂O and with Al³⁺ to form solid Al(OH)₃:

$$ {\mathrm{OH}}^{-}+{\mathrm{H}}^{+}\to {\mathrm{H}}_2\mathrm{O} $$

and

$$ 3{\mathrm{OH}}^{-}+{\mathrm{Al}}^{3+}\to \mathrm{Al}{\left(\mathrm{OH}\right)}_3\left(\mathrm{solid}\right) $$

Thus, liming eliminates toxic Al³⁺ 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 Ca²⁺ (if CaCO₃ is used) as well as Mg²⁺ (if dolomite [CaMg(CO₃)₂] is used) or even K⁺ (if wood ash [K₂O, KOH, CaO, MgO] is used).

Silicates can be used as liming materials that do not contain carbon and therefore do not release CO₂ 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 is composed mostly of K₂O, CaO, CaCO₃, and 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 Fig. 5.14.

Fig. 5.14

Exchangeable Al of 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 5.3. 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 CaCO₃, which is given a value of 100.

Table 5.3 Common liming materials, their chemical names and formulas, and relative neutralizing values (modified version of Weil and Brady 2017)

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 (Fig. 5.15). 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.

Fig. 5.15

Soil pH changes in time as affected by different particle sizes of a liming material

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).

5.4.2 Lime Requirements of Acid Soils

5.4.2.1 Titration Curves with Commercially Available CaCO₃ 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 CaCO₃ and a local lime source is shown in Fig. 5.16.

Fig. 5.16

Lime titration curves of a Hawaiian Oxisol using pure CaCO₃ and a local lime source

5.4.2.2 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 drops in the buffer solution equilibrated with a soil are 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: (1) buffer-solution pH drop and (2) 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 KH₂PO₄; Huluka 2005) for low CEC soils of the Southeast region of the United States could be well suited for acid weathered Oxisols and Ultisols of the tropics.

5.4.2.3 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).

5.4.2.4 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 (CaSO₄.2H₂O) is much more soluble than lime (CaCO₃), 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 Al³⁺ and H⁺ of the acid surface soil, preventing Ca²⁺ from moving downward, SO₄²⁻ of the dissolved gypsum can accompany Ca²⁺ cations in leaching. Once the Ca²⁺ and SO₄²⁻ ions move down to the subsoil, Ca²⁺ can replace Al³⁺ ions from the exchange site, and the released Al³⁺ can react with SO₄²⁻ to form Al-SO₄ solids (e.g., basaluminite mineral) or soluble, but non-toxic AlSO₄⁺ ion pair (Hue et al. 1985; Kinraide 1997). Furthermore, SO₄²⁻ can replace terminal OH of Fe and Al oxyhydroxides, releasing some OH⁻ and raising soil pH and precipitating Al (Hue et al. 1985).

and

$$ 3\mathrm{Ca}{\left(\mathrm{OH}\right)}_2+2{\mathrm{Al}}^{3+}\to 2\mathrm{Al}{\left(\mathrm{OH}\right)}_3+3{\mathrm{Ca}}^{2+} $$

Figure 5.17 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).

Fig. 5.17

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)

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 Fig. 5.17 and Table 5.4. Such acidity ameliorating effects of organic materials are convincingly explained by Weil and Brady (2017) as quoted below:

• 1. High molecular weight organic matter can bind tightly with aluminum ions and prevent them from reaching toxic concentrations in the soil solution.

• 2. Low-molecular-weight organic acids produced by microbial decomposition or root exudation can form soluble complexes with aluminum ions that are nontoxic to plants and microbes.

• 3. Many organic amendments contain substantial amounts of calcium held in organic complexes that can leach quite readily down the soil profile. Therefore, if such amendments as legume residues, animal manure, or sewage sludge are high in Ca, they can effectively combat aluminum toxicity and raise Ca and pH levels, not only in the surface soil where they are incorporated, but also quite deep into the subsoil.

Table 5.4 Effects of organic residues and lime on soil acidity, soil aluminum, soluble carbon and the growth of a legume forage plant, Desmodium intortum in acid tropical soils

5.4.3 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), and 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: (1) minimizing Al uptake by exclusion or avoidance and (2) 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 (Fig. 5.18).

Fig. 5.18

Hydrangea (Hydrangea macrophylla) petals show red in low Al and blue in high Al conditions (adapted from Weil and Brady 2017)

In the case of tea, Sun et al. (2020) reported that root growth was stimulated in the presence of Al: better growth in 200 and 1000 μM Al solutions than in the treatments of 100 μM Al or no Al solutions. Furthermore, the length of new roots in 1000 μM Al was twice that of new roots in the 200 μM Al treatment (Fig. 5.19). Tea shoots can contain as much as 3% Al, perhaps as Al-oxalate complexes (Morita et al. 2011).

Fig. 5.19

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).

5.5 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., Ca²⁺, Mg²⁺, K⁺) are easily leached and are replaced with H⁺ and Al³⁺. 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 CaCO₃, CaMg(CO₃)₂ or CaSiO₃ 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.