Influence of Soil Ph and Microbes on Mineral Solubility and Plant Nutrition: A Review Author's Details

Abstract

Soil pH is one of the essential vital features that increases or decreases the nutrient availability in soil. The lower pH lessens the secondary macronutrient availability while higher pH limits the available micronutrient in soil. Furthermore, the pesticides efficiency, use of organic and inorganic fertilizers sources to soil also required proper pH for maximum utilization by plants. Therefore, soil pH is termed as "principal soil indicator" that affect the biogeochemical cycles and has broader effects on the soil microbial community. Mineral approaches used to alter the soil pH had demonstrated drawbacks that it is too difficult to change. That’s why alteration in rhizospheric pH can be a practical approach. Hence, a microbial-breeding technique such as genome replication of microbes may be a suitable approach to alter rhizospheric pH. It might be possible that microbes genetic product releases too much acidic or basic compounds that increase or decrease the pH of rhizosphere. Greater exploitation of microbes in this respect would be essential to pursue, as they have the ability to resolve several stresses in a more sustainable manner. In order to breed the microbes selectively for optimal nutritional interactions with plants, the genetic components of different traits must first be practiced.

Full text

International Journal of Agriculture and Biological Sciences- ISSN (2522-6584) Jan & Feb 2021
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Influence of Soil Ph and Microbes on Mineral Solubility and Plant Nutrition: A Review
Author’s Details:
Aqarab Husnain Gondal1,*, Irfan Hussain1, Abu Bakar Ijaz1, Asma Zafar1, Bisma Imran Ch1, Hooria
Zafar1, Muhammad Danish Sohail1, Humaira Niazi2, M Touseef1, Asim Ali Khan2, Maryam Tariq1,
Hamza Yousuf1, Muhammad Usama1
1Institute of Soil and Environmental Sciences, University of Agriculture, 38000, Faisalabad, Punjab, Pakistan
2College of Agriculture, University of Sargodha, 40100, Sargodha Punjab, Pakistan
Corresponding Author* Email: aqarabhusnain944@gmail.com
Received Date: 09-Feb-2021 Accepted Date: 27-Feb-2021 Published Date: 28-Feb-2021
_____________________________________________________________________________________________
Abstract
Soil pH is one of the essential vital features that increases or decreases the nutrient availability in soil. The
lower pH lessens the secondary macronutrient availability while higher pH limits the available micronutrient in
soil. Furthermore, the pesticides efficiency, use of organic and inorganic fertilizers sources to soil also required
proper pH for maximum utilization by plants. Therefore, soil pH is termed as "principal soil indicator" that
affect the biogeochemical cycles and has broader effects on the soil microbial community. Mineral approaches
used to alter the soil pH had demonstrated drawbacks that it is too difficult to change. That’s why alteration in
rhizospheric pH can be a practical approach. Hence, a microbial-breeding technique such as genome
replication of microbes may be a suitable approach to alter rhizospheric pH. It might be possible that microbes
genetic product releases too much acidic or basic compounds that increase or decrease the pH of rhizosphere.
Greater exploitation of microbes in this respect would be essential to pursue, as they have the ability to resolve
several stresses in a more sustainable manner. In order to breed the microbes selectively for optimal nutritional
interactions with plants, the genetic components of different traits must first be practiced.
Keywords: Soil pH, Rhizosphere, Organic acids, Basic compounds, Nutrient availability, Growth promotion,
Soil health, Genes transfer
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INTRODUCTION
Soil is the essential component of life support systems since it supplies several goods and services that have
positive or negative impacts on human well-being such as water regulation, carbon preservation, food
production, and soil fertility including pH (FAO, 2015; FAO and ITPS, 2015; Jones et al., 2013). The pH is a
negative logarithm of hydrogen ion activity in the soil-water continuum (Hong et al., 2018) and is a critical
component of nutrient availability. Hydrogen ion activity in soil is considered as the dominant phenomenon; at
elevated pH value, the hydrogen ion concentration is low and vice versa (Cushman, 2015). A logarithmic pH
scale is used when hydrogen ion concentration ranges over a broad range; with a pH decrease of 1, the acidity
increases by a factor of 10 (Gethin, 2007). The pH scale varies between 0-14 (Schneider et al., 2007) and
differentiates the soil types with different ranges of pH worldwide. For instance, the pH value of ordinary soil
ranges from 3.5 to 9 and in precipitated areas ranges from 5 to 7 and in dry regions varies between 6.5 to 9
(Queensland Government, 2016).
Upper and lower level of pH in soil solution significantly affect the nutrient uptake and all other
phenomenon’s that occurred in soil. Soil pH directly influences the cation exchange capacity (CEC) (Sparks,
2003). The presence of negative charges on soil colloids tends to develop the CEC of the soil, and the CEC of
soil fluctuate significantly due to change in negative charge (Weil and Brady, 2017). Negative charges on soil
particles (allophones, organic colloids, sequiooxides, and 1:1 types silicates) are increased due to an increase in
pH that also tends to increase the CEC of soil and vice versa (Sollins et al., 1988). Commonly, variable and

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permanent charges are the two types of soil charges (Cunha et al., 2014). Variable charges are completely pH-
dependent (vary with variation in pH), while permanent charges are independent (constant) (Cunha et al., 2014).
Furthermore, the pH explains the chemical behaviour of protons, the main player in redox reactions, and
precipitation, surface complexion, crystal degradation, and other geochemical processes (Bethke et al., 2011).
These processes determine the factors such as salinity, nutrients avaibility, pH, and micronutrients association
in soil solution. It also affect the enzymatic activities, and organic matter (Lauber et al., 2009). However,
increase or decrease in pH significantly influence the crop growth, quality and yield in terms minimum nutrient
uptake.
Furthermore, each pesticide has its prerequisite for proper functioning. Simultaneously, the soil pH
reaches the inappropriate standard; it may either become ineffective and may not degrade as expected (Nicholls,
1988), resulting in difficulties for the subsequent crop growth period. The intensive cultivation and climatic
changes significantly modify the soil characteristics including pH (Guo et al., 2010; Yang et al., 2012). Hence,
plant growth, pesticide efficacy, soil productivity, microbial activity, and nutrient availability are adversely
affected by soil pH; plant growth problems are common in too alkaline or too basic soils. Besides, many heavy
metals become more water-soluble under acidic conditions and can move down to the soil with water and, in
some instances, move to aquifers, surface streams or reservoirs and soils with a pH of 5.5 or less are likely to be
incredibly corrosive to concrete (USDA Natural Resources Conservation Service, 1998).
Various mineral soil conditioners such as limestone, dolomite, and potassium feldspar etc. are generally
used to overcome the problem of soil acidity and these conditioners are rich source of calcium (Ca+2), silicon
(Si), magnesium (Mg+2), and potassium (K) (Yang et al., 2020). Addition of sulfur (S) may acidify the alkaline
soil to the desirable pH range (McCauley et al., 2009). In addition, natural community disrupt the pH
significantly because of their metabolism (Ye et al., 2012). Microbial population change their metabolism in
several ways. Various microbes such as acidophilic (Thiobacillus acidophilus (a type of bacteria), Vorticella (a
type of eukaryote), and Crenarchaeota (a type of archaea)) maintain the pH towards neutral by secreting basic
compounds in the soil solution (Gemmell and Knowles, 2000) and are resistant to salinity and other abiotic
stresses as prescribed in Table 1. Similarly, Alkaliphilic (Thiohalospira alkaliphila) microbes released acidic
compounds to adjust the pH towards neutral and are also resistant to stresses by adopting various mechanisms
(Kulshreshtha et al., 2012). For instance, the applied S is converted into hydrogen sulfate or sulfuric acid by
particular kind of microbes that helps the soils to bring the pH down a bit (Kopecky, 2014).
Table 1. Tolerance of microbes in acidic and basic environment;
Microbe
Acidity Tolerant
Alkalinity Tolerant
Reference
Rhizobia
yes
-
(Watkin et al., 2003).
Rhizobium tropici
yes
-
(Muglia et al., 2007;
Wang et al., 2018)
Arbuscular mycorrhiza (AM) Fungi
yes
-
(Clark, 1997; Bloom et al., 2006)
Alkaliphilic Bacteria
-
yes
(Torbaghan et al., 2017)
Bacillus
yes
-
(Shin et al., 2017)
Paenibacillus
yes
-
(Shin et al., 2017)
Alicyclobacillus
Yes
-
(Shin et al., 2017)
Burkholderia bannensis sp.
yes
-
(Aizawa et al., 2011)
Sulphur oxidizing bacteria
yes
(Bao et al., 2016)
– sign show that data is not available.
Fragile effects of the acidic soil environment

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The soil acidity influence plant development in several ways. Hydrogen (H+), iron (Fe+2 or Fe+3), and aluminum
(Al+3) are common acid-forming cations, while sodium (Na+), K+, Ca+2, and magnesium Mg+2 are base-forming
cations (Diriba, 2018). The solubility of manganese (Mn), Al, and Fe become maximum at lower pH. The
excessive soluble Al in soil solution restricts root growth, reduces the supply of macronutrients, which also
affects microbial development (Cornell University, 2010). These nutrients, however, become too poisonous for
plants when their volume reaches the cap. On the other hand, the availability of macronutrients increases as
certain micronutrients and phosphorus decrease. The lower level of these primary and secondary nutrients
negatively affects plant growth, especially by disrupting the many plant characteristics such as biomass, flower
size and number, lateral spread, and pH-induced pollen production (Jiang et al., 2017). Higher amounts of H+
ions dissolve basic cations at a lower pH level, remove them from exchange sites, release them into the soil
solution, and very small concentration of these nutrients is utilized by plant and remaining is lost by leaching
(University of Hawai'i, 2021). In acidic conditions, microbes and their counts are responsible for transforming
nitrogen (N), phosphorus (P), and sulfur (S) into the usable type of plants (Jacoby et al., 2017), thus reducing
the concentration of minerals. In acidic soils, Mg+2, Ca+2, nitrate ions of nitrogen (N), boron (B), P, and
molybdenum (Mo) availability is reduced (Extension Service of Mississippi State University, 1914; Maathuis,
2009; McCauley et al., 2009). The Ca+2 and Mg+2 ions become inaccessible to the plants by reacting with soil
matrix or adsorption by clay particles and leached to some extent (Maathuis, 2009; McCauley et al, 2009). The
symbiotic nitrogen fixation may be impaired in legume crops after alteration in soil pH. Rhizobium is mainly
responsible for N fixation in legumes, which demands more N (Mabrouk et al., 2018), and its activities have
decreased under acidic conditions.
Fragile effects of the alkaline soil environment
At very alkaline pH levels, mineralization of organic matter is delayed or halted due to weak microbial behavior
(Diriba, 2018) associated with nitrification and nitrogen fixation that is also hindered. The pH of soil influence
the degradation and mobility of insectides and hercides and also influence the heavy metals solubility (Smith
and Doran, 1996). The pH affects cation exchange reactions that modify soil aggregation (Sumner and Miller,
1996), e.g. Ca+2 ions serve as a barrier between clay particles and organic colloids in alkaline or acidic
environments. In addition, the Gaeumannomyces graminis fungus grows well at alkaline pH and thus affects
barley, rye, wheat and various other grasses (Smith and Doran. 1996). At higher pH, denitrification process
become limited due to inhibition in microbial community and accumulation of nitrite (NO2-) occur (Albina et
al., 2019). Mineralization of organic matter is slowed down at alkaline pH and organic matter is pH dependent
(McCauley et al., 2009).
Correlation of soil nutrients and pH
Soil nutrients are essential for vigorous plant growth. Macronutrients (potassium (K), P, and N) are needed by
crop pants in greater quantity and can be handled and applied by fertilizers on crop-based requirements (Rosse
et al., 2011). Fertilizers (organic and inorganic) are a more incredible nutrient source and are pH-dependent
(Neina, 2019). Furthermore, pesticides stability is adversely affected by pH (Schilder, 2008). Soil pH can
change the available type of nutrients in soil solution (Jensen and Thomas, 2010). Changing pH to the indicated
value greatly influences the essential plant nutrient, and plants typically grow well above 5.5 pH. As a rule, 6.5
is the most appropriate pH level for optimum nutrient absorption (Cornell University, 2010). The supply of
nutrients, solubility, microbial population, and various other processes depend on pH. For instance, a higher pH
value promotes micronutrient availability than neutral or alkaline soils that favour plant growth (Lončarić et al.,
2008). Similarly, soil chemical, biological, physical, and other processes are closely interlinked with
biogeochemical cycles and positively affected by soil pH, which eventually influences plant growth, yield, and
biomass (Neina, 2019; Minasny et al., 2016).
pH specification of macro and micronutrients

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A pH range of either 7 or nearly 7 is most suitable for plant growth, since all plant nutrients are readily
available (Hayman and Tavares, 1985) and at pH 5.5 poor solubility of phosphorus, molybdenum, calcium, and
magnesium while solubility of Fe, Al, and B is high. Furthermore, calcium and magnesium become more
abundant at pH 7.8 and pH of 6.6 to 7.3 is optimal for microbial activities that contributes towards plant
available nutrients (N, S, and P) (USDA Natural Resources Conservation Service, 1998). The availability of B
to the plants decreases with an increase in soil pH (Marx et al., 1996), particularly above pH 6.5. However,
highly acidic soils (pH less than 5.0) often appear to be poor in usable soil B due to boron sorption of iron and
aluminium oxide on the soil mineral surfaces (Goldberg, 1997). The K fixation between clay layers tends to be
lower under acidic conditions and is believed to be attributed to the presence of soluble Al occupying the
binding sites, while the accessible type of S had no effect on soil pH (Stanford, 1947). The pH had a crucial
impact on the solubility of Fe (Lindsay and Schwab, 1982). Less than 50% of Fe is available to the plants at pH
7 and due to the precipitation of iron hydroxide at pH 8; none of the Fe is to be found in the soil solution. More
than 90% of Fe become accessible to plant as pH tends toward acidic (< 6.5) (Fageria et al., 2014). With each
unit rise in pH lessens the solubility of Fe approximately by 1000-fold (Lindsay, 1979; Fageria et al., 2014).
The activity of Mn, Cu, and Zn is decreased by 100-fold approximately with increase in each unit of pH in the
range of 4-9 (Lindsay, 1979). Hydrated copper (Cu) exhibit the process of hydrolysis as the pH of soil increases
(pH >6.0), that tends to increase the adsorption of Cu to the organic matter (OM) and clay minerals (Fageria and
Nascente, 2014).
Role of pH in microbial growth
In natural environments, microbes are widespread biota; from hot springs to deep aquifers, in the natural
habitats and also commonly support the microbes in ocean floor (Edwards et al., 2012). They modify a number
of biogeochemical cycles ranging from global carbon cycling and redox reaction to weathering (Maguffin et al.,
2015). A wide variety of environmental factors such as temperature, supply of nutrients, salinity and pH
regulate their metabolism (Amend et al., 2013). Among these factors, pH has greater influence (Chen et al.,
2004). The pH is the indication of managing the microbial community, their activities, and composition (Lauber
et al., 2009). On maximum basis, microbes are classified into three groups namely, alkaliphiles grow fastest
above pH 9, acidophiles grow best at pH <5, neutrophils grow optimally at pH between 5 to 7 (Baker-Austin
and Dopson, 2007). One unit increase or decrease the pH reduce the microbial growth 50% (O'Flaherty et al.,
1998; Kotsyurbenko et al., 2004).
Management of soil pH
Various approaches, methods, and strategies are being used to mitigate the problem of soil pH. Generally for the
management of acidic soils, calcitic limestone is used to maintain the pH towards neutral and is more effective
than the dolomitic limestone (Pennisi and Thomas, 2015) and for high pH elemental sulfur is recommended in
certain cases. Gypsum is more effective to enhance electrical conductivity (EC) of soil than S but S is effective
to reduce the pH of soil than that of gypsum (Turan et al., 2013). Both application pulls the pH towards neutral
as shown in Figure 1. Unfortunately, to change the soil pH is a complicated phenomenon because of all the
artificially applied nutrients sources are pH dependent. Therefore, management of soil pH is necessary to
achieve successful production of horticultural and agronomic crops (Shober et al., 2019). The microbial
breeding approaches can be suitable alternative.

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Figure 1. The mineral application drag the pH towards neutral.
Mechanism of innovative microbial approach
The fluctuation of pH in rhizosphere could be a suitable phenomenon that increase or decrease the pH by
several folds in rhizosphere. The microbial approach can be beneficial if manage properly. In this technique, the
collected microbes (that release organic acids or essential compounds) may be multiplied through genome
transferring until their characteristics become too acidic or basic in rhizosphere to help to alter soil pH where
nutrients are readily available to the plants. The microbial community collected from different sites, alkaline
and acidic medium can be helpful for this purpose because every bacteria has its own characteristics collected
from various locations. The Figure 2 clearly explains the innovative mechanism. Furthermore, in recent years,
this view has helped to begin to answer some common evolutionary concerns regarding how bacteria, along
with their host species, have evolved from their early ancestors. Furthermore, it is of vital significance to
consider how plant tolerance has been affected by their encounters with microbes, although much remains
unclear.
Soil pH
Acidic soil
Calcitic limestone
application
Sulfur, gypsum
application
pH maintenance (neutral
Basic soil
Mineral compounds
addition

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Figure 2. Innovative microbial approach towards neutral soil pH.
In addition, the next challenge is to identify the primary genetic elements that support how different plant
genotypes interact with rhizospheric bacteria. Decades of study have shown that the vulnerability to pathogenic
microorganisms is heavily dependent on the genome of plants, between various species as well as on accessions
of the same species (Zhang et al., 2013). Similarly, Arabidopsis accessions have demonstrated considerable
variance in support for the development of the rhizospheric bacterium Pseudomonas fluorescens in the
hydroponic system (Haney et al., 2015).
Microbial efficiency towards soil pH
Plants do not live on their own; they still have dynamic relationships with microbes (Knack et al., 2015).
Plants allow the microbes (fungi, archaea and bacteria) in all over their tissue and the subsequent accumulation
of microbes is called as phytomicrobiome (Knack et al., 2015; Smith et al., 2017). Various organic acids such as
malic acid, gluconic acid, citric acid, oxalic acid, tartaric acid, lactic acid, and succinic acid are produced by
microbial biota in which both anions and cations serve as chelating agents and anions trap positively charged
ions (Ca+2, Al+3, and Fe+3) present in the soil (Mardad et al., 2013). Plant roots, decay of organic matter, and
bacteria may be the cause of acids in the soil. Previous studies agreed that microbes are the primary cause of
soil organic acid production and therefore the problems associated with the formation of organic acids are
becoming important (Adeleke et al., 2017). From wider variety of ecosystem, the organic acids concentration
Microbial
community
Microbial genes
transformation
Alkaliphilic
Variation in heredity
material of basic
microbes
Variation in heredity
material of acidic
microbes
New microbial
characteristics
Neutrophilic
Acidophilic
Thiobacillus
Vorticella
Natronomonas
pharaonis
Thiohalospira
alkaliphila
Basic compounds
released
Acidic compounds
released
New compounds
developed
pH maintenance
Plant growth promotion
Soil productivity
Crop yield

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varies between 0 to 50 μM for tri or dicarboxylic acids such as tartaric acid, citric acid oxalic acid, malic, and
succinic acid while these concentrations varies greatly ranging from 0 to 1 mM in monocarboxylic acids
including formic, valeric, lactic acid, acetic acid, propionic acid, and butyric acids. (Strobel, 2001). However, it
should be focused that these concentrations are extremely variable based on the soil composition, organic matter
degradation, root exudates, and microbes. Microorganisms including bacteria, fungi, and lichen species contain
large quantities of soil organic acids (Ryan et al., 2001, Lian et al., 2008, Aoki et al., 2012).
Conclusions
Increasing or decreasing pH significantly influence the nutrients in soil and ultimately affect the growth and
yield of plants. Various methods are used to solve these problems but it is very complex phenomenon.
Therefore, breeding of microbes may be suitable option. Microbes release organic acids that take part in much
of the physico-chemical processes that make the soil ecological system working. Future crop production may
entail more breeding for pH stress resistance and the introduction of microbial technologies that have improved
tolerance to pH stress. However, the underlining theory that organic acids synthesis in the soil setting is one
process involved in the mineralization and solublization of poorly available and complex minerals, and that it
leads to the carbon cycle, the detoxification of soil metals, among other functions, is possible, although it also
needs further study. Comparing of current and previous genes characteristics by these microbes should be
checked through experiments. In order to selectively breed the microbes for optimal nutritional interactions for
plants, the genetic components of this trait must first be established.
Conflict of interest
The authors do not have any conflict of interest with each other.
Acknowledgment
All the authors are highly thankfull to Institute of Soil and Environmental Sciences, University of Agriculture,
Faisalabad, Punjab, Pakistan and College of Agriculture, University of Sargodha, Sargodha Punjab, Pakistan.
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