pH Affect on Ion Sorption (part 2)
H+ Ions

Cations are leached away by various means. They can be leached from the soil and rhizosphere due to rainfall. Cations can also be leached from their exchange site due to soil microbial activity. Rainfall causes alkaline nutrients to remain adsorbed on the surface only by a weak static electrical charge. The nutrients are pushed and pulled by other charged particles (ions) as they oscillate on the surface of exchange sites. Plant roots and soil microbes exude hydrogen ions (H+ ions). The exuded H+ ions begin to surround nutrient cations and, in some cases, fill the exchange sites as the H+ ion concentration in the soil increases, which can neutralize the negatively charged exchange sites. Once the H+ ions neutralize the negatively charged exchange site, the nutrient cation becomes available for plant or microorganism uptake as it is freed of its static bond. This process works specifically with plant roots and microbes because they release carbon dioxide into the soil. The carbon dioxide (CO2) then mixes with moisture in the soil, forming carbonic acid (H2CO3); the H+ ions from the formation of carbonic acid replace the cation nutrient on the exchange site. One example of this would be when a calcium ion sorbed to an exchange site has a double-positive charge, expressed as Ca++. As the concentration of H+ ions encroach on the exchange site and begin to outnumber the Ca++ ions, two H+ ions will then replace the Ca++ ion, and the plant or microbe is now free to take the Ca++ up as a nutrient.

pH Impact on Biochemical Processes

Source: Buerkert & Joergensen, 2016

Naturally, soil pH tremendously impacts biochemical processes in the soil, making pH the master link. Soil pH is the critical life support element that delivers carbon storage, water regulation, and soil fertility, affecting plant growth and biomass yield.

One study by Neina (2019) went as far as to say that “pH is compared to a patient’s temperature during medical diagnoses because it readily hints at the soil condition and the expected direction of many soil processes.” The soil pH leaches basic cations (Ca, Mg, K, and Na) as minerals become weathered, leaving H+ and Al3+ ions as the dominant exchangeable cations in the soil. As CO2 is released into the soil, creating carbonic acid, as mentioned earlier, dissociates and releases H+ ions into the environment. Neina (2019) also states that humic residues from the humification of soil organic matter produce high-density carboxyl and phenolic groups that dissociate to release H+ ions, and nitrification of NH4+ to NH3- produces H+ ions.

Source: Google Images

Modern research has elicited a higher understanding regarding the contributions of pH to soil processes. Soil biology and biological processes are controlled by pH; however, the biogeochemical processes greatly impact soil pH. Furthermore, this exchange is important for nutrient recycling and availability for crop production, allocation of toxic substances in the environment, and their disposal or translocation. Substance transaction involves the simultaneous functions of biochemical and physiochemical processes that affect leachate quality, adsorption, dilution, and volatilization. Trace elements’ solubility, mobility, and bioavailability are regulated through soil pH, which determines their translocation in plants, and is largely dependent on the element phase (solid-liquid).

Trace Element Availability

The dissolution due to precipitation causes reactions when pH-dependent charges dominate in high pH. Conversely, positive charges prevail at low pH values. Also, concentrations of dissolved organic carbon are controlled by pH, which directly affects the availability of trace elements within the soil. Lower pH means high desorption–the release of an adsorbed ion from a surface–of soluble trace elements. Oscillation from low to high pH within a narrow pH range is known as the pH-adsorption edge.

An example of this is posited in a study from Bradl (2004), which found that at pH 5.3, Cd, Cu, and Zn adsorption onto a sediment composite comprised of 60% Al-, 62% Fe-, and 53% Si-oxides. On the other hand, Bradl noted that 50% of Cd and Zn could sorb onto humic acids between pH 4.8—4.9 more readily. These trace elements rely heavily on the ionic species and properties of each species that form in the soil solution, as soil pH influences the chemical systems within the soil. As soil pH increases, the trace element solubility will decrease, leaving a low concentration of trace elements in the soil. Metal solubility is very sensitive to any pH increase or decrease, and the effect depends on the specific ionic species of the metals and the charge direction of pH. Research has shown that divalent metal solubility decreases up to one hundredfold and trivalent metal solubility decreases approximately one thousand-fold. Other research has stated that a decrease in the soil pH by a unit of one elicits a ten-fold increase in metal solubility.

Organic Matter and pH in the Soil

The organic matter in soil ranges from simple molecules (amino acids, monomeric sugars, etc.) to larger polymeric molecules (cellulose, protein lignin, etc.). These molecules exist together with undecomposed and decomposed plants and microbes within the soil. Soil pH increases the dissociation of acid in functional groups, thereby increasing the solubility of soil organic matter and begging the question of alkaline soil pH and if there is an effect on alkaline soil pH conditions which leach soluble organic carbon and nitrogen in soils with high concentrations of organic matter. The answer is yes. Dissolved organic matter content continues to increase with the increase of soil pH and mineralizable carbon and nitrogen. As the pH goes beyond pH 6, it will depend more on dissolved organic carbon concentration.

The soil offers microbial eco-physiological indicators within the soil’s biological processes. Ecophysiology is the interlinkage between cell physiological functioning under the influence of biological factors. When there is a decrease in overall carbon released by the microbes, there is an increase in biomass production. The higher biomass yield indicates a higher metabolic function within the soil and is, therefore, an indicator of the environmental conditions of the soil. Microbial metabolic activity is over two times higher in low pH soils than at a neutral pH. Normally, pH requirements from microbial activity reside between 5.5–8.8. Soils with a pH in this range will yield higher microbial biomass as soil respiration increases due to the optimal soil pH. 

As the microbial community increases, there is an increase in metabolic waste. The microbes produce enzymes for the biogeochemical cycling of nutrients. The soil pH is crucial for proper functioning enzyme activity in the soil and, therefore, indirectly regulates the enzymes produced by its effect on the microorganisms that produce them.

The chemicals produced by the enzymes eventually break down over time, known as biodegradation–the chemical dissolution of organic and inorganic pollutants produced by microorganisms. Biodegradation is another biological process in the soil controlled by pH through its effect on metabolic activity, diversity, enzymatic production, degradation, and substance degradation properties of the microbial community. Generally, alkaline or slightly acidic soil pH enhances biodegradation, while acidic environments limit biodegradation.

Microbial Impact on pH in the Rhizosphere

Living organisms in the soil undergo biochemical transformations when they die and long after they are dead, which generates changes in soil pH. This biogenic regulation of soil pH directly affects biochemical functions within the living microorganisms and occurs primarily through rhizosphere processes.

  The rhizosphere is the soil volume in the surroundings of roots that are impacted by root and microbial activities. There are a few factors that induce pH changes in the rhizosphere. 

  1. Ion uptake and inorganic ions that maintain electroneutrality.
  2. Organic acid anion excretion.
  3. Root exudation and respiration.
  4. Redox-coupled processes.
  5. The release of root carbon inducing microbial acid 
  6. Plant genotype.

Roots have a high propensity for causing a rhizospheric pH increase versus lowering it. The uptake of anion and cation nutrients is the main factor responsible for the pH increase or decrease. NH4+ and NH3- are two forms of inorganic nitrogen that are the main culprits sorbed in large concentrations. The primary forms of nitrogen taken up by the plant are ammonium (NH4+), nitrate (NO3-), and molecular nitrogen (N2), although the plant can also take up amino acids. Bicarbonates (HCO3-) or hydroxyl ions (OH-) are released as nitrogen is taken up, which maintains the electroneutrality in the soil increasing rhizosphere pH. Plants also release protons in reply to NH4+ uptake, causing a decrease in rhizosphere pH. Plant species and growth stages are other factors that control processes and pH in the rhizosphere.

Further Reading

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Aciego Pietri, J. C., & Brookes, P. C. (2008). Relationships between soil ph and microbial properties in a UK arable soil. Soil Biology and Biochemistry, 40(7), 1856–1861. https://doi.org/10.1016/j.soilbio.2008.03.020

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