The pH of soil has been often called the master variable since it can have an overriding effect on numerous processes and properties - chemical, physical and biological.
— Brady & Weil, 1999
Photo by montiannoowong/iStock / Getty Images
Photo by montiannoowong/iStock / Getty Images

Soil pH has a major influence on the following abiotic and biotic factors:

  1.  Decomposition of mineral rock into essential elements that plants can use: 

Soil pH has a major effect on the solubility of minerals or nutrients. Fourteen of the seventeen essential plant nutrients are obtained from the soil. Before a nutrient can be used by plants it must be dissolved in the soil solution. Most minerals and nutrients are more soluble or available in acid soils than in neutral or slightly alkaline soils Bickelhaupt, D. 2016 (1).

Nitrogen (N), Potassium (K), and Sulfur (S) are major plant nutrients that appear to be less affected directly by soil pH than many others elements. At alkaline pH values greater than pH 7.5, phosphate ions tend to react quickly with calcium (Ca) and magnesium (Mg) to form less soluble compounds. At acidic pH values, phosphate ions react with aluminum (Al) and iron (Fe) to again form less soluble compounds Jensen, T.L 2010 (2)

    2.   Plant nutrient availability: 

At pH levels that are too high or too low, certain nutrients become too available and toxic to the crop while others become less available and show up as crop deficiencies.

A pH range between 6.2 and 7.3 (between heavy lines) is considered ideal for most plants (Figure 1). At this pH range, most of the soil nutrients will be most soluble in the soil water and thus more available for plant roots for uptake Hahn, K. 201 (3).  

Figure 1: How soil pH affects the availability of plant nutrients:

 Hahn, K. 2012 blog: Soil test results are back - now what? Food Plotting. Heartland Outdoors

Hahn, K. 2012 blog: Soil test results are back - now what? Food Plotting. Heartland Outdoors

     3.   Conversion of fertilizer bag contents to a plant available form: 

The ratio of nitrate to ammoniacal nitrogen in a fertilizer determines the rate of substrate pH change and can even be used to correct pH during production. In general, ammoniacal and urea nitrogen are acidic, and tend to drive the media pH down, whereas nitrate nitrogen is basic and tends to drive the media pH up.  The pH changing property is known as a fertilizer’s potential acidity or basicity and is listed on a fertilizer’s label Mattson, N et al. 2009 PDF (4).

      4.   Activity of soil microorganisms to convert organic matter:

The soil pH can also influence the activity of beneficial microorganisms and thus, effect plant growth. Chemical transformation of organically bound N into simple inorganic forms (NH4+ and NO3-) that are available to the plant is called mineralization. A pH of between 7.0 and 8.0 has been suggested as optimum for denitrification Knowles R. 1982 (5). At very acid or alkaline pH levels, organic matter mineralization is suppressed because of poor microbial activity. Total bacteria and actinomycete populations steadily decline at pH below 5.5. Fungi, effected oppositely at pH below 5.5, dominate while bacteria and actinomycete populations decline (see #8: fungal-bacteria competition) Carrow, R. N. et al. 2001 (6).  Studies have shown that when highly acidic soils are limed, the pH increase is accompanied by a release of labile organic matter, correlating closely with C and N mineralization Curtin, D et al. 1998 (7). Very acid or alkaline pH levels prevent organic matter from breaking down, suppressing ammonification and nitrification, thereby immobilizing nutrients, particularly nitrogen Bickelhaupt, D. 2016 (1).

     5.   Activity of soil microorganisms that degrade toxic pesticide soil chemicals:

pH also affects the mobility and degradation of herbicides and insecticides, and the solubility of heavy metals that are pH dependent.  In an Australian research study, the soil degradation rate of the pesticide chlorpyrifos is strongly related to soil pH, concluding that the degradation was microbial, not due to abiotic hydrolysis, degrading the pesticide as a sole source of carbon and energy Singh, B.K et al. 2003 (8).

    6.   Root–microorganism competition:

Because root N uptake can alter rhizosphere pH, the ratio between NO3- and NH4+ uptake by roots could change the competition with microorganisms.  A deviation of pH from the optimal (6–7) has impacts on both roots and microorganisms. Acidification suppresses bacterial activities more strongly than those of roots: decreased soil pH therefore increases N uptake by plants. By contrast, alkalinization strongly limits N uptake by roots vs microorganisms, and the latter utilize more N than at neutral soil pH Raven et al. 1976 (9); Allen, 1988 (10) PDF

     7.  Root growth and elongation:

Rhizosphere pH changes may be responsible for the differential patterns of root growth observed under NH4+ and NO3- nutrition Bloom, 1997a (11); Bloom et al., 2002 (12).

When roots take up charged molecules, such as ammonium or nitrate, they typically release an oppositely charged molecule to maintain a balanced pH inside the plant cells. Rhizosphere pH changes as roots absorb and assimilate inorganic nitrogen; the assimilation of NH4+ strongly acidifies, whereas absorption of NO3- slightly alkalizes the media near the root apex Mattson, N et al. 2009 PDF (4)

Figure 2: Conversions between nitrogen form and effect of nitrogen uptake on root-zone pH:

       8.  Fungal–bacterial competition:

One of the most influential factors affecting the microbial community in soil is pH. The highest fungal - bacterial growth ratio was at about pH 4.5, resulting in a 30-fold increase fungal growth. Bååth and Arnebrant (13) investigated the influence of pH on bacterial growth in forest soils treated with lime and ash, showing a fivefold increase in bacterial growth in pH changes from about pH 4 to 7 Bååth, E. et al. 1995 (13). A pH of between 7.0 and 8.0 has been suggested as optimum for denitrification (Knowles, 1982). 

One potential explanation could be the pH physiological limitations of both decomposer groups (fungi and bacteria); i.e., low hydrogen ion concentrations limit fungal growth, and high hydrogen ion concentrations limit bacterial growth, with no direct causal connection between the groups of organisms.

      9.   Increased microbial activity and soil CO2 respiration:

 A soil CO2 respiration study (a measurement of the total activity of the soil microbial community) including 19 different soils from areas with various land uses, spanning a pH range from 4 to 8, showed that there was an increase in bacterial growth at higher pHs. Bacterial growth increased fourfold between pH 4 and pH 8 (Figure 3) Bååth, E. 1998 (14).

Figure 3: Respiration, a measurement of microbial growth rates, was  strongly affected by pHs be-tween 4.5 and 8.3:

 Bååth, E. 1998. Growth rates of bacterial communities in soils at varying pH

Bååth, E. 1998. Growth rates of bacterial communities in soils at varying pH

Respiration, a measurement of microbial growth rates, was  strongly affected by pHs between 4.5 and 8.3.


References:

  1. Bickelhaupt, D.  2016. Soil pH: What it Means. Environmental Learning Resources.  State University of New York College of Environmental Science and Forestry, SUNY-ESF
  2.  Jensen, T. L.  2010. "Soil pH and the Availability of Plant Nutrients," IPNI Plant Nutrition TODAY
  3. Hahn, K. 2012 blog: Soil test results are back - now what? Food Plotting. Heartland Outdoors
  4.  Mattson, N., Leatherwood, R., Peters, C. 2009. Nitrogen: All Forms Are Not Equal, Reprinted from the June, 2009. Edition of GMPro Magazine. Cornell University Cooperative Extension
  5. Knowles R. 1982. Denitrification. Microbiol. Mol. Biol. Rev. 46 43–70 
  6. Carrow, R. N., Waddington, D. V., Rieke. P. E.  2001. Turfgrass Soil Fertility & Chemical Problems: Assessment and Management. John Wiley & Sons
  7. Curtin, D., Campbell, C.A., Jalil, A. 1998. Effects of acidity on mineralization: pH-dependence of organic matter mineralization in weakly acidic soils. Soil Biology and Biochemistry. Volume 30, Issue 1, Pages 57–64
  8. Singh, B.K., Walker, A., Morgan, A.W. and Wright, D.J. 2003. Effects of Soil pH on the Biodegradation of Chlorpyrifos and Isolation of a Chlorpyrifos-Degrading Bacterium. Appl Environ Microbiol. 2003 Sep; 69(9): 5198–5206. doi:  10.1128/AEM.69.9.5198-5206.2003
  9. Raven JA, Smith FA. 1976. Nitrogen assimilation and transport in vascular land plants in relation to intercellular pH regulation. New Phytologist 76: 415–431
  10. Allen S. 1988. Intracellular pH regulation in plants. ISI Atlas of Science: Animal and Plant Sciences 1: 283–288
  11. Bloom A.J. 1997a. Interactions between inorganic nitrogen nutrition and root development. Zeitschrift für Pflanzennährung und Bodenkunde 160: 253–259
  12. Bloom A.J., Meyerhoff P.A., Taylor A.R., Rost T.L. 2002. Root development and absorption of ammonium and nitrate from the rhizosphere. Journal of Plant Growth Regulation 21: 416–431
  13. Bååth, E., and K. Arnebrant. 1995. Growth rate and response of bacterial communities to pH in limed and ash-treated forest soils. Soil Biol. Biochem. 26:995–1001
  14. Bååth, E. 1998. Growth rates of bacterial communities in soils at varying pH: a comparison of the thymidine and leucine incorporation techniques. Microb. Ecol. 36:316–327.