Soil pH is a key indicator because it correlates directly with nutrient availability/solubility and also affects microbial activity. Thus, assessment of pH allows to predict nutrient availability potential in a given production system.
— Sousa et al., 2007

 

Factors that influence soil pH:

  1. Parent material:

Rocks from which parent material originally formed vary from acidic to alkaline in reaction. Soils formed from sandstone or shale are more acidic than soils formed from limestone. Soil acidity or alkalinity (pH) is extremely important because it has an effect on the decomposition of mineral rock into essential elements that plants can use.

 The parent materials slowly release soil elements (macro and micronutrients) over time as part of the natural weathering process. These elements move back and forth (soluble and insoluble) between several chemical forms, depending on soil pH, texture, soil aeration and the presence of other ions. In their soluble form, they are dissolved in soil solution as ions (molecules with a positive or negative charge) Plant & Soil Sciences eLibrary 2016 (1)

   2. Precipitation:

As annual precipitation increases, leaching of Ca and Mg increases, allowing the pH to decrease. It is the interaction of H+ from acid precipitation with soil cation exchange capacity (C.E.C.) that determines the effect of the added acidity on soil properties.

While acid rain (from burning fossil fuels) is not a serious problem for most agricultural production areas, the acidifying effect of nitric and sulfuric acids can seriously influence the vegetation in industrial areas Plant & Soil Sciences eLibrary 2016 (1).

    3.  Flooding:

After a soil is flooded, regardless of its original pH before flooding, the pH will approach neutrality (pH 6.5 to 7.5). The pH of alkaline soils declines and the pH of acid soils increases. The change in pH upon flooding may take up to several weeks, depending on the soil type, organic matter levels, microbial population, temperature, and other soil chemical properties (Figure 1) Snyder, C.S et al. 2002 PDF (2).

Figure 1: Effect of flooding on soil pH. Adapted from The Chemistry of Submerged Soils Ponnamperuma, Advances in Agronomy, 1972.

  Snyder, C.S. and Slaton, N.  2002. Effects of Soil Flooding and Drying on Phosphorus Reactions

Snyder, C.S. and Slaton, N.  2002. Effects of Soil Flooding and Drying on Phosphorus Reactions

    4.  Irrigation water:

Irrigation or well water pumped from groundwater sources often has high concentrations of calcium Ca2+, magnesium (Mg2+) and bicarbonate (HCO3-) dissolved in it which makes it “hard water.”  Long-use has led to increased soil pH. This is a significant source of cations which can offset the acidifying effects of fertilizer or organic matter Vossen. P. 2002 PDF (3).

    5.  Decayed organic matter and silicate clays that control cation exchange capacity (C.E.C.):

The ability of organic matter and silicate clays to store cations and to resist rapid changes in pH is due to their negatively charged particles which behave like cation exchangers. The composition of the cations attracted to the humus and clay particles determines the soil pH and availability of many plant nutrients. A high C.E.C. results in a high nutrient storage capacity and a resistance to pH shifts. If most of the cations are basic in nature such as Ca++, Mg++, K+ and Na+, the soil has a high "percent base saturation". Therefore, the pH is controlled by the degree of base saturation on the exchange sites.

It is the interaction of H+ from acid precipitation with this cation exchange capacity (C.E.C.) that determines the effect of the added acidity on soil properties. Cation exchange capacity in soils is almost exclusively a property of decayed organic matter and silicate clays (Table 1) and is usually quantified in milliequivalents per 100 g of soil (meq/100 g) McFee, W.W et al. 2007 PDF (4).

Table 1. Cation Exchange Capacities of Soil Components C.E.C. --- meq/100 g.

  McFee, W.W., Kelly, J.M., Beck, R. H. 2007. Acid Precipitation Effects on Soil pH and Base Saturation of Exchange Sites.

McFee, W.W., Kelly, J.M., Beck, R. H. 2007. Acid Precipitation Effects on Soil pH and Base Saturation of Exchange Sites.

     6.  Soil Texture:

Soils with high clay and organic matter content are more able to resist a drop or rise in pH (have a greater buffering capacity) than sandy soils. Although clay content cannot be modified, organic matter content can be changed by management. Sandy soils commonly have low organic matter content, resulting in a low buffering capacity, high rates of water percolation and infiltration making them more vulnerable to acidification.

In order to select the correct application rate use a soil test to determine both the soil texture group and the current pH. As the percentage of clay in a soil increases, it requires proportionately more limestone to raise the pH (Table 2). This means it is much harder to change the pH of clay soil than sandy soil Vossen. P. 2002 PDF (3).

Table 2. Approximate Amount of Finely Ground Limestone Needed to Raise the pH of a 7-inch Layer of Soil: 

 Vossen. P. Changing pH in Soil. 2002. University of California Cooperative Extension

Vossen. P. Changing pH in Soil. 2002. University of California Cooperative Extension

Some soils are alkaline and have a pH above 6.5. Some fertilizers (ammonium sulfate, urea, and ammonium nitrate) create an acid reaction in the soil, so they aid in lowering or maintaining a specific pH Vossen. P. 2002 PDF (3).

Table 3. Tons of sulfur needed to lower pH to 6.5.

  Vossen. P. Changing pH in Soil. 2002. University of California Cooperative Extension

Vossen. P. Changing pH in Soil. 2002. University of California Cooperative Extension

    7.  Native vegetation:

Soils formed under prairie (grasses) tend to be less acid than soils formed under forests.  Residue from coniferous(evergreen) trees is more acidic than from deciduous (broadleaf) ones. Further, roots of growing plants produce CO2 and small amounts of organic acids which increase soils acidity.

   8.  Type of crops grown:

The highest soil cation exchange capacity (CEC) is found in legume-based rotation systems with the highest organic matter content and humus. Especially systems with pigeon peas and lablab resulted in a 70 percent increase of the CEC, compared to fallow/maize system. CEC is closely linked to the organic matter content of the soil and thus gradually increases with soil depth.

Apparently, when conservation agriculture is practised for a number of years, the organic matter content of the soil increases. The soil reaction is different and recovers its natural buffer capacity; the pH increases as is shown in Figure 2 Mielniczuk, J. PDF 1996 (5).

Figure 2. Effect of tillage regime and cover crops on the pH of the soil (Mielniczuk, 1996). Original pH level was 5.3.

Conservation of natural resources for sustainable Agriculture. What you should know about… Soil fertility.

After 20 years of conservation agriculture Crovetto (1997) reports an increase in CEC of 136 percent (from 11 to 26 meq 100g-1 soil) due to humus increase in the soil Crovetto, C. 1997 PDF (6).

     9.  Fertilizers and amendments (manure, crop residue and soil organic matter):

These are all potential sources of H+ which lowers the pH. During the nitrification process, two H+ cations are liberated which can accumulate and significantly reduce the pH of the soil. The application of any fertilizer which carries part of its N in the ammonium form will ultimately decrease the soil pH due to the nitrification process Hodges, S.C. 2000 PDF (7).

In alkaline soils, with a pH above 6.5, these fertilizers (ammonium sulfate, urea, and ammonium nitrate) can create an acid reaction in the soil that aids in lowering or maintaining a specific pH. In order to not induce problems with soil acidity through the use of ammonium fertilizers, it is recommended that a well-planned liming program be an integral part of the farming enterprise Hodges, S.C. 2000 PDF (7).

Amending the soil with organic matter, on average, will lower pH. Peat or sphagnum peat moss are highly acidic and will lower soil pH more than other organic amendments.


References:

  1. Plant & Soil Sciences eLibrary. Soils - Part 4: Soil pH. 2016. USDA National Institute of Food and Agriculture, University of California-Davis, National Science Foundation (NSF), University of Nebraska
  2. Snyder, C.S. and Slaton, N.  2002. Effects of Soil Flooding and Drying on Phosphorus Reactions. News & Views. Potash & Phosphate Institute (PPI). Crop, Soil and Environmental Sciences Department, University of Arkansas
  3. Vossen. P. Changing pH in Soil. 2002. University of California Cooperative Extension 2604 Ventura Ave. Santa Rosa, CA 95403
  4.  McFee, W.W., Kelly, J.M., Beck, R. H. 2007. Acid Precipitation Effects on Soil pH and Base Saturation of Exchange Sites. Departments of Agronomy, Forestry and Natural Resources, and Agronomy, respectively, Purdue University, West Lafayette, Indiana
  5. Mielniczuk, J. 1996. A sustentabilidade agrícola e o plantio direto. In: Plantio direto: ocaminho para uma agricultura sustentável. Palestras do I Congresso Brasieleiro de Plantio Direto para uma Agricultura Sustentável. Ponta Grossa 1996. Eds. R. Trippia dos Guimarães Peixoto, D.C. Ahrens e M.J. Samaha. 275 pp.
  6. Crovetto, C. 1997. La cero labranza y la nutrición del suelo. In: Agricultura Sustentable de Alta Producción, ya! 5o Congreso Nacional de AAPRESID, Mar del Plata, Argentina: p73-78.
  7. Hodges, S.C. 2000. Soil Fertility Basics. NC Certified Crop Advisor Training. Soil Science Extension. North Carolina State University