Discussion Examples for Soil pH Measurement | Acidic Soils, Buffering Capacity, and Measurement Errors

What Is Soil pH Measurement?

Soil pH measurement is an analysis in which soil is mixed with water, potassium chloride solution, or another extraction solution and the pH of the resulting suspension or supernatant is measured.
pH is a value related to hydrogen-ion concentration and indicates whether the soil is acidic, neutral, or alkaline.
Soil pH is important when judging whether plants can readily absorb nutrients.

Soil is not simply a collection of sand and clay but is a complex system containing organic matter, clay minerals, humus, microorganisms, metal ions, carbonates, exchangeable cations, and other components.
Therefore, soil pH is not as simple as the pH of water and is affected by soil components, sampling location, fertilizers, rainfall, plant roots, microbial activity, and other factors.

Main Items to Include in the Results

In the results of soil pH measurement, organize the sampling location, sampling depth, condition of the soil, whether it was dried, soil-to-water ratio, type of extraction solution, measurement method, pH value, and other information.
Because soil pH readily changes depending on measurement conditions, it is important to clearly state the conditions under which the measurement was performed.

Main Items to Include in the Results

• Soil sampling location
• Sampling date and time
• Sampling depth
• Color and odor of the soil
• Moisture condition of the soil
• Whether drying or sieving was performed
• Soil-to-water mixing ratio
• Type of extraction solution
• Stirring time
• Standing time
• Measurement method
• pH-meter calibration conditions
• Measurement temperature
• Measured pH value
• Average value from multiple measurements
• Comparison among samples
• Sources of error and points for improvement

Reference Experimental Values and Calculation Examples for Soil pH Measurement

Here, reference experimental values are organized for measuring soil pH and discussing acidic soil, alkaline soil, buffering capacity, and errors caused by measurement conditions.

Soil pH is an indicator affected by hydrogen-ion concentration and exchangeable acidic components in the soil.
In soils with low pH, acidity is strong and may affect nutrient uptake by plants and the leaching of metal ions.
In addition, the obtained pH value also changes depending on the extraction solution used and the ratio of soil to water.

Reference Experimental Conditions

pH Measurement Results by Soil Type

A reference example is shown in which 25.0 mL of distilled water was added to 10.0 g of soil and the pH was measured after stirring and standing.

In this reference example, forest soil showed the lowest pH and lime-treated soil showed the highest pH.
In forest soil, acidic components produced by decomposition of organic matter and the leaching of basic components by rainfall may have contributed.

Comparison Between Water-Extraction pH and KCl-Extraction pH

Soil pH may differ when measured with distilled water and when measured with KCl aqueous solution.
In KCl extraction, exchangeable H+ and Al3+ retained on soil particles are more readily released into the solution, so the pH may be lower than with water extraction.

When KCl-extraction pH is lower than water-extraction pH, acidic components retained on soil particles are considered to have been released into the solution.
Therefore, water-extraction pH can be treated as an approximation of the soil-solution condition, while KCl-extraction pH can be treated as a guide that includes exchangeable acidity.

Relationship Between pH and Hydrogen-Ion Concentration

pH is expressed as the logarithm of the hydrogen-ion concentration.
When pH decreases by 1, the hydrogen-ion concentration becomes approximately 10 times higher.

Differences in pH may appear small numerically, but they correspond to large differences in actual hydrogen-ion concentration.
Therefore, forest soil at pH 4.8 is considered much more acidic than sandy soil at pH 6.8.

Changes in pH Due to Lime Addition

In acidic soil, pH increases when calcium carbonate, dolomitic lime, or similar materials are added.
Here, a reference example is shown for adding calcium carbonate to acidic forest soil.

The soil pH increased as the amount of calcium carbonate added increased.
This is considered to be because calcium carbonate neutralized acidic components and reduced the acidity of the soil.

Example Calculation of the Increase in pH

The change in pH is determined by subtracting the pH before treatment from the pH after treatment.

Increase in pH = pH after treatment − pH before treatment

When 100 mg of calcium carbonate was added, the pH before treatment was 4.8 and the pH after treatment was 6.2.

Increase in pH = 6.2 − 4.8 = 1.4

From this result, it can be organized that the addition of 100 mg of calcium carbonate increased the pH of the acidic soil by 1.4.

Comparison of Soil Buffering Capacity

Soil has a buffering capacity that suppresses changes in pH to some extent even when acids or alkalis are added.
Soils containing large amounts of organic matter or clay minerals tend not to undergo rapid pH changes.

In sandy soil, the pH change was large when acid or alkali was added.
In contrast, soil containing large amounts of humus showed a small pH change and was considered to have strong buffering capacity.

Differences in Measured Values Depending on the Soil-to-Water Ratio

Soil pH also changes depending on the mixing ratio of soil to water.
When a larger amount of water is used, ion concentrations in the solution become diluted and the measured pH may become slightly higher.

When measurement conditions differ, it becomes difficult to directly compare pH values.
Therefore, when comparing soil pH, it is important to standardize conditions such as soil amount, water amount, and standing time.

Changes in pH Depending on Standing Time

Immediately after soil and water are mixed, soil components may not yet have sufficiently dissolved into the water.
A reference example of pH changes with different standing times is shown below.

When the standing time is short, the pH may not be stable.
Allowing a fixed standing time and measuring under the same conditions makes the results easier to compare.

Comparison Between a pH Meter and pH Test Paper

Soil pH can be read in finer detail using a pH meter, but pH test paper can also be used to roughly determine whether the soil is acidic, neutral, or alkaline.

pH test paper is affected by individual differences in color reading and by the color and turbidity of the soil extract.
When making detailed comparisons, using a pH meter is more appropriate.

Example of How to Write the Results

A 25.0 mL portion of distilled water was added to 10.0 g of soil, and the pH was measured after stirring and standing for 30 minutes.
The pH was 4.8 for forest soil, 5.9 for field soil, 6.4 for potting soil, 6.8 for sandy soil, and 7.5 for lime-treated soil.
From these results, the forest soil was strongly acidic, while the lime-treated soil was weakly alkaline.

When water-extraction pH and KCl-extraction pH were compared, the KCl-extraction pH was lower for all samples.
In particular, forest soil had a water-extraction pH of 4.8 and a KCl-extraction pH of 4.2, a difference of 0.6.
This was considered to be because exchangeable H+ and Al3+ retained on soil particles were released into the solution by KCl.

When calcium carbonate was added to acidic soil, the pH increased as the amount added increased.
In forest soil, the addition of 100 mg of calcium carbonate increased the pH from 4.8 to 6.2.
This was considered to be because calcium carbonate neutralized acidic components in the soil and reduced the acidity.

Points for Connecting the Results to the Discussion

In a discussion of soil pH measurement, it is important to explain not only the magnitude of the pH value but also differences caused by soil components, extraction solutions, buffering capacity, and measurement conditions in relation to one another.

• Can the reason pH differs depending on soil type be explained in relation to organic matter, basic components, and acidic components?
• Can the difference between water-extraction pH and KCl-extraction pH be explained in relation to exchangeable acidic components?
• Is it understood that a difference in pH corresponds to a large difference in hydrogen-ion concentration?
• Can the reason pH increased after lime addition be explained in relation to the neutralization reaction?
• Can it be explained that soil buffering capacity may suppress pH changes even when acid or alkali is added?
• Can the effects of the soil-to-water ratio, standing time, and stirring conditions on the measured value be discussed?
• Can insufficient calibration of the pH meter, contamination of the electrode, temperature differences, and turbidity of the extract be explained as sources of error?

Example Discussion

In this experiment, the pH of several soils was measured by water extraction and their acidity and alkalinity were compared.
The pH of the forest soil was 4.8, the lowest among the samples measured.
In contrast, the pH of the lime-treated soil was 7.5 and showed weak alkalinity.
In forest soil, organic acids produced by decomposition of organic matter and the leaching of basic components by rainfall were considered possible causes of the lower pH.

When the water-extraction pH and KCl-extraction pH were compared, the KCl-extraction pH was lower.
This was considered to be because K+ replaced exchangeable H+ and Al3+ on the surfaces of soil particles, causing acidic components to be released into the solution.
In particular, the difference between water-extraction pH and KCl-extraction pH was large in forest soil, suggesting that the soil may have relatively large exchangeable acidity.

When calcium carbonate was added to acidic soil, the pH increased as the amount added increased.
Calcium carbonate was considered to have raised the pH by neutralizing acidic components in the soil and reducing exchangeable H+.
However, even when the same amount is added, the degree of pH increase differs depending on the soil.
This is because the strength of buffering caused by organic matter and clay minerals differs among soils.

In the comparison of buffering capacity, sandy soil showed a large pH change when acid or alkali was added.
In contrast, soils containing large amounts of humus showed smaller pH changes and were considered to have stronger buffering capacity.
Because soil buffering suppresses rapid changes in pH, when improving soil it is important not to determine the amount of lime to add all at once but to adjust it while checking the measurement results.

Possible sources of measurement error include the soil-to-water ratio, stirring time, standing time, calibration of the pH meter, and contamination of the electrode.
Because the measured pH changed when the soil-to-water ratio was changed, soils must be measured under the same conditions when comparing soil pH.
In addition, because pH may not be stable when the standing time is short, it is desirable to allow the sample to stand for a fixed period before measurement.

Summary

In soil pH measurement, soil is mixed with water or KCl aqueous solution at a fixed ratio and the pH of the supernatant or suspension is measured.
Water-extraction pH reflects a condition close to the soil solution, while KCl-extraction pH can be treated as a value more strongly affected by exchangeable acidic components.

In this reference example, forest soil was strongly acidic, while lime-treated soil was weakly alkaline.
In addition, the pH increased with calcium carbonate addition, and soils containing large amounts of humus showed smaller pH changes because of buffering capacity.
In a report, it is useful to discuss soil type, differences in extraction solution, lime addition, buffering capacity, and errors caused by measurement conditions in relation to one another.

What Is Acidic Soil?

Acidic soil is soil whose pH is lower than neutral.
In regions with high rainfall, basic components such as calcium and magnesium in the soil are readily leached away, which may cause the soil to become acidic.
Plant roots, microbial activity, types of fertilizers, and decomposition of organic matter are also related to soil acidification.

When soil becomes acidic, aluminum ions and manganese ions may dissolve more readily and may damage plants.
Phosphate may also combine with iron or aluminum and become less available.
Therefore, in acidic soils, the availability of nutrients and effects on plant growth must be considered.

What Is Alkaline Soil?

Alkaline soil is soil whose pH is higher than neutral.
Soils may have high pH in areas with calcareous parent materials, arid regions, soils containing large amounts of calcium carbonate, and soils that have received excessive lime application.
Alkaline soils may develop nutrient deficiencies different from those in acidic soils.

In high-pH soils, trace elements such as iron, manganese, zinc, and copper become less soluble and may become difficult for plants to absorb.
Therefore, alkaline soils may also cause problems for plant growth.
When the pH is high, the effects of carbonates and lime components should be considered.

What Is Soil Buffering Capacity?

Soil buffering capacity is the property by which soil resists rapid changes in pH even when acid or alkali is added.
Clay minerals, humus, carbonates, exchangeable cations, and other components in the soil suppress pH changes by adsorbing or neutralizing acids and bases.
For this reason, soil pH may change less readily than the pH of an aqueous solution.

In soils with strong buffering capacity, pH does not immediately change greatly even when lime or fertilizer is added.
In contrast, soils with weak buffering capacity undergo pH changes more readily when small amounts of acid or alkali are added.
In a discussion of soil pH, not only the measured value but also how strongly the soil resists pH change is important.

Relationship Between pH and Plant Growth

Soil pH affects root elongation, nutrient uptake, and microbial activity.
Many plants are considered to grow readily in weakly acidic to near-neutral soil, but the suitable pH range differs depending on the plant species.
Some plants prefer acidic soil, while others prefer neutral to weakly alkaline soil.

If pH is too low, excessive dissolution of aluminum and manganese, phosphate fixation, calcium deficiency, and other problems may occur.
If pH is too high, deficiencies of trace elements such as iron and zinc are more likely to occur.
Therefore, soil pH is not simply an indicator of acidity or alkalinity but is closely related to plant nutrition.

pH and Nutrient Availability

The solubility and availability to plants of nutrients in soil change depending on pH.
Nutrients such as nitrogen, phosphorus, potassium, calcium, magnesium, iron, manganese, and zinc are each affected by pH.
In particular, phosphate readily combines with iron and aluminum under acidic conditions and with calcium under alkaline conditions, which may make it difficult for plants to use.

In other words, even if nutrients are added as fertilizer, plants may not be able to absorb them sufficiently when the soil pH is inappropriate.
Soil pH measurement provides a clue for judging the effectiveness of fertilizers and the need for soil improvement.

Relationship Between pH and Microbial Activity

Many microorganisms are present in soil and are involved in organic matter decomposition, nitrogen cycling, mineralization of nutrients, and other processes.
Microbial activity is also affected by soil pH.
Under extremely acidic or alkaline conditions, the activity of some microorganisms may decrease, and organic matter decomposition and nitrogen conversion may proceed less readily.

For example, nitrifying bacteria may become less active under strongly acidic conditions.
Therefore, in acidic soil, conversion from ammoniacal nitrogen to nitrate nitrogen may be suppressed.
Soil pH is related not only to chemical properties but also to biological properties.

Difference Between Water-Extraction pH and Salt-Extraction pH

Soil pH can be measured as water-extraction pH by adding water or as salt-extraction pH by adding a solution such as potassium chloride.
Water-extraction pH reflects a condition close to the soil solution.
On the other hand, in salt extraction, hydrogen ions and aluminum ions adsorbed on soil particles are more readily released into the solution, so the pH may be lower than the water-extraction pH.

The meaning of the pH value changes depending on the measurement method.
Therefore, a report must clearly state whether the value is a pH measured with water or a pH measured with a salt solution such as KCl.
It is important not to directly compare values obtained by different measurement methods.

Effect of the Soil-to-Water Mixing Ratio

In soil pH measurement, the mixing ratio of soil to water affects the measured value.
When the amount of water is large, soluble components in the soil are diluted and the pH may change.
On the other hand, when the amount of water is small, the suspension becomes concentrated and it may become difficult to stabilize the electrode and obtain a uniform measurement.

Therefore, in soil pH measurement, it is important to follow the soil-to-water ratio specified in the laboratory manual.
For example, a soil-to-water ratio of 1:2.5 or 1:5 may be specified.
Directly comparing measured values obtained at different mixing ratios may lead to an incorrect discussion.

Effects of Stirring Time and Standing Time

If soil and water are not stirred sufficiently after mixing, soil components may not transfer uniformly into the water and the measured value may not stabilize.
In addition, immediately after mixing, soil particles are suspended and the pH-electrode value may be difficult to stabilize.
Allowing the mixture to stand for a fixed period allows the suspension to settle.

If stirring time or standing time differs among samples, comparison of pH values becomes inaccurate.
It is important to stir all samples for the same period and measure them after the same standing time.
Standardizing measurement conditions allows differences among samples to be compared correctly.

Effect of Soil Drying Conditions

Soil pH may change depending on the drying condition of the soil.
Freshly collected moist soil and dried soil may differ in microbial activity, carbon dioxide, organic acids, salt distribution, and other factors.
Drying may also oxidize some components or make them more readily redissolved.

Soil analysis may use either air-dried soil or fresh soil.
Because the meaning of the measured value changes depending on which is used, the pretreatment of the sample must be clearly stated in the report.
Samples being compared should be treated under the same pretreatment conditions.

Differences Due to Sampling Depth

Soil pH may change depending on the depth at which the sample is collected.
Surface soil is readily affected by fallen leaves, organic matter, fertilizers, rainfall, and plant roots, so its pH may fluctuate readily.
In deeper layers, parent material, groundwater, and mineral components may have a stronger effect.

Even at the same location, the pH may differ between the surface and deeper layers.
Therefore, the sampling depth must be recorded in the report, and samples at the same depth should be used when making comparisons.
If samples collected at different depths are compared, the depth difference should also be included in the discussion.

Discussion of Differences Among Sampling Locations

Soil pH varies greatly depending on the sampling location.
Fields, forests, parks, school grounds, flower beds, rice paddies, roadsides, and other locations differ in vegetation, organic matter, fertilizers, lime, drainage, compaction, and parent material.
Therefore, pH may differ among locations even within the same region.

Fields and flower beds are readily affected by fertilizers and liming materials, while forest soils may become acidic because of fallen leaves and organic acids.
Roadsides may be affected by dust, drainage, deicing salts, and components originating from concrete.
Relating the characteristics of the sampling location to the results leads to a good discussion.

Effects of Acid Rain and Rainfall

Rainfall affects soil pH.
As rainwater passes through the soil, basic components may be leached out and the soil may become acidic.
Acid rain may also contribute to soil acidification.
In regions with high rainfall, calcium and magnesium in the soil are more readily washed away.

However, because soil has buffering capacity, rainfall does not necessarily cause a rapid change in pH.
The ease of acidification differs depending on the type of soil, amount of clay and humus, and presence of carbonates.
Recording the weather before sampling can also be useful for discussion.

Effects of Fertilizers and Liming Materials

Fertilizers and soil-improvement materials greatly affect soil pH.
Some nitrogen fertilizers may cause acidification during nitrification in the soil.
On the other hand, liming materials are used to neutralize acidic soil and act to increase soil pH.

If the pH of field or flower-bed soil differs from that of surrounding soil, the effects of fertilizer or lime application can be considered.
However, pH does not necessarily change uniformly immediately after application, and differences may occur depending on soil buffering capacity and mixing conditions.

Discussion of Measurement Using a pH Meter

A pH meter is an instrument that measures pH using a glass electrode.
In soil pH measurement, the electrode is immersed in the soil suspension or supernatant.
A pH meter can measure accurately, but calibration, electrode cleaning, temperature correction, and confirmation that the value has stabilized are important.

Because soil suspensions contain fine particles, soil particles may adhere to the electrode and make the value difficult to stabilize.
The electrode is washed after each measurement and calibrated with standard solutions before use.
The value must be read only after it has stabilized.

Discussion of Measurement Using pH Test Paper

pH test paper is a simple method for reading pH from a color change.
Measurement can be performed without special equipment, but the accuracy is lower than that of a pH meter.
If the soil suspension is strongly colored or turbid, the color change of the test paper may be difficult to determine.

With pH test paper, the comparison with the color chart is performed visually, so the result is affected by individual differences and lighting conditions.
It is useful for roughly determining acidity, neutrality, or alkalinity, but a pH meter is more appropriate when comparing small differences in pH.

Sources of Error in Soil pH Measurement

Sources of error in soil pH measurement include nonuniformity of soil samples, differences in sampling depth, deviations in the soil-to-water mixing ratio, insufficient stirring, differences in standing time, insufficient pH-meter calibration, contamination of the electrode, temperature differences, differences in drying conditions, and changes during sample storage.
Because soil is a more nonuniform sample than an aqueous solution, representativeness is important.

In addition, if stones, roots, fallen leaves, fertilizer granules, lime particles, or similar materials are mixed into the soil, the measured value may become biased.
Collecting soil from multiple locations, mixing it thoroughly, and sieving when necessary makes it easier to obtain a representative sample.

Discussion When Measured Values Vary

If pH values vary even when the same soil is measured, possible causes include insufficient mixing of the soil, insufficient cleaning of the electrode, reading before the value stabilizes, sedimentation of soil particles, and differences in measurement position.
Because soil suspensions settle over time, differences in measurement timing may change the conditions around the electrode.

If multiple measurements give similar values, the reproducibility can be considered high.
If the variation is large, the measurement conditions should be reviewed and not only the average value but also the cause of the variation should be discussed.
Because soil samples are inherently nonuniform, it is necessary to devise a method for preparing a representative sample.

When the Results Can Be Considered Good

Soil pH measurements can be considered to have produced good results when the measurement conditions are clear, variation among multiple measurements is small, and the pH values do not contradict the characteristics of the sampling location.
For example, results showing slightly acidic forest soil and near-neutral or slightly alkaline soil in a limed field can be readily explained in relation to environmental factors.

In addition, if the pH meter has been calibrated with standard solutions and the soil-to-water ratio and standing time have been standardized, the reliability of the results increases.
In the discussion, not only the magnitude of the values but also the reproducibility of the measurement conditions should be checked.

Example Discussion When the Experiment Did Not Go Well

When soil pH measurement does not go well, possible causes are considered from results such as unstable measured values, variation in values for the same sample, unexpectedly extreme pH values, or differences among samples that are difficult to explain.
Organizing the causes according to sampling, pretreatment, mixing ratio, stirring and standing, measuring instruments, electrodes, and reading makes the discussion easier.

How to Write Points for Improvement

In a discussion of soil pH measurement, including not only sources of error but also points for improvement makes the report easier to organize.
Points for improvement can be divided into sample collection, pretreatment, preparation of the suspension, pH measurement, and analysis.

Improvements to Sample Collection and Pretreatment

• Collect soil from multiple locations
• Standardize the sampling depth
• Remove stones, roots, and fallen leaves
• Air-dry when necessary
• Perform sieving
• Mix the sample thoroughly to prepare a representative sample
• Record the sampling location, date and time, and weather

Improvements to Suspension Preparation

• Accurately weigh the amount of soil
• Accurately add the amount of water or extraction solution
• Standardize the soil-to-water mixing ratio
• Keep the stirring time constant
• Keep the standing time constant
• Standardize the conditions before measurement

Improvements to Measurement and Analysis

• Calibrate the pH meter with standard solutions
• Wash the electrode between samples
• Prevent soil particles from adhering to the electrode
• Read the measured value only after it stabilizes
• Perform multiple measurements and calculate the average value
• Clearly state whether the value is water-extraction pH or salt-extraction pH
• Discuss the results in relation to the sampling location and soil management

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of soil pH measurement, simply writing that “the soil was acidic” or “the pH was low” results in a superficial discussion.
A good discussion relates the causes of acidification, buffering capacity, nutrient availability, plant growth, measurement conditions, and sources of error.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of soil pH measurement.
Adjust the necessary parts according to your own experimental results.

• Soil pH is an indicator of whether soil is acidic, neutral, or alkaline.
• Soil pH affects plant nutrient uptake and microbial activity.
• In acidic soil, Al3+ and Mn2+ may become more readily dissolved.
• Under acidic conditions, phosphate may combine with iron and aluminum and become less available to plants.
• In alkaline soil, trace elements such as iron and zinc readily become insoluble.
• Soil has buffering capacity caused by clay minerals and humus.
• Soil pH is affected by sampling location, sampling depth, fertilizers, liming materials, and rainfall.
• If the soil-to-water mixing ratio differs, the measured pH may change.
• Insufficient calibration of the pH meter and contamination of the electrode are sources of measurement error.
• Because soil is a nonuniform sample, it must be collected from multiple locations and thoroughly mixed.

Points to Check When Discussing Soil pH Measurement

Checking the following points before writing the report makes the discussion easier to write.

• Is it explained what soil pH represents?
• Are the characteristics of acidic and alkaline soils described?
• Is soil buffering capacity explained?
• Is the relationship with plant growth and nutrient availability considered?
• Are effects on microbial activity considered?
• Are the sampling location and sampling depth recorded?
• Is the soil-to-water mixing ratio stated?
• Are the effects of stirring time and standing time considered?
• Is water-extraction pH distinguished from salt-extraction pH?
• Are pH-meter calibration and electrode cleaning considered?
• Is nonuniformity of the soil sample considered?
• Do the points for improvement correspond to the sources of error?

Summary

Soil pH measurement is an analysis used to evaluate whether soil is acidic, neutral, or alkaline.
Soil pH is closely related to plant nutrient uptake, microbial activity, leaching of metal ions, phosphate fixation, the effectiveness of fertilizers, and other factors.
Therefore, it is an important indicator for understanding soil properties.

In acidic soil, leaching of Al3+ and Mn2+, phosphate fixation, and deficiency of basic components may become problems.
In alkaline soil, trace elements such as iron and zinc may become insoluble and difficult for plants to use.
In addition, because soil has buffering capacity, it has the property of resisting rapid changes in pH even when acid or alkali is added.

In a report, rather than simply writing that “the pH was high or low,” organize and discuss the causes of acidification and alkalization, soil buffering capacity, effects on plant growth and nutrient availability, sampling location, measurement conditions, sources of error, and points for improvement.
Soil pH measurement may appear to be a simple experiment, but because soil is a nonuniform and complex sample, it is important to standardize the measurement conditions and evaluate the results carefully.