Chapters 4.1.2
4.1.2 - Litmus paper and liming for adjusting soil pH
Dylan P. Harding, University of Guelph, Canada
https://sciencestockphotos.com/free/chemistry/ph_paper.jpg
Suggested citation for this chapter.
Harding,DP (2022) Litmus paper and liming for adjusting soil pH, The Encyclopedia for Small Scale Farmers. Editor, M.N. Raizada, University of Guelph, Canada. http://www.farmpedia.org
Introduction
Acidic soil is generally undesirable for the growth of crops for multiple reasons. Acidity can interfere with the availability of some nutrients, can cause aluminum (which is found naturally in some soils) to change to a form that is toxic to plants, and can destroy or shift soil microbial populations.
What is acidity? When a new material is dissolved into water, the mixture is considered a “solution”. Often, water (H2O ) molecules will change and become positively or negatively charged when a solution is made. If a water-based (or “aqueous”) solution develops an internal electrical charge then it will be considered either acidic (positively charged) or basic/ alkaline (negatively charged). As a solution becomes acidified, H2O molecules lose one hydrogen atom and one oxygen atom and become H+ ions, which are positively charged. As the concentration of H+ ions increases in a solution, it becomes more acidic. Conversely, as a solution becomes more basic, H2O molecules lose one hydrogen atom and become OH- ions, which are negatively charged. As the concentration of OH- ions increases in a solution, it becomes more basic (also referred to as alkaline).
Some soils are acidic, some are basic and some are neutral.It is important to note that there can be a lot of variation in soil pH within a small area, and hence testing soil pH is important (see below). In general, soils near the equator are often acidic (van Straaten, 2002).
In situations where soil acidification has occurred, lime (calcium carbonate) can be applied to the soil in order to neutralize acidity. This is a well-established practice that has been exercised globally for hundreds of years.
Understanding the pH Scale
Acidity and alkalinity are indicated using the pH scale. This scale uses values ranging from 0 to 14, with a value of 7 representing a “neutral” environment, or one in which there are neither H+ nor OH- ions, but simply pure H2O. Values less than 7 are considered “acidic” and values above 7 are considered “basic” or “alkaline”. pH is measured according to a logarithmic scale, which means that each unit represents a tenfold increase over the previous unit. For example, if the pH in a given environment changes from 6 to 5, this indicates that there is ten times the hydrogen ion concentration when pH is 5 than when the pH was 6. If the pH drops to 4, then the hydrogen ion concentration is 10 times greater than it was at 5, and 100 times greater than what it was at 6. Another way to consider pH is that an increase of 0.1 units represents a halving in the hydrogen ion concentration, and a drop of 0.1 units represents a doubling in hydrogen ion concentration.
⇦ Increasing H+ ion concentration ⇦ Pure Water ⇨ Increasing OH- ion concentration ⇨
⇦ 0 to 6.9 ⇨⇦ 7 ⇨⇦ 7.1 to 14 ⇨
pH Effect on Crops
Every crop has a range of pH that will optimize its growth. Most crops prefer mildly acidic pHs of between 6 and 7 however this will vary between crop varieties.
Cation Exchange Capacity and Buffer Capacity
Soil generally has an overall negative charge, which causes it to attract and hold positively charged particles or “cations” (van Straaten, 2002). The quantity of cations that the soil can hold at a given time is referred to as the “cation exchange capacity” or CEC. As most plant nutrients are cations (nitrate and sulphur are notable exceptions), the CEC can be considered a measure of the quantity of soil nutrients a soil can hold at a given time. In general, clay soils will have high CECs and sandy soils will have lower CECs.
H+ is a cation and thus will be held on the CEC of the soil. In the soil environment, a balance will naturally form between H+ ions on the CEC and H+ ions in the soil water. As high CEC soils can hold more H+ ions, they will take longer to become acidified, however they will also require more lime to ameliorate acidity when it becomes necessary. Conversely, low CEC hold fewer H+ ions and thus will become acidified more quickly, but will require less lime to raise the soil pH when it becomes necessary to do so. The property of having a high CEC and thus resisting rapid pH changes is referred to as “buffer capacity”.
A map of soil characteristics including CEC (use the “Soil Nutrient Retention Capability” overlay) is available through the Centre for International Earth Science of Columbia University and can be accessed at http://www.ciesin.columbia.edu/afsis/mapclient/ . Unfortunately there is currently no data on soil pH available through this service. High soil nutrient retention capability would indicate a high buffer capacity and thus a higher requirement for lime when its application becomes necessary.
Liming
Calcium carbonate, commonly known as lime, can be applied to a soil in order to counteract acidity. When introduced to the soil environment, lime will react with H+ ions in such a way that they are converted to H2O, thus decreasing acidity.
Step 1) CaCO3(lime) + H2O (soil water) ⇨ Ca2+ + 2 OH- + CO2
Step 2) 2 OH- (from lime breakdown) + 2 H+ (cause of soil acidity) ⇨ 2 H2O (acidity neutralized as water is formed)
The nutrient holding capacity of the soil must be considered when determining the appropriate amount of lime to add. Ideally the nutrient holding capacity of a soil will be measured before lime application so that a specific lime rate can be formulated. In general, soils that are predominantly composed of clay will have a high CEC and soils and that predominantly sandy will have a low CEC. If the equipment to determine buffer capacity is not available, then the CEC can be estimated based on soil texture.
Relieving soil acidity through the application of lime frequently has a strong effect on improving crop growth (Bailey & Laidlaw, 1999). Because of their reliance on soil bacteria that are commonly sensitive to acidic conditions, legumes crops often demonstrate the most benefit following the application of lime (Quaggio et al., 2004). The availability of the micronutrient Molybdenum, which is especially important to legumes is also very sensitive to changes in pH and will become available at pH less than 5.2 (Franco & Munns, 1981). See Chapters 5 and 6 for further detail on the interaction of legumes with acidic soil conditions.
It is important to avoid over-applying lime, as this may cause some micronutrients to become unavailable for crop uptake (Bambara & Ndakidemi, 2010)
Lime Production
Lime is produced from calcium or magnesium based bedrock materials. Lime is a “blanket term” that can refer to several different chemical compounds, however it is most commonly used to refer to CaCO3, or calcium carbonate. Lime is also used to refer to Calcium Oxide (aka Quick Lime, Burnt Lime) and Calcium Hydroxide (aka hydrated lime). These materials are used to make concrete and are not appropriate for agricultural use because their highly reactive nature presents danger to those handling the materials. Calcium Carbonate will effectively raise soil pH and does not require specific handling precautions, thus it is the choice material for improving acidic soils. Production of agricultural lime is a fairly simple process. Essentially, agricultural lime is produced by extracting limestone material from the bedrock and grinding it into a fine powder.
Lime should be graded according to purity and fineness. Fineness refers to the size of the individual particles in the powder. Commercially produced lime should have a specific fineness rating based on what percentage of the liming powder will pass through a series of progressively finer sieves. Fineness rating are used to adjust liming rates to increase the accuracy of its effect on pH. More finely ground material will be more effective. If fineness ratings are not available then lime should be ground as finely as possible before application. The purity of lime should also be considered. A “pure” agricultural lime would be 100% calcium carbonate, and would be given a neutralizing value of 100. Magnesium based carbonates will have a similar neutralizing value. The presence of other materials (non-lime mineral compounds, etc.) will decrease this purity.
Assessing pH
Soil pH can be most accurately tested using a simple pH meter. Basic pH meters usually cost from $15 to $30 although this price may increase with shipping charges. A pH meter is a good investment that can be used for many seasons if properly cared for. These tools are available from a number of suppliers, some of which are listed at the end of this chapter.
Litmus paper can also be used as an approximate indicator of acidity. Litmus papers are small strips of paper than contain a dye that will change colour when exposed to change in pH. Although litmus paper will indicate acidic conditions it will not indicate a specific pH value. Blue litmus paper is most appropriate for soil testing as it will turn red at pH below 4.5, indicating extremely acidic conditions. It should be noted however that as even pH values around 5 can be problematic for plants, acidic conditions may be limiting to crop growth but undetectable with litmus paper.
More advanced pH indicator strips are also available that will gradually change colour in reaction to pH. The final colour of the strip can be compared to a chart (provided with the indicator strips) which will show the approximate pH of the solution the strip was exposed to. These strips are more appropriate for soil testing as they can provide more accurate pH readings.
To test soil pH with either litmus paper or pH indicator strips, enough rainwater should be added to the sample area to make it very muddy, just short of causing water to pool on the surface. The indicator strip should then be placed in the muddy soil for the period of time specified for the variety of indicators being used (usually a few minutes). The soil can then be gently rinsed or wiped from the indicator and its colour can be observed. Rinsing the indicator strip will not influence the reading as the colour changing reaction should be complete by the time the indicator is removed from the soil.
When testing pH (by any method) at least three areas of the field should be tested to decrease the likelihood of an isolated area with a significantly different pH value causing an inaccurate conclusion regarding soil acidity. As well, both surface soil and soil approximately 1 foot beneath the surface should be tested in order to recognize potential pH variation between soil strata.
Picture Based Lesson to Train Farmers
For the South Asian version (pictures only, text for you to insert), click this link for lesson 5.1:http://www.sakbooks.com/uploads/8/1/5/7/81574912/5.1_south_asian.pdf
For the East/South Asian version (pictures only, text for you to insert), click this link for lesson 5.1:http://www.sakbooks.com/uploads/8/1/5/7/81574912/5.1e.s.a.pdf
For the Sub-Saharan Africa/Caribbean version (pictures only, text for you to insert), click this link for lesson 5.1:http://www.sakbooks.com/uploads/8/1/5/7/81574912/5.1subsaharan_africa_carribean.pdf
For the Latin-America version (pictures only, text for you to insert), click this link for lesson 5.1:http://www.sakbooks.com/uploads/8/1/5/7/81574912/5.1latin_america.pdf
For North Africa And Middle East version (pictures only, text for you to insert), click this link for lesson 5.1:http://www.sakbooks.com/uploads/8/1/5/7/81574912/4.1n._africa_middleeast.pdf
Source: MN Raizada and LJ Smith (2016) A Picture Book of Best Practices for Subsistence Farmers. eBook, University of Guelph Sustainable Agriculture Kit (SAK) Project, June 2016, Guelph, Canada.
Useful Links
pH meter suppliers:
[Hach International (based in India) also ships to many African and Latin American countries: http://www.hach.com/AfricaDistributors
[pH indicator strip suppliers Sigma Aldridge (will ship internationally) http://www.sigmaaldrich.com/catalog/product/fluka/37109?lang=en®ion=D
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