What is pH?
Soil pH is the acidity or alkalinity of soil and is an important indicator of soil health. The pH of soil impacts the availability of nutrient uptake and the activity of soil microorganisms.
The pH scale ranges from 0 to 14, 7 is neutral pH and values below 7 indicate the soil is acidic, while values above 7 indicate that the soil is alkaline. Low pH indicates the soil is very acidic, with a high hydrogen ion concentration and high pH values tell us the soil is alkaline with a low hydrogen ion concentration. The majority of plants prefer a neutral to slightly acidic pH range of 6.0 to 7.5. However, some plants have very specific preferences. Blueberries thrive in acidic soil of pH 4.5 to 5.5), while asparagus prefers alkaline soil of pH 7.0 to 8.0.
Various factors influence soil pH including rainfall as well as the types of minerals and organic matter present. Areas with high rainfall generally have more acidic pH as rainwater can leach away basic elements such as calcium and magnesium. Arid regions tend to be more alkaline.
How do hydrogen ions form in water?
Water molecules are made up of two hydrogen atoms and one oxygen atom (H2O) that are bonded together. When a water molecule (H2O) donates a proton (H+, which is essentially a hydrogen atom without its electron) to another water molecule, it becomes a hydroxide ion (OH-). This happens during the self-ionisation of water, where two water molecules react to form a hydronium ion (H3O+) and a hydroxide ion (OH-).
This occurs due to the constant collisions between molecules in a liquid or gaseous state. This type of interaction is common in many chemical reactions.
What does pH mean?
The term “pH” stands for “potential of hydrogen” or “power of hydrogen.”It measures the concentration of hydrogen ions (H⁺) in a solution. The pH scale is logarithmic, which means that each whole pH value below 7 is ten times more acidic than the next higher value. For example, a pH of 4 is ten times more acidic than a pH of 5 and 100 times more acidic than a pH of 6. Similarly, each whole pH value above 7 is ten times more alkaline than the next lower value.
- > 7.5—alkaline
- < 6.5—acidic (soil with pH less than 5.5 is strongly acidic)
Factors that determine soil pH
- Parent material: The original rock material that formed the soil, called parent material, influences the initial pH of soil. Rocks such as limestone create more alkaline soil, while granite can lead to more acidic soil.
- Rainfall: Areas with high rainfall generally have more acidic soils. This occurs because water can leach basic ions, such as calcium and magnesium, out of the soil, leaving behind more acidic ions like aluminium and iron.
- Plant and microbe activity: Certain plants and soil microbes can alter the soil pH. For example, legumes work with bacteria to fix nitrogen, which, in turn, makes the soil more alkaline. When plant material decomposes, organic acids can be produced, making the soil more acidic.
- Human activity: Fertilisers, lime, or sulfur, can significantly change soil pH. For example, applying lime (calcium carbonate) raises soil pH, making it more alkaline, while adding sulfur can lower the pH, making the soil more acidic.
How does soil pH impact plants?
Soil pH levels play a significant role in a plant’s health and growth. When soil pH is too high or low, it can affect the availability of nutrients essential for plant growth.
Low (acidic) pH soil (high hydrogen ions) impacts the availability of phosphorous, calcium and magnesium.
- Phosphorous can react with aluminium and iron in highly acidic soil to form iron phosphate and aluminium phosphate, both of which are insoluble, and therefore unavailable to plants.
- Calcium and magnesium carry a positive charge (cations). In highly acidic soil, high levels of hydrogen ions that carry a positive charge are present. Hydrogen ions can outcompete calcium and magnesium for binding sites on negatively charged soil particles. This results in less calcium and magnesium available for plant uptake. Additionally, high acidity can increase the leaching of calcium and magnesium, which further reduces their availability in the soil. Once displaced from the soil particles, the calcium and magnesium ions become part of the soil solution. When rainfall or irrigation exceeds the soil’s ability to hold water, this excess water drains through the soil, carrying with it soluble nutrients, including the displaced calcium and magnesium.
- Aluminium is abundant in the earth’s crust and is normally locked up in minerals such as feldspars and clays. When soil pH drops below 5, these aluminium-containing minerals begin to dissolve, releasing aluminium ions (Al3+) into the soil solution which can be taken up by plant roots. High concentrations of Al3+ can damage root tips and limit the ability of the roots to take up water and nutrients.
- Manganese is an essential micronutrient for plants but can be toxic in excess when present in excess. Under acidic conditions, the Mn2+ ion becomes highly available and can be taken up in large amounts by plant roots. This can lead to an accumulation of manganese in the plant tissues, interfering with the plant’s metabolic processes, particularly photosynthesis.
In alkaline (low hydrogen ions) soil iron, manganese, boron, copper, and zinc become less soluble. On the other hand, nutrients like calcium and magnesium can become more available.
- Iron (Fe) and manganese (Mn) can oxidise (combine with oxygen), to form iron (III) oxide and manganese (IV) oxide. These compounds are less soluble and hence less available to plants as they form a type of rust in the soil, which the roots are unable to absorb.
- Phosphorus (P) can combine with calcium (Ca) to form calcium phosphate, a compound that is not easily dissolved in water and thus not readily available for plants to absorb.
- Iron, copper, and zinc can form insoluble chelates (a compound that contains a ligand bonded to a central metal atom), that are not available to most plants.
- When the soil pH is too high (alkaline), these enzymes produced by beneficial bacteria, fungi and other microbes may not function effectively, reducing the microbes’ ability to metabolise nutrients and reproduce.
- Sodium, lead, copper, cadmium, nickel and zinc are more soluble and thus more bioavailable at high pH levels, which can be harmful to some microbes.
How to test for pH
Testing soil pH is important to ensure plants are grown under optimal conditions to ensure they thrive. The good news is that pH testing is easy and cheap, starting from $10.00 for a pH meter.
pH meters are readily available at garden centres for a few dollars. They consist of two metal prongs that are inserted into the soil, an internal electric reference electrode and circuitry, battery compartment and a display screen.
pH meters with metal prongs measure the voltage potential between the prongs when they are inserted into the soil. This is created due to the difference in hydrogen ion concentrations between the soil and a reference electrode inside the meter. The meter converts this voltage difference into a pH reading based on the relationship between voltage and pH.
How to use:
pH meters are simple to use. Just insert the prongs into the soil and the needle will show you the pH of the soil. When you are finished, wipe any soil debris from the prongs and rinse in distilled water. Distilled water has a neutral pH of 7, which means it won’t interfere with the pH reading of the next sample.
Vinegar and baking soda test:
The vinegar and baking soda test is a rudimentary and qualitative method to get a rough idea of whether the soil is acidic, alkaline, or neutral. This test is not precise and should not be relied upon for accurate pH measurements.
- Alkalinity: White vinegar can test the alkalinity of a soil sample. Pour a small amount of white vinegar onto a sample of soil. If it starts to fizz or bubble, the soil is alkaline, with a pH above 7.
- Baking soda: Take a moist soil sample and place it in a small container. Sprinkle a small amount of baking soda onto the soil. If it fizzes or bubbles, it indicates that the soil is acidic, with a pH below 7.
If neither tests produce any fizzing or bubbling, the soil has a pH of around 7.
A soil sample is collected from multiple spots and a soil:water suspension is made with distilled water to create a slurry. The most common ratio is 1:5 (soil:water). The mixture is allowed to settle, which takes between 30-60 minutes. Once this has occurred, the soil will have settled, leaving a layer of coloured (supernatant) water at the top. A pH paper strip is dipped into the supernatant and then removed. The soaked pH strip will change colour based on the acidity or alkalinity of the soil solution. This is compared to a colour chart provided to determine the pH.
Professional testing services:
A professional soil testing service can provide the most accurate result. The most common method is the 1:5 (soil:water) suspension method. This method involves creating a soil-water mixture, allowing it to settle, and then measuring the pH of the water. Here’s a step-by-step description of the process:
A small amount of soil is collected from multiple spots within an area to get a representative sample. This soil is then mixed with distilled water in a ratio of 1:5 (soil:water). So, for every one part of soil, you add five parts of water. The sample is mixed thoroughly. After mixing, the soil-water suspension is allowed to settle for a specific period. This could be anywhere from half an hour to several hours, depending on the specific protocol being followed. After the soil settles, a pH meter is used to measure the pH of the water. The electrode of the pH meter is inserted into the supernatant (the clear liquid above the settled soil), and the pH reading is taken.
Correcting pH balances
Soil pH can be adjusted to suit the requirements of specific plants. Adding lime to the soil can raise its pH (make it more alkaline) while adding sulfur or peat moss can lower soil pH. Commercially available products are also available to help adjust soil pH.
It is important for gardeners and farmers to test and, if necessary, adjust the pH of their soil to ensure that it is suitable for the plants they are growing, as it can significantly affect plant health and yield.