How to Measure the pH of Soil

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Whether you have three green thumbs or cannot keep a succulent alive, plants need more than proper sunlight and water. The soil plays a large role in the health and wellness of a plant. Ensuring a proper soil pH will allow plants to thrive to their highest potential. In other words, this article will show you how to measure the pH of soil as well as understand the optimal soil pH for plants.

What is pH and how to measure it?

pH is a concentration measurement. It measures the concentration of the hydrogen ions in a water-based solution and defines the words acid and base. This pH scale ranges from 0 to 14, packed with insight about the nature of the test sample. Most commonly, pH and water are discussed together since pure water is the poster boy with a neutral pH of 7 (middle-point on the scale). As the ion concentration increases, an acid is produced and has pH values below 7—vice versa for bases with pH values higher than 7 on the pH scale. 

For drinking water, healthy values of pH span from 6.5 to 8.5 according to the Environmental Protection Agency (EPA) on municipal drinking water. Plants thrive in these more neutral ranges since they need water too. Water quality can be affected by many factors like pollution. Water outside these ranges can produce unwanted health effects such as skin irritation in households or animal poisoning in natural waterways, depending on how severe the pH becomes. That said, it’s best to understand and measure pH to prevent any unwanted side effects from compounding over time.

Beyond qualitative pH test strips that give a general color-scale range, pH is best measured with a pH probe. These probes use a reference and test electrode to relate the ion activity between the sample and the reference to produce a measurable, but small, voltage. With calibration, this voltage measurement directly correlates with a pH value. To be thorough, neutral 7 on the pH scale should have a 0 mV reading, and then acids or bases will have an increasingly larger positive or negative voltage as the solution drifts farther from neutral 7. A few different types of pH probes exist, but the basic principle of measuring pH with a pH probe remains the same. With this knowledge, let’s now define soil and how to measure the pH of soil.

What is the optimal pH of soil?

Soil is the underappreciated cousin of life. Consisting of water, minerals, air, organic matter (dead stuff), and microorganisms to break down the organic matter, it provides a base for all life to grow. Similar to the natural world, soil does fine on its own. However, it can be thrown out of balance by many factors (human, climate, etc) that we will discuss later. First, how can soil have a pH?

Due to the necessary water component, the other minerals and organic matter in soil will combine with water and create an overall pH (using our definition above about ion concentration). In this way, pH can be used as an indicator of soil quality for optimal growing conditions because plants cannot absorb nutrients if the pH is too high or low. For example, if the pH is too high certain nutrients become locked into the soil and do not reach the plants—imagine a dry, cracking soil where water cannot permeate as easily. In this scenario iron could be locked up and the plants will lose their green appearance due to the lack of chlorophyll. 

It is also important to note that aqueous solutions have a property called alkalinity, or the ability to resist acidification. Soil has a similar property called buffering capacity. A higher buffering capacity, like clay, will resist changes to pH compared to low buffering capacity soils like sand. Since soil is most commonly a mixture of sand, silt, and clay, the buffering capacity is more a general insight compared to a specific value like pH. These soil types will come into play more when adjustments to soil pH are necessary. 

The most optimal soil pH is slightly acidic (slightly under 7) for most plants. Different types of plants will thrive in a variety of soils; but overall, the slightly acidic pH indicates a great balance of water and nutrients from the decaying organic matter. Evergreen trees are a great example of acidic soil loving plants. To illustrate, when plants and other life die, they slowly decompose (thanks to the microorganisms) and become nutrients. This process releases CO2 which mixes with water to form a weak acid, hence dropping the pH. Therefore when overwatering occurs, it washes out all these nutrients from the soil, the plants don’t receive them, and the overall pH increases. A delicate, but important, balance.

How to Measure the pH of Soil

A few different approaches exist, but the overall concept involves taking soil, making a slurry (mud), and using a pH test device to measure the pH of soil. Certain test strips and pocket probes will provide a general pH value (i.e. acidic or basic). Even using household items like vinegar and baking soda mixed with soil will indicate an acidic or basic soil type. However, with an Atlas Scientific pH probe this measurement can be done in 4 easy steps to produce an accurate pH value. 

Step #1: Dig a hole deep enough for the pH electrode, and place the soil in a bucket or bowl.

How to Measure the pH of Soil

Step #2: Add water to the soil and remove any large rocks or sticks.

How to Measure the pH of Soil

Step #3: Place the calibrated pH probe into the newly dug hole.

How to Measure the pH of Soil

Step #4: Pour the mud back into the hole, around the probe, and wait 25 minutes to achieve the most accurate pH reading.

How to Measure the pH of Soil

This process can be repeated several times over multiple spots on the land plot of interest. Or you can leave the probe in the ground indefinitely for continuous soil pH measurement. It is also best to use deionized (DI) water for making mud to ensure the water is not affecting your soil pH measurement. Since most pH probes have delicate housing over the electrodes, be sure to sift out all large rocks and sticks to protect your pH probe or choose a more heavy duty pH probe for more severe environments.

Lastly, if the soil is soft enough, or you don’t want to dig a large hole, one can create a pencil sized hole, pour in water, and place the probe in the ground. Similar outcome but we recommend the first approach to avoid breaking a pH probe on rocks or sticks in the soil.

What Affects Soil pH?

Many factors come into play that affect the pH of soil. Namely:

  • pollution
  • irrigation water pH
  • environment & climate
  • organisms
  • geology
  • topography, types of soil
  • time

As one can expect, if pollutants enter a water system the soil will be impacted along with the essential microorganism and overall pH. Measuring the pH of soil can be a useful warning sign to pollution in your area if pH is changing drastically. In a similar vein, the irrigation water used to water plants will have its own pH before it enters the soil, and will therefore change the pH of soil upon watering.

As discussed above, high precipitation and overwatering will wash an abundance of nutrients from the soil and away from plants. This is why climate is so important; because on the other end, too little water and droughts will cause soil to dry out and nutrients become concentrated—unable to reach plants for proper growing conditions. Then enters the balance of microorganisms breaking down dead plants for nutrients (which again creates a weak acid to lower soil pH). If plants grow in an area with large plants there will be more dead plants and more acid release. Say it with me: more decay, more acidic soil.

Geology and topography also affect the pH of soil. For example, shale and coal are rich in sulfates and can produce a strong sulfuric acid that will lower pH. On the other hand, limestone (calcium carbonate) produces a basic solution when mixed with water to increase the pH. In a like manner, the topography and type of soil affect the ability to alter soil pH as discussed previously with buffering capacity. 

Sand soil types have a low buffering capacity, meaning pH can change a lot easier than clay-type soils which have a high buffering capacity. This will come into play with fertilizers and ultimately adjusting soil for optimal growing pH. To understand more details about your soil type and texture, use a simple mason jar test which involves mixing your soil in water and letting it separate out in a mason jar (like an oil and water separation). The general percentages of clay, sand, and silt will inform you about the type of soil. Use this graph below by the USDA’s Natural Resources Conservation Service to understand your soil type more specifically, if required.

How to Measure the pH of Soil

Lastly, over time all these factors may or may not come into play and the pH of your soil will change. In light of this knowledge, understanding how to measure the pH of soil will best aid efforts to create the optimal growing conditions for plants.

How to Correct Soil pH

After measuring the pH of soil, fertilizers can help adjust the pH to a necessary level, in most cases a slightly acidic level. But proper research should be done to determine which pH will be best for the plants.

Generally, organic and nitrogen-based fertilizers add more nutrients and decaying organic matter to lower the pH once the microorganisms get to work. Frequent watering can also lower the pH to release locked up nutrients, but beware overwatering. On the other hand, adding limestone/calcium carbonate will increase the soil pH if it becomes too acidic. Small garden plots can use eggshells for this calcium grab purpose. 

However, in either scenario one must consider the cost of correcting pH to the land size, as most options will not be economically viable for large farm land. Additionally, it is important to know there are multiple factors that affect soil pH (as discussed above) and fertilizers are not the cure all solution for soil pH correction. Many fertilizers can cause harm to neighboring environments or even burn out your own plants.

Therefore by understanding the pH of your own soil, and the target pH value, altering the pH can be done with various organic and chemical materials, watering, and time; but the most important step is to first understand and measure the pH of soil before you correct it.

Proper Equipment for Measuring the pH of Soil

As discussed in the step by step process, a pH probe is the best measurement approach. Certain test strips and pocket probes can provide a general pH measurement, but a pH probe is recommended for the most accurate results. 

Before testing the soil, be sure to calibrate the pH probe in the various calibration solutions, normally ranging from 4-10 on the pH scale. Atlas Scientific recommends using an acid, nuetral, and basic solution with every calibration to ensure accuracy. If you choose to plant the probe for continuous monitoring, re-calibration should be performed often along with cleaning.

Using water to rinse soil from the probe is an easy cleaning procedure. Once clean and dry, be sure to recalibrate the probe and make sure the soil is still moist enough for probe placement. 

Conclusion

With these tools and knowledge, any grower will be set up for success. Start with measuring the current pH of soil, then understand the target pH for your plants. Using fertilizer, organic material, and water, the desired pH should be achievable over time. Since time is always a factor, continuous and/or frequent monitoring with a pH probe will aid efforts to, time and time again, create the most optimal growing conditions.   
If you are unsure exactly which pH probe or testing approach will best suit your needs, do not hesitate to reach out to the world-class team at Atlas Scientific.

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