If pH doesn’t top the list of keywords that make you pull a book off the library shelf or sidetrack your daily newsfeed scroll, you’re not alone. For those of us just trying to remember to water our plants, pH seems like something better left to people wearing lab coats. If it’s not even a Scrabble word, it must be a technical topic best left to the master gardeners. Not true.
A Little Chemistry Makes a Big Difference
Resist the urge not to care, fellow gardeners! If you can stomach just a little bit of chemistry, you can learn simple pH solutions that will make a huge difference in the health of your soil and plants. (Alternatively, if you’re more of a “what” person than a “why” person, scroll down to the solutions below. They’ll often work even if you don’t understand why!)
What is pH?
Before you start to care about pH, it helps to know what it is.
pH is a measurement. More specifically, it’s the negative logarithm of the concentration of hydrogen ions in a given substance. Or, in human terminology, it’s how we tell if a solution is acidic or alkaline (an acid or base). A solution with a high concentration of hydrogen ions (H+) registers as acidic.
Understanding the Scale
The pH scale ranges from 1 to 14. 7, which is the score of pure water, is neutral. Everything on the scale works outwards from neutral.
One Small Step for pH…
If you remember nothing else from the technical jargon, remember the word logarithmic. On a logarithmic scale, every whole number represents a tenfold increase or decrease. So a solution that scores a 6 is ten times more acidic than a solution that scores a neutral 7. This will come into play later.
Acids tend or have a sour taste, like citric acid in lemons and oranges. They also tend to be corrosive. Vinegar is a mild household example. Hydrochloric acid, which can burn a hole through the floor, is a more dramatic one.
Alkalines, or bases, are also corrosive, but in a different way. Bases often have a soapy feel, and are present in many household cleaning agents. Familiar examples include dish soap (a mild 7-8), Borax (a much stronger 10), and chlorine bleach (11-13).
Like familiar household products, soils fall all over the pH spread. In the New Jersey pine barrens, you’ll find super acidic soils that test at pH levels lower than 3.5 (pine needles actually increase soil acidity as they build up). Arid grasslands throughout the southwest might test as high as 10, pushing the opposite end of the spectrum.
How soil pH Forms
But how do soils form their pH levels, and how can they change? It’s all about those hydrogen ions. The presence of hydrogen and aluminum increase the acidity of soil, while other cations (positively charged particles), like calcium (Ca++) or sodium (Na+), increase the alkalinity.
In soil, decaying material (roots, organic matter, small organisms) constantly releases hydrogen ions into the soil, which can increase a soil’s acidity. “Things usually balance out, however” organic soil experts Jeff Lowenfels and Wayne Lewis add, “because root surfaces also take up negatively charged anions.”
The amount of rainfall a particular soil receives can really change that. Rainfall is naturally a little acidic, and it also washes away loose cations (which would make soils more alkaline if they remained). This means rainfall is often an excellent predictor of soil pH.
In areas of heavy, continuous rainfall, like the Pacific Northwest, soils trend toward the acidic end of the spectrum. In areas of low rainfall, soils tend to be more alkaline because nothing is washing away those cation nutrients.
Cation Exchange Capacity
Particles like clay and organic material have a negative electrochemical charge, so they attract positively charged particles like hydrogen (H+) and other cations (Mg++, Ca++, K+), many of which are important plant nutrients.
Plant roots “eat” by swapping out cations attached to root hairs for cations attached to soil particles. “Roots use hydrogen cations (H+) as their exchange currency, giving up one hydrogen cation for every cation nutrient absorbed,” Lowenfels and Lewis explain in Teaming with Microbes.
pH & Plant Growth
The reason pH is often talked about in the plant world, is it can have a dramatic effect on the health of plants.
pH affects plant growth indirectly by changing the availability of mineral nutrients. In acid conditions, extra hydrogen ions will bind more tightly to negatively charged particles of clay or organic matter than the nutrients that were previously hanging on.
When hydrogen knocks those nutrients free, they become available for plants to take up.
Overly Acidic Soil
While slightly acidic soil frees up helpful plant nutrients, too much of a good thing can be a problem. If the soil becomes highly acidic, cation plant nutrients become too mobile.
Rainfall sweeps these “loose” nutrients away in a process called leaching. The nutrients pool in groundwater, which isn’t much help to your nutrient deficient garden.
Acidic soil can also reduce the activity of bacteria and other microorganisms that break down organic matter and make nutrients available.
In The Soul of the Soil, Grace Gurshuny and Joseph Smillie add, “Acid soils are low in fertility because too much of their cation exchange capacity is occupied either by hydrogen, which is not a plant nutrient, or aluminum, which is toxic to plants.”
On the other hand, if the soil is too alkaline, those nutrients stay stuck to soil particles and don’t become available.
It’s the old cliché of the stranded mariner—surrounded by water and dying of thirst. There may be lots of nutrients in your soil, but if they’re not available to your plants, what does it matter?
Alkaline soils can also lead to salt build-ups and soil crusting, both of which damage crop plants.
The Sweet Spot: Slightly Acidic Soil
Nutrient availability is why most crop plants thrive in slightly acidic soil. Fortunately, this is also the baseline for most (though certainly not all) soils.
According to Gurshuny and Smillie (true sustainable soil experts), “the most fertile soil generally has a pH range of 6.3 to 6.8…Many of the soil organisms involved in making nutrients available are most active in this range.”
Microbial activity is key here. When you adjust soil pH to fit a plant’s liking, most of what you’re doing is selecting for the range at which microbial life that supports that particular plant is most active.
While the decimal may seem like splitting hairs, remember that on the logarithmic scale a full point value is a tenfold change. A decimal point on the pH scale is a meaningful difference.
Plants on Acid
Most forest trees are tolerant of acidic soils, and pine trees in particular add to soil acidity.
Other acid-loving plants include:
Many desert plants full under this category. On the extreme end, grasses and plants like creosote brush thrive in soils with a high pH. Other “basic” plants include:
Testing Soil pH
Anything so fundamental to gardening success is sure to have many solutions. Testing pH is no exception. In general the options follow an increasing slope of time, expense, and accuracy. Let’s start with cheap, easy, and less accurate and work our way up.
Meters and Probes
pH meters and testing probes are without question the easiest and fastest way to check your soil pH. In most cases, they are also the cheapest available option. For most probes, all you need to do is plunge them into the soil you want to test and a readout will be ready in seconds.
Unfortunately, cheap meters tend to be the least accurate option available. Especially at lower price points, readouts are unreliable, which makes the whole exercise pointless.
Test your test
If you do choose to test your soil with a probe or meter, it is best practice to first check the instrument’s accuracy with a few baseline readings.
Mature compost should fall between 6-8. Peat moss is more acidic, around 4.5. Pure water should be close to 7, and mixing in some baking soda should raise the result.
If you aren’t seeing substantial change between these materials, the meter won’t give you accurate enough results to be helpful.
You Get What You Pay For
Easy doesn’t always mean inaccurate. There are many probes and meters that have lab-quality accuracy. You will have to pay for the difference, though. While a cheap (but potentially inaccurate) probe may set you back $5-10, expect to spend $50 or more for higher quality results.
Look for a meter that will provide a pH reading to at least one decimal point. For speed and accuracy, market gardeners may find an investment in a quality meter pays off quickly.
Taking a Soil Sample
The rest of the test options will require taking a soil sample. Gathering a clean, uniform sample is key to obtaining accurate results. So let’s talk about sampling technique before moving forward.
Collect Multiple Samples
Whether you’re using test strips, chemical dyes, or sending a sample to a lab, the soil sample must represent the tested area of soil as a whole. To make sure your sample does, take at least a dozen samples. Space the samples out evenly across your whole garden area.
Lee Reich makes an excellent point that outliers should be avoided. That means don’t sample from the spot where you used to have your compost pile. The corner where you pour fertilizer into your spreader is also out. Uniformity is key to an accurate sample.
Timing is Important
You want to take soil samples after crop harvest and before the next planting. This means early fall (when the soil is still dry) is the time most gardeners take soil samples for pH testing. Spring may also be an option, but that leaves less time for adjustments before the growing season begins.
If you plow or cultivate your garden, sample to the same depth that you turn the soil (roughly 6-8 inches). Pastureland or lawns can be sampled at 3-4 inches. In any case, keep the depth consistent throughout your sampling. Different soil horizons can have different pH levels, even in the same spot.
Soil Sampling Probes
A soil sample probe is the best way to gather uniform core samples. This is how lab-quality samples are taken. If you’re going to test pH each year, a sampling probe is a worthwhile investment.
3 foot probes can be pricier. Handheld probes only run about $20.
If you do use a probe, don’t sample when the soil is too wet. Your sample will be soggy and compressed. Likewise, if the soil is too dry, the core won’t hold together. After a few experiments, you’ll figure it out, Goldilocks!
You can absolutely take a quality sample with tools you already have. Push the top layer of soil away and dig straight down, taking a thin, deep sample. You don’t need a whole trowel full of dirt, so use a knife to cut away from each side until you just have a core in the center of your trowel.
For further reading, the USDA provides an excellent overview of Sampling Soils for Nutrient Management.
In a clean bucket or bag, mix all of your samples. Don’t use a bucket you’ve previously used for compost, fertilizer, or lime. These will alter the pH of your sample. Once combined, spread and allow the soil mixture to dry before testing.
Lab and Extension Samples
Specific soil-sampling instructions will depend on the lab or extension service that will analyze your soil sample. For instance, Penn State’s extension guidelines closely mirror the above general sampling advice. But check with your service for precise instructions before taking your sample.
To use a pH test strip, you’ll need to first take and mix your soil samples as described above. Then suspend the sample in distilled water (you don’t want your water influencing the results). Wait for the soil to settle. Then dip your test strip and compare it to the provided color code.
Each kit is a little different, so you’ll need to refer to the manufacturer for specific instructions. In general, the readouts are only whole numbers, but it will give you a rough idea and you won’t need to wait long.
A dye-based test kit operates in essentially the same way. The key difference is you’ll add a chemical dye to your liquid soil solution and then wait for the whole solution to change color. Most kits have the grading scale attached to the sample container. You just need to match the color of the solution to the sample scale.
Again, low cost and a short wait time. Dye kits are useful for periodic checks.
Extensions and Labs
For the most accurate results, send your sample into your local agricultural extension office or a lab that offers soil testing.
You’ll have to wait for the results and may pay a fee (some counties still offer this service for free), but you’ll know you can trust the results as long as your sampling skills are up to par.
Adjusting Soil pH
Ok, soil pH changes with cations and rainfall and compost. Different plants like different levels of pH in soil and the corresponding change in microbial activity that results. But how, oh how, do you change soil pH to a plant’s liking?
Excellent question. It depends on which direction you’re correcting towards.
In areas of low rainfall, or where the soil parent material is limestone, the pH will likely be too high. Iowa State University Extension recommends several solutions: sphagnum peat, elemental sulfur, aluminum sulfate, iron sulfate, acidifying nitrogen, and organic mulches.
Remember how organic matter releases hydrogen ions as it decays? This makes organic material the safest and most long-term solution for lowering your soil’s pH.
However, it’s going to take A LOT of compost or mulch to make a difference. To quote Purdue University, “enormous amounts of organic matter must be turned into the soil to have a significant effect on pH.”
But as you add, you’ll also increase your soil’s water holding capacity and nutrient levels. You’re feeding the whole soil food web.
Pine bark mulch is an especially potent option for correcting alkaline soil.
Sphagnum peat has a pH between 3.5-4, so it can dramatically lower alkaline soil pH. It also adds organic matter and has a high water holding capacity (which can help sandy soils). Unfortunately, swamps and wetlands are often drained to harvest sphagnum peat, making it a potentially unsustainable choice.
Ammonium Fertilizers (Read: Urine)
Soil bacteria process ammonium and release acidic compounds. Looking for a natural source of ammonium? Look no further than urea.
Livestock bedding that has been thoroughly urinated on can bring soil pH back down to earth and fertilize at the same time. Other chemical ammonium fertilizers can also accomplish this; but that’s less natural and less fun.
Elemental Sulfur is oxidized by soil bacteria which creates sulfuric acid. This will lower the pH in soil more dramatically than ammonium fertilizers, but as this great article by Agvise explains, “the rate must be high and the change may not be permanent.”
For the home garden with alkaline soil, focus on increasing soil organic matter and collecting as much animal urine and bedding as possible. Your plants will thank you.
Raising the pH of an acidic soil is the more common, and in many ways, simpler process. The key in most cases is neutralizing acid compounds and adding cations.
Adding lime is the traditional method for raising soil pH. As Phil Nauta, author of Building Soils Naturally explains, “The calcium and magnesium in the lime will probably knock some of the hydrogen out of the way. That will give us less hydrogen and more of these minerals, therefore raising the pH, at least in the short term.”
When choosing between limes, take a look at neutralizing values (NV). This is a measurement of the ability to neutralize acids. “Pure calcium carbonate has an NV of 100, which is the standard. Ideally, NV for any liming agent should be over 95.”
Agricultural lime is simply crushed limestone, which is mostly composed of calcium carbonate. Quality lime ranges from about 37–40% calcium.
Dolomitic Lime is crushed rock that contains both calcium carbonate and magnesium carbonate (right around 22% calcium and 12% magnesium). This can be helpful for addressing nutrient imbalances. Or, if used incorrectly, can create them.
The Case Against Liming
While a pH test may indicate you need to make an adjustment, a pH test alone won’t describe what type of adjustment is needed. Adding any form of lime will change the nutrient profile of your soil. Without testing for nutrient levels, you won’t know if that’s a good thing or a bad thing.
As Phil Nauta explains, “the problem is not that dolomite lime won’t raise the pH, but that our pH test did not tell us if we actually needed calcium and magnesium. Perhaps we already have too much magnesium, or too much calcium. It’s almost certain that we don’t need both in the ratio that dolomite lime gives us. Adding more of the wrong nutrient is just going to make things worse.”
Test for Nutrient Levels
Before raising your pH, have your soil sample tested for nutrient levels so you can adjust pH and nutrient levels accordingly.
Wood ash, particularly the fine material found in a wood stove, can be substituted for lime. Wood ash is high in calcium and potassium, and also contains lesser amounts of phosphorous, magnesium, zinc, and copper.
In general you’ll need more wood ash to accomplish the same pH change as lime, but it will occur more quickly.
The source material matters. If using a home source, be sure you’re not burning wood containing lead paints or toxic materials.
The University of New Hampshire extension recommends applying a maximum of 20 pounds of ash per 1,000 square feet of garden soil with an average pH and mixing the ash into the top 2-4 inches of soil. This is roughly the amount in a 5-gallon bucket.
Wait, how can compost be a solution to acidic soil and alkaline soil? Another top-notch question. You’re staying razor sharp 3,000 words in.
Finished compost is more alkaline than organic matter that hasn’t broken down. So organic material in the form of un-processed mulch can help lower soil pH. Adding mature, finished compost can help raise it.
In both cases, the materials being composted will affect the pH. Bark manures can record pH scores in the 5s. Mixed animals manures top out close to 8 according to a compost quality analysis.
Testing your soil is key to long-term success. This means taking a quality sample and analyzing it for nutrient levels in addition to pH. Whether you use a pH meter or lab test to determine pH values, make sure you trust the results. That information will guide your amendment decisions.