Gardener’s Guide to Soil pH: Why it Matters and How to Correct pH Problems
Have you struggled with your veggie plants growing poorly and not doing much, even though you put out fertilizer and took good care of them? If you’ve seen mysterious signs of nutrient deficiency in your garden, like yellowing or discolored leaves, stunted growth, and low production, your soil’s pH level could be part of the problem.
Vegetable plants need a soil pH that’s mildly acidic to neutral in order to access the right balance of nutrients for healthy growth. If your soil’s pH is way out of whack, some nutrients will become unavailable, and some may become too available, leading to toxicity.
Soil pH problems can be a little tricky to deal with, depending on the extent and the cause. With a little time and patience, though, you can usually bring an off-kilter soil pH back into the proper range.
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What Does Soil pH Actually Mean?
Since most folks probably just want to grow nice plants, not discuss pH down the molecular level, we’ll skip the scientific jargon and get down to the basics. Essentially, pH is a way to describe how acidic or alkaline something is. It’s measured on a scale of 1.0-14.0, with 7.0 being neutral. Anything below 7.0 is acidic, while anything above is alkaline.
Though some vegetables can tolerate acidic or alkaline soil conditions better than others, most of your garden plants will be happiest when the soil’s pH is between 6.0 and 7.0, give or take a little. Serious extremes in pH can cause nutrient deficiencies and, in severe cases, crop failure.
Testing Your Soil’s pH Level
To get the most accurate pH reading, take some soil samples from around your garden area and have them tested by the pros. All you need is a basic soil test, which you can usually get for the best deal at your local cooperative extension office. The soil test kits you order online tend to be a little pricey.
You can also find soil pH test kits and sometimes pH meters at garden centers, but they’re not as trustworthy. Sometimes, I think the type of soil you have can throw off the accuracy of the do-it-yourself pH tests. With our soil, they tend to show a lower pH than our “official” soil test results from the lab.
Acidic Soil
If the pH is way too low (generally below 5.5 to 6.0, depending on the crop), plants will have trouble getting important nutrients like phosphorous, potassium, and magnesium. At the same time, they can absorb too much of certain minerals like iron and manganese and metals like aluminum, causing toxicity symptoms and poor root development.
Beneficial soil microbes that help with nutrient availability are also less active in highly acidic soils, while disease-causing fungi and naturally-adapted weeds will thrive and multiply.
What causes acidic soil? Overuse of conventional fertilizers and lack of organic matter in the soil can cause high acidity. Soil in areas that get lots of rainfall tends to be naturally more acidic.
Alkaline Soil
Plants often show symptoms of mineral deficiencies such as iron chlorosis (yellowing of new growth) if the soil pH is too high. High pH also lowers the amount of phosphorous they can absorb.
Overly alkaline soil is a less common problem than acidic soil, unless you live in a dry climate where native soils are naturally alkaline. Otherwise, high pH can be caused by hard water, overuse of alkaline garden amendments, and growing in greenhouses or hoop tunnels where the ground doesn’t get rained on for long periods of time.
Adjust Your Soil’s pH
For milder pH imbalances, beefing up the organic matter in your soil can bring things back into line. Add compost, worm castings, and/or composted manure, and mulch your garden with natural mulches. Shredded leaves are a particularly good option, since they’ll break down quickly. If it’s not practical to make or buy a lot of compost, you can try trench composting and cover cropping as a cheaper way to add more organic matter.
Finished compost and other fully decomposed organic matter, called humus, has a pH close to neutral. This helps balance overall soil pH regardless of whether your initial pH is too high or too low.
Plentiful organic matter also raises your soil’s buffering capacity. A high buffering capacity helps the soil withstand big pH swings due to things like hard water or fertilizing. The humic acids in compost and organic matter do this by binding with or “capturing” acidic or alkaline compounds in the soil, limiting how much they can change the pH.
Balancing pH with organic matter takes time. It might take a couple years of consistent amending, before conditions are perfectly ideal. In the meantime, compost’s unique properties help offset the effects of pH on your plants’ nutrient intake. If your pH is off balance enough to cause major problems, though, you could look into using pH-specific amendments to adjust it a little more quickly.
Raising Soil pH With Lime
Using lime to raise soil pH is a long-standing practice, and it’s fine for organic gardening as long as you use an approved lime product. Calcite lime and dolomitic lime are the most practical organic options for backyard gardeners. If your soil test results also showed your soil to be low in magnesium, dolomitic lime helps boost Mg levels.
Lime takes a matter of months to raise soil pH, which is why it’s a good idea to put amending for pH on your pre-winter garden to-do list. That way, it’ll have some time to work before you plant spring crops. Lime works fastest when tilled in, but you can also spread it on the surface if you use no-till methods. For no-till gardens, it works best to apply about half as much lime twice per year, instead of just once in the fall.
Speaking of which, how much lime do you need to add?
That’s the tricky part, because it depends on your soil type and how much you want to raise the pH. Lowering soil pH is a lot harder than raising it if you overdo the lime, so don’t go spreading it all willy-nilly. I’d recommend asking someone at your county extension office for region-specific advice. The folks at our local feed-and-seed store are also really knowledgeable about how to use the garden products they carry, including lime.
Raising Soil pH With Wood Ash
Using hardwood ash as a garden amendment goes back a long way. Folks in areas with naturally acidic soils figured out it improved their crops way before modern soil science and pH tests were a thing. On top of raising pH, wood ash doubles as a good fertilizer. It’s rich in potassium, calcium, and a bunch of other trace nutrients. Some gardeners also say it helps keep pests away.
The official recommendation for applying wood ash is to spread about 15 to 20 pounds per 1000 square feet of garden. To simplify, that works out to a THIN (1/4 inch) layer on the soil surface. If the pH is above 5.5 and you only want to raise it a little, stick to a lighter sprinkling of ash. Ash is more forgiving than lime, since the effects don’t last as long, but you still don’t want to bump the pH too much.
Because it’s more soluble, wood ash has a quicker effect on soil pH than lime. Still, it’s best to add it in the fall, if possible. Fresh ashes can be toxic to young seedlings, if they come in contact with them.
When using wood ash in the garden, stick to ashes from natural firewood. Don’t use ashes from pressure-treated or painted wood, which could contain heavy metals and chemicals you don’t want in your soil. Also, don’t collect ashes from a burn barrel or firepit where trash or non-woody things may have been burned. If you have a friend or family member who heats with a wood stove, they can probably save plenty of ashes for you.
Wood ash is powdery and easily picked up by the wind, so don’t spread it on a blustery day. You don’t want to get a face full of ash, since it’s fairly irritating to the lungs, nose, and eyes. I like to spread it in the morning or evening when the wind is calm and humidity is high.
Lowering Soil pH With Sulfur
Bringing down the pH of overly alkaline soil is a harder ask than raising the pH of acidic soil. One possible option to lower pH naturally is to use elemental sulfur. Soil microbes convert this into sulfuric acid over time, gradually lowering pH. You can find elemental sulfur at garden centers, nurseries, and farm supply stores.
The caveats:
How well elemental sulfur will work to lower pH depends on a few factors, like your soil composition and why your soil is too alkaline in the first place. Clay soil is resistant to pH changes because of its high buffering capacity. That means you have to add a lot more sulfur to lower the pH of alkaline clay soil. Too much sulfur can be toxic, so it’s hard to strike the right balance.
If your soil naturally has a pH of above 7.5, it’s probably because it contains high amounts of alkaline minerals like limestone (calcium carbonate). Soil like this is also harder to amend with sulfur, since those alkaline substances will constantly be working against your efforts.
Using sulfur to lower pH works best for looser, sandy soils and soils that have a high pH due to hard water or overuse of lime or other high-pH amendments. Even still, it’ll take a while. You’ll need to make several applications over several seasons to significantly lower that pH.
If you live in a region with naturally alkaline soil, especially if it’s heavy on clay, building up organic matter with compost and natural mulch is likely your most practical approach to getting the pH more balanced. Grow things that have a better tolerance for high pH in-ground, and stick with containers for acid-loving plants.
What Can You Grow if Your Soil pH is Too High or Too Low?
Changing your soil pH takes time and often repeat applications of pH amendments. In some situations, especially if your native soil is highly alkaline, getting it into that preferred 6.0 – 7.0 range is a hard ask no matter how much amending you do.
The good news is that many veggie plants can handle soil pH outside of the perfect range. If you’re dealing with high or low soil pH, find out what plants will grow well with the current soil conditions so you don’t end up with a bust on your harvest. For crops that won’t jive with your soil’s pH, consider container growing or a raised bed instead. That way, you can grow everything you want and still get the benefits of growing in-ground.
Vegetables for Acidic Soil
The vegetable crops listed below can tolerate some pretty acidic soil conditions and still produce a good harvest:
- Peppers
- Potatoes
- Tomatoes
- Tomatillos
- Eggplant
- Pole or Bush Beans
- Carrots
- Corn
- Cucumbers
- Winter Squash
- Summer Squash
- Melons
- Cauliflower
- Parsnips
- Turnips
- Radish
- Garlic
- Dill
- Parsley
- Cilantro
How low is too low? Most of the above plants can handle a pH down to 5.5 and produce well, even if they develop slight nutrient deficiencies. Potatoes can go lower, down to 5.0. At pH levels below 5.0, iron, manganese, and aluminum toxicity will become a problem, seriously hindering root growth.
Want to grow some backyard fruit? Blueberries, blackberries, alpine strawberries, raspberries, and elderberries like acidic soil. Blueberries need a pH range from 4.5 – 5.5, while the latter four prefer 5.5 – 6.5. These are low-effort crops for folks with naturally acidic soil, since no tedious pH amending is needed.
Vegetables for Alkaline Soil
These crops can grow and produce well in alkaline soil conditions, in a range between 7.0 to 8.0:
- Asparagus
- Beets
- Tomatoes
- Pole or Bush Beans
- Carrots
- Corn
- Garlic
- Okra
- Swiss Chard
- Kale
- Cauliflower
- Cabbage
- Brussels Sprouts
- Broccoli
- Peas
- Spinach
- Pumpkins
- Winter Squash
- Summer Squash
- Cucumbers
- Celery
- Parsley
Note that some of these crops are on both lists, like carrots, garlic, beans, cucumbers, and tomatoes. Those are less pH-picky plants that can tolerate a wide range of soil conditions.
Some More Tips for Handling Soil pH Problems
Even for really experienced gardeners, adjusting soil pH is a fickle business. Proceed carefully, or you might end up creating a worse problem than the one you had to start with! Here are a few things to keep in mind if you want to raise or lower your soil’s pH:
Don’t amend for pH without hard evidence!
Many different issues can cause poor growth and nutrient deficiency in your vegetable plants, so don’t toss out some lime based on a guess your soil might be too acidic. If it turns out pH isn’t the cause, you could end up with too-alkaline soil on top of the original problem. Always test the soil to confirm your suspicion before using pH adjustors.
Start small.
The pH effects of things like lime and sulfur differ widely based on what your soil is made up of. That means the same amount of lime might raise the pH a lot more dramatically in sandier soil than it does in soil that’s high in clay. When you add pH amendments, err on the side of less, and test the soil pH 3 to 6 months later before you apply more.
Consider pH influence when choosing fertilizers.
Some fertilizers and amendments have a significant impact on pH. If your pH is already off, you don’t want to unknowingly add something that could worsen the issue. For example, putting out lots of composted chicken manure can raise pH, while conventional nitrogen fertilizers like ammonium nitrate can make your soil more acidic.
For soil testing, make sure to take your samples properly.
Take samples from a handful of different areas in your garden, dig down to get soil from at least 6 inches deep, and combine those for your final test sample. It’s also a good idea to test any “problem areas” you’ve noticed separately.
I like to take samples for a soil test later in the fall, when most of my harvests are done and I haven’t added any fertilizer in a while. Getting a handful of some fresh amendment or fertilizer in your test sample can seriously skew the results.
Soil pH is just another one of the challenges that come along with growing annual vegetables in different climates and conditions most are not naturally adapted to. Once you’ve identified it as the problem, you can make adjustments and find the right workarounds to keep your veggie garden thriving.
