It is easy to ignore soil pH when planting a tomato garden, or at least to take it for granted. It is not a mineral or nutrient, after all, and it is not water that has such an obvious impact on your plants.
The level of your soil’s pH is a critical component to growing tomatoes successfully, though. And it certainly does affect how and if your tomatoes can access those minerals and the nutrients that it needs.
Jump to:
- What is Soil pH?
- Why pH Matters in the Tomato Garden
- What Is the Right pH Level for a Tomato Garden?
- How can you find out what your tomato garden’s pH is?
- Can you change the pH of your tomato garden if it's wrong?
- How can you change the pH of your tomato garden’s soil?
- How often should I test the pH of my tomato garden soil?
- What time of year is best for testing tomato soil?
- No pH Level is “Bad”, but It Can Be “Bad” for the Crop You Are Growing (Tomatoes!)
- Further Reading, References, and Resources
What is Soil pH?
Soil pH is defined as “the negative logarithm of the hydrogen ion concentration” (SUNY College of Environmental Science and Forestry).
That is a complicated explanation, but practically speaking, pH is how we measure how acidic or alkaline or basic something is. In the case of gardening, like tomato gardening, usually this is in reference to the garden’s soil.
- The scale for measuring pH ranges from 0 to 14
- Seven (7) is neutral
- Below 7 is acidic
- The lower the number, the more acidic the soil is
- Higher than 7 is alkaline
- The higher the number, the more alkaline the soil is
- Most vegetable plants, including tomatoes, like a slightly acidic soil in which nutrients are available to the plant, nutrients are soluble, and can be taken up and used by the plants
Why pH Matters in the Tomato Garden
The right pH range is important for tomatoes so that they can grow properly, access minerals and nutrients vital to growth, fruit development, and yields, and stay strong enough to fight off diseases with a healthy, supported immune system.
When the pH of the soil is wrong, crucial nutrients become locked up and trapped in the soil. The plants simply can’t take them up. This can happen to different nutrients when the soil pH is high and others when it is too low.
The effects of improper soil pH on tomatoes include:
- Calcium and magnesium become unavailable to the plant, even though they may be abundant in the soil
- This results in diseases like blossom end rot
- Growth rates are slowed
- High pH makes phosphorus, iron, and manganese unavailable
- Photosynthesis is disrupted (blocked)
- Yellowing leaves result in a condition called chlorosis
- Fruit set is poor
- Yields are low
- Microorganisms may slow or stop functioning in acidic soils
- This impedes the breakdown of organic matter
- It keeps nitrogen tied up in the soil, so the tomatoes can’t use it
- Poor growth and yellowing result
What Is the Right pH Level for a Tomato Garden?
Tomatoes like a soil that is slightly acidic -- but only slightly so.
- The optimal pH for soil for growing tomatoes is between 6.5 and 6.8
- A range of 6.2 to 6.8 gives you more leeway and is a good range for growing tomatoes (in fact, many experts list this as the optimal range)
How can you find out what your tomato garden’s pH is?
There are several ways you can determine the pH of your tomato garden soil.
The most accurate measure would be to have a soil test performed by an accredited laboratory, such as an extension service, university lab, or private lab that performs soil testing. These resources should give you a very accurate result as well as a report to tell you where your levels lie, including essential minerals and components.
They should also tell you exactly how much and at what rate you should apply the recommended amendments. This should be tailored to the results of your soil test so that you hit the right target pH range without overshooting it.
Other home and DIY options can give you a quite good result that may not be as minutely accurate, but should get you a result within a close range. Some options include:
- Home pH soil test kits
- Probe-style pH meters (often combined with moisture and sometimes other components)
- Basic DIY home tests like the baking soda test or the vinegar test -- these will not give pinpointed results, but can reveal if your soil is alkaline or acidic
You may want to employ a combination of testing options.
For example, it may be worth paying for a soil test to begin with to see where your starting point is and to get a full report with corrections. Then, you may switch to a home test strip or soil meter for ongoing monitoring.
This can help you gauge the success of your efforts without constantly paying for new soil tests, which, while beneficial, can add up and become expensive.
Can you change the pH of your tomato garden if it's wrong?
Yes. Soil amendments can be added to either raise or lower your tomato garden’s soil pH, depending on your needs.
This is good news.
Even if your soil is not in a good range for growing tomatoes, you can change that!
How can you change the pH of your tomato garden’s soil?
- Apply lime to raise soil pH (for soils that are too acidic)
- Different types of lime can be used, but dolomite lime is often recommended because it adds both calcium and magnesium as it works to raise the pH of your soil
- If you don’t need magnesium, calcitic lime is recommended
- Apply elemental sulfur to lower pH (for soils that are too alkaline)
If you are looking for a more DIY home solution for adjusting tomato soil pH, there are a couple of options.
- Wood ash raises soil pH
- It acts more quickly than lime, so ash can be good to apply if you need to change your soil pH faster (or if you have an abundance of ash and want to save money)
- Wood ash also lends other minerals and nutrients, and can be a good source of calcium, potassium, and, to a lesser degree potassium, magnesium, and traces of copper and zinc
- Powdered eggshells are made up of calcium carbonate and can be used like lime to raise soil pH
- If you have a large area, it may require more eggshells than you can access
- The eggshells need to be ground into a powder to make them accessible to work on the soil
- Lowering pH with home products can be more difficult
- Sphagnum peat moss is the most effective natural amendment for lowering soil pH
- Wood chips can be beneficial over time
- Compost can help lower pH, but this may depend on the composition of your compost (which can also be tested)
- Adding soil at a different pH level is an option, too, though it is typically considered more of a second line of action because it is expensive, and bringing in soil also risks bringing in spores, diseases, chemicals, and pests
- You can also grow your tomatoes in raised beds or containers and bring in soil to do so -- this is less costly than trucking in soil for a whole large garden area
How often should I test the pH of my tomato garden soil?
You can test your soil as often as you like, but amendments that change soil pH do need time to work. On the other hand, those adjustments should last for a significant amount of time.
Experts recommend testing your soil
- Before planting
- 3 to 6 months after amendments have been made to see if the pH level is corrected and/or determine what more you may need to do (or if you need to do anything at all)
- Every 3 years, after correction, to determine if the pH needs readjustment
What time of year is best for testing tomato soil?
Soil can be tested at any time of the year, but the best times to take a soil sample and perform a soil test are either in the fall or in the spring.
Fall versus spring doesn’t matter as far as the pH level and result. The soil will not change in that amount of time. It can be helpful to test your soil in the fall and apply amendments then, if you need to, because then whatever you apply will have several months to work before it is planting time again.
A spring soil sample is still beneficial, though. The earlier in the spring you are able to perform this, the sooner you can apply amendments and the more time they will have to work before planting begins. And what is in the soil will continue to work in the growing season.
Three to six months is the range of time that it takes before you’ll see a measurable change in soil pH. But you can test your soil as early in the season as you can dig into it. So, if you are waiting to plant until the last frost date, there is a lot of time there to test and begin correcting your soil to an optimal pH level for tomatoes.
No pH Level is “Bad”, but It Can Be “Bad” for the Crop You Are Growing (Tomatoes!)
It should be noted that a high or low pH level is not necessarily bad. It just may not be right for the tomatoes you are growing. That same soil pH might be perfect for a different type of plant or crop. A berry or bramble, perhaps.
It should also be noted that while you should strive to get as close to the ideal pH as possible, a slight variation will not break your tomato growing. Tomatoes will live in a range of soil types, but will do best if that soil’s pH is close to the 6.2 to 6.8 range.
All of this is to say, don’t give up on tomato gardening just because you don’t have the perfect soil for growing them. Amendments can be made; garden methods can be adjusted. Even during the growing season, top dressings and liquid amendments can be used to adjust the pH of your tomato soil.
The key is to keep working towards optimizing the soil pH for your tomato garden, but enjoy growing in the meantime.
Further Reading, References, and Resources
- Gardening.org – Ways to Use Wood Ash in the Garden
- Gardening.org – 15 Fantastic Ways to Use Eggshells in the Garden
- Gardening.org – 14 Uses for Sawdust in the Garden (With Tutorials)
- Ohio State University Extension – Growing Tomatoes in the Home Garden
- Growfully – The Easiest and Most Reliable Way to Test Soil pH
- University of Georgia CAES Field Report – Balance Your Tomato Garden’s Soil pH and Fertilization for a Bountiful Homegrown Harvest
- A Garden Patch – Acid Test: Understanding Soil pH for Tomato Plants
- UNH Cooperative Extension – Growing Vegetables: Tomatoes (Fact Sheet)
- Bonnie Plants – Why Does pH Matter?
- Ask Extension (Cooperative Extension) – Tomato Container Soil pH Too High (FAQ #912682)
- Iowa State University – Can Ground Eggshells Be Used as a Liming Source?
- University of Wisconsin–Madison Division of Extension – Using Wood Ash in the Home Garden
- University of Minnesota Extension – Coffee Grounds, Eggshells and Epsom Salts in the Home Garden
- Kellogg Garden – How to Make Soil More Acidic Organically
- Michigan State University Extension – Should I Use Dolomitic or Calcitic Lime?
- SUNY College of Environmental Science and Forestry – Soil pH: What it Means
- USDA Natural Resources Conservation Service – Soil Quality Indicators
