To grow the best tomatoes, you need the right soil acidity. Soil texture and nutrient levels are also important, but without the proper soil pH, tomato plants will suffer. If you get the soil pH right, you will see healthier tomato plants and better harvests.
So, do tomato plants like acidic soil? Tomato plants like slightly acidic soil, with a pH of 6.0 to 6.5. If your soil is too acidic (low pH) or too alkaline (high pH), nutrients become less available, which slows tomato plant growth. You can use sulfur to acidify soil (lower pH) or lime to “sweeten” soil (raise pH) for tomato plants.
Of course, there are other factors that affect soil pH. For example, the use of certain fertilizers can also acidify soil.
In this article, we’ll take a closer look at the ideal acidity (soil pH) for tomatoes. We’ll also look at ways to make the soil better for growing tomatoes.
Let’s get started.
Do Tomato Plants Like Acidic Soil?
According to Rutgers University, tomatoes like soil that is slightly acidic, with a pH of 6.0 to 6.5. (For reference, a pH of 7.0 is neutral).
Make no mistake – tomatoes can still grow in soil with a pH outside of this ideal range. For example, the University of New Hampshire Extension suggests that tomato plants grow best in soil with a pH between 6.2 and 6.8.
However, tomato plants will be healthier with a soil pH close to the ideal range. The farther your soil strays from this range, the worse your results will be when growing tomatoes.
There are several ways that an improper pH affects tomato plant growth.
How Does Soil pH Affect Tomato Plant Growth?
Soil that is too acidic (low pH) will slow tomato plant growth and reduce your harvest. Soil that is too acidic (or too basic) will also lead to nutrient deficiencies, causing problems such as blossom end rot on tomato fruit.
This paper from PubMed suggests that tomato plants grown in acidic soil show slower growth and reduced ability to absorb nutrients from the soil.
The experiment in this paper compared tomato plants grown in two different soils with a pH of 6.4 and 4.2. From the abstract:
“The growth of tomato plants was seriously affected by the soil acidity and lowering of uptake of elements was observed for the plants cultivated on acidified soil.”https://pubmed.ncbi.nlm.nih.gov/10800716/
This suggests that a soil pH of 4.2 is far too low (too acidic) for tomato plants to grow well. In fact, pH works on an exponential scale, so soil with a pH of 4.2 is about 100 times as acidic as soil with a pH of 6.2!
What about pH values that are somewhat higher, but still acidic?
“In a ferrallitic sandy loam soil, tomato fruit yield and dry matter accumulation were significantly increased by liming at a pH of 5.2 while at a pH of 5.7 and above liming had little effects.”https://www.jstor.org/stable/42934377?seq=1
This suggests that a soil pH of 5.2 is still too low (too acidic) for tomato plants to grow their best. In fact, a pH of 5.2 is still 10 times as acidic as a pH of 6.2!
The table below gives a guideline on soil pH levels for tomato plants.
|Effect On |
|4.2 or |
|Far too acidic. Severely |
reduced plant growth
due to impaired
uptake of nutrients.
|Too acidic. Drastic |
reduction in plant
growth & fruit yield.
|A little too acidic. |
growth & fruit yield.
|Slightly below the |
|Ideal range. Optimal |
uptake, & fruit yield.
|Slightly above the |
|7.0 or |
|Too alkaline. A higher |
pH will cause more
pH ranges for tomato plants.
It also suggests that using soil amendments (such as lime) plays an important role in improving tomato yield (more on this later).
On the opposite end of the spectrum, a soil pH that is too high will also reduce growth and yield of tomato plants. For example, the availability of iron in soil starts to drop off as soil pH rises above 6.5.
To see this in more detail, check out this chart from Research Gate, which shows the effect of soil pH on nutrient availability in soil.
As we can see from the chart, a pH range of 4.0 to 5.0 makes the “big three” nutrients (NPK – nitrogen, potassium, and phosphorus) less available to plants. In addition, the nutrients sulfur, calcium, and magnesium become less available to plants in this pH range.
Even in the higher range of pH 5.0 to 6.0, some nutrients become less available to plants, especially phosphorus. Calcium also becomes less available, which can lead to blossom end rot in tomato plants.
How To Adjust Soil pH For Tomatoes
The best way to adjust the soil pH for your tomato plants is to use additives that do double duty. Add amendments that move pH in the right direction while also adding nutrients that your soil needs.
Just remember to get a soil test before you add anything to your soil. A soil test will tell you the pH and nutrient levels in your soil.
You can learn more about how to do a soil test in my article here. If you send a soil sample to a local agricultural extension office and tell them what you are growing, they will give you recommendations.
The information from a soil test will tell you whether you need to add anything to your soil. It will also give you a better idea of what to use.
Remember: a soil pH in the range of 6.0 to 6.5 is ideal for growing tomato plants. Outside of this range, you should consider some soil additives.
Start With Compost
One of the best ways to maintain good soil for gardening is to add compost to your garden every year. Compost provides nutrients for your plants, and it also adds organic material to your soil.
This organic material encourages earthworms and beneficial bacteria in your garden. In addition, it improves soil structure, helping soil to drain better (if heavy clay) or retain more water (if sandy).
You can make compost from yard waste and kitchen scraps, including:
- Grass clippings
- Fallen leaves
- Fruit and vegetable scraps
- Coffee grounds
How To Raise Soil pH For Tomatoes
If your soil pH is too low (acidic soil), then you will need to use an additive that raises pH. Some of the more common soil amendments to raise pH include:
- Lime – lime (or limestone) is another name for calcium carbonate, which is also found in chalk and antacid tablets. As you can guess, it counters acid in soil and increases pH. As an added bonus, it provides calcium for your plants. Follow the instructions on the label, though – it is possible to add too much lime to your garden.
- Dolomitic lime – dolomitic lime is similar to lime, but it also contains magnesium. Much like lime, it counters acid in soil to raise the pH. Dolomitic lime provides both calcium and magnesium for your plants, which are important nutrients for tomatoes.
- Wood ash – wood ash contains lots of calcium, so it will have an effect similar to lime. Just be careful about using wood ash from older wood treated with chemicals.
- Calcium sources – this includes bone meal, clamshells, and eggshells. They will take effect faster if they are ground up into a fine powder.
How To Lower Soil pH For Tomatoes
If your soil pH is too high (alkaline soil), then you will need to use an additive that lowers pH. Luckily, there are a few ways to acidify soil for tomatoes.
Some of the more common soil amendments that lower pH include:
- Sulfur – elemental sulfur lowers soil pH. However, it works slowly, and can take several months to have the full effect. As an added bonus, sulfur is a nutrient that plants need, so adding this amendment will help to avoid a sulfur deficiency.
- Sulfates – iron or aluminum sulfate both lower soil pH. They also work faster than pure elemental sulfur. However, using too much of them can leave you with excessive iron or aluminum in your soil. This will cause other problems for your plants, so be careful with the dosage that you use!
- Ammonia based fertilizers – fertilizers such as ammonium nitrate and ammonium sulfate will both lower soil pH quickly. However, be careful – they work so fast that the quick change in soil pH can burn your plants.
How Do You Acidify Soil Naturally?
One way to acidify soil naturally is to add peat moss (decayed sphagnum moss) to your garden. According to the Iowa State University Extension, Canadian peat moss has a pH of 3.0 to 4.5 – other types may not be quite so acidic.
According to the University of Vermont, peat moss is much more expensive than compost, since it is often shipped from Canada (or from other places where peat moss is harvested).
(If you want to try this water-absorbing material, you can find a large bale of peat moss online from Ace Hardware).
Keep Soil pH Steady
In some cases, your soil pH will be in the proper range, but you still need to add some nutrients that are lacking. In that case, you will want to use soil amendments that add nutrients and maintain a steady soil pH.
Here are a couple of options for you if that is the case:
- Gypsum – gypsum, or calcium sulfate, is often used in construction. However, it can also be used in your garden to add nutrients! Gypsum adds both calcium and sulfur for your soil. However, adding it won’t change the soil pH, so if your soil is already in the right pH range, consider using this to add calcium or sulfur (or both).
- Epsom salt – Epsom salt, or magnesium sulfate, is often used to provide magnesium, sulfur, or both to plants. Epsom salt can be mixed into your soil, or dissolved in water and sprayed on the leaves of plants (as a foliar spray). Epsom salt won’t change soil pH, so use it to add magnesium or sulfur if the pH is already ideal. Just make sure not to use too much Epsom salt.
For the most part, you should avoid adding nitrogen fertilizers to soil if you want to keep the pH stable. Using nitrogen fertilizer will make soil more acidic.
What To Avoid When Amending Soil
When amending your soil, be careful not to add anything with a high concentration of salt. For example, adding manure that is not fully decomposed can expose your plants to high levels of salt, which will burn them.
Also, avoid adding too much of anything with high amounts of any one nutrient.
For example, high-nitrogen fertilizer will burn your plants if you add too much at once. You can learn more about low nitrogen fertilizer in my article here.
Finally, remember that adding some amendments when you don’t need them can lead to too much of a given nutrient.
For example, let’s say you add too much Epsom salt (magnesium sulfate) to your garden soil. With too much magnesium, any calcium in the soil will become less available to your plants.
This is because calcium and magnesium “compete” for uptake by a plant’s roots. The reason is that calcium and magnesium are in the same column in the periodic table (both have a +2 charge as ions), meaning that they behave similarly.
You will then see symptoms of calcium deficiency in your plants, including blossom end rot in tomatoes.
Now you know that tomato plants like slightly acidic soil, with a pH in the range of 6.0 to 6.5. You also have some methods you can use to adjust soil pH and nutrient levels as needed.
If you are preparing for the upcoming gardening season, check out my article on what to do before planting tomatoes.
I hope you found this article helpful. If so, please share it with someone who can use the information.