What to do with old tomato soil?

Tomato plants are heavy feeders, and require very fertile soil. So, it’s common for growers to ask what to do with old tomato soil from the previous year. There are several options for what you can do with your old tomato soil, including how best to reuse the soil in pots and garden beds.

The key factors that determine if you should re-use tomato soil are:

  • the soil pH,
  • the amount of organic materials & nutrients, &
  • whether or not any pests have grown in it.

If the old tomato soil has a lot of nutrients, is mostly acidic, and has no pests, then go ahead and re-use it.

However, there are a few reasons why you wouldn’t want to re-use soil that tomatoes have grown in. So, let’s take a look at those reasons, along with whether tomatoes grow better in alkaline or acidic soil, what to do if you have clay soil, and other questions.

What to do with old tomato soil?

What to do with old tomato soil?

If you’re facing the dilemma of whether to reuse your old tomato plant soil or replace it, there are a few things to consider.

The key factors that determine if you should re-use tomato soil are:

  • the soil pH,
  • the amount of organic materials & nutrients, &
  • whether or not any pests have grown in it.

If the old tomato soil has a lot of nutrients, is mostly acidic, and has no pests, then go ahead and re-use it.

Otherwise, you might need to amend the soil to a bit to make it more suitable for growing tomatoes more productively.

Specifically, after growing tomatoes, soil can often get depleted of some key nutrients, such as the following:

Nitrogen
  • Nitrogen: is a key component of protein, which helps plants produce more fruit and vigorous growth.
  • Phosphorus: used in photosynthesis to form chlorophyll, needed for plant growth and development.
  • Potassium: is used by plant to grow and produce fruit.
  • Sulfur: helps plants reduce stress, maintain healthy growth, resist diseases.
  • Boron: the plant hormone that controls flowering and fruiting; it also aids in root development.
  • Zinc: an essential micronutrient for tomato plants to grow well, zinc is involved in various biochemical processes.

The best way to see if your old tomato soil is deficient in any of these nutrients is to have it tested by a reputable lab.

Typically, county extension offices and universities will do soil testing for a small fee.

Once you receive your soil test results, they’ll indicate what nutrients your soil is lacking.

Once you know what nutrients are needed, the next step is to find a fertilizer that contains those specific nutrients and apply it as directed in the instructions.

How to reuse soil from tomatoes?

How to reuse soil from tomatoes?

In general, unless your soil is very fertile, if you want to reuse old tomato plant soil, then you’ll need to amend it first. Specifically:

  1. A simple way to amend tomato soil is by adding a few handfuls of compost or manure per square foot of old tomato dirt. For larger gardens, you may want to calculate how much manure or compost to add per acre.
  2. To balance out the pH (if necessary), add a bit of sulfur (to make soil more acidic) or lime (to make soil less acidic) as well. If you’re unsure, contact your local extension service for assistance.

So, is your soil quality poor?

To improve this, start by adding wood ashes and pine needles to the soil.

The wood ashes are higher in potassium, which tomatoes need a lot of.

Wood ash also raises the pH of highly acidic soils. Pine needles can help increase organic matter in the soil for more nutrients and organic content.

Next, add bone meal or rock phosphate to get back some phosphorus that was lost from the soil.

If you know that there’s plenty of phosphorus already present (as indicated by a high pH level), then put on as much nitrogen as you can afford to buy from your local hardware store or garden center.

How deep soil do tomato plants need?

How deep soil do tomato plants need?

Tomatoes need deep, well-drained soil. Specifically, if you’re using standard potting soil, then that’s typically good enough for tomatoes–at least for the first year.

However, if you have extra rich (fertile) soil, then there are some things to consider to see how deep (to the end of the root) your tomato plants need to be planted.

For best results and highest yields, tomatoes must have at least 12 inches of good quality soil in their pot.

If you plant tomatoes in shallow, compacted soil, they will grow slowly and be prone to diseases.

So, If your tomato plants are planted in standard potting soil, they should be planted 4 inches deep.

Ideally, the soil should be around 12 inches deep. Tomatoes need deep soil to grow and produce fruit. With deep soil, the roots can fully absorb the nutrients needed for plant growth.

For example, tomato roots absorb nitrogen from the soil.

In addition, deep soil helps keep your tomato plants cool, by allowing roots to absorb more water, which can cool the plant through transpiration.

Also, deep (12-inch) soil also allows you to use tomato cages, which allows your tomatoes to grow more productively.

You’ll be curious to see our best tips & tricks for growing more tomatoes.

Do tomatoes grow best in acidic or alkaline soil?

Do tomatoes grow best in acidic or alkaline soil?

Tomatoes grow best in soil that has a pH level between 5.5-6.8, which is slightly acidic. When the pH is too low (below 5.5), or too high (above 6.8), it can compromise growth and development of tomato plants because it affects the plant’s need for nutrients like potassium, nitrogen, sulfur and zinc.

Those nutrients are needed for plant growth and development. Without them, tomato plants will produce less fruit because of the lack of these nutrients.

So, what can you do if you have highly acidic soil?

Add wood ashes to the soil.

Wood ash also raises the pH of highly acidic soils and pine needles can help increase organic matter for more nutrients.

In addition, if you have sandy loam or clay loam that is too acidic for most plants, you can add organic material or ammonium sulfate to raise the pH level of the soils.

If you have alkaline soil, you can lower the pH level of your soil by adding plant materials with a high acid content to the soil.

Specifically, you can put pine needles or citrus fruit rinds into your soil to lower the pH level.

Also, If you maintain a soil pH that is too high for tomatoes (above 6.8), then the plant will have a more difficult time absorbing essential nutrients like potassium, nitrogen, sulfur and zinc which are needed for plant growth and development and also produce less fruit.

We’ve got a complete article on what type of soil is best for tomatoes.

Will tomatoes grow in alkaline soil?

Will tomatoes grow in alkaline soil?

Tomatoes grow best in soil that has a pH level between 5.5-6.8, which is slightly acidic. When the soil pH is above 6.8, it can compromise growth and development of tomato plants because nutrients like potassium, nitrogen, sulfur and zinc are less available to plants. The result is tomatoes will grow less vigorously and will produce less fruit because due to these nutrient deficiencies.

Tomatoes will grow in soil that is alkaline if you decrease its pH level by adding plant materials with a high acid content to the soil (specifically pine needles or citrus fruit rinds).

The best way to decrease soil pH is by adding a mix of finely ground rock phosphate and clay, or sulfur. When you add these substances into the soil, it lowers the pH level.

Check out our full article on how to lower soil pH.

Besides the nutrients involved in growing tomatoes (nitrogen, phosphorus and potassium), other important elements for growing tomatoes include calcium (for establishing strong stems and fruit production) and sulfur (which helps defend against disease).

Without these nutrients, tomato plants won’t be as productive.

Amending clay soil for tomatoes

Amending clay soil for tomatoes

Tomato plants need deep soil, so they are not typically able to grow and produce good fruit in clay soil.

Tomato plants require soil that has good drainage and depth for best growth.

Clay soils are often more compacted, which handicaps the plant’s ability to get necessary nutrients from the soil.

For these reasons, tomato plants do not grow well in clay soils.

To amend clay soil for growing tomatoes:

  1. Soak the clay soil thoroughly
  2. Add manure or compost to loosen the soil to help it drain better and increase soil nutrients (such as potassium, nitrogen, phosphorous, and other micronutrients).
  3. Amend with sand or perlite to increase soil drainage.
  4. Add ammonium sulfate to reduce pH if needed
  5. Ensure that you plant a deep enough container (at least 12 inches) for tomatoes

In general, soil needs to be a mix of sand, clay, and organic matter. For plants to grow effectively in soil that is highly compacted, heavy and dense, they need to be planted deeply.

With compacted clay soil for tomatoes, it is possible to improve the condition by planting deep containers with a mix of sand (up to 3″ in depth), manure or compost (up to 1 inch in depth) with added ammonium sulfate (1/2 teaspoon per 10 gallons of water).

This mix can be added directly into the potting mix, or if you’re growing outside in raised beds, simply sprinkle the mixture over the base of the bed and cover with soil.

Can you use miracle grow potting soil for tomatoes?

Can you use miracle grow potting soil for tomatoes?

Yes, you can use Miracle-Grow potting soil for tomatoes. However, when it’s too rich for your tomatoes, then you should dilute it first with some regular potting soil to balance out the nutrients in the miracle-grow soil.

Also, if you have a continuous supply of Miracle-Grow tomato soil and no ready source of compost or manure in your area, I would suggest mixing up a batch of homemade compost and putting most of it aside for the plants to use during their first 2 years.

Miracle-Gro Potting Soil is a soil amendment and potting mixture that contains 40% of organic matter. Miracle-Gro Potting Soil also contains 2% of agricultural lime as well as other ingredients that are listed below:

Sphagnum peat moss
  • Sphagnum peat moss,
  • redwood bark,
  • perlite and
  • vermiculite.

Word of caution: Miracle-gro potting soil should not be used in the garden or directly on plants outdoors because it has a high pH level due to the presence of agricultural lime.

Remember: tomatoes prefer slightly acidic soil.

Besides Miracle-Gro, another product on the market that’s suitable for tomato plants would be Pro-Mix by Plax.

The major difference between the two products mentioned above is that Pro-Mix only contains 20% organic matter, while Miracle-Gro contains 40%.

What to do with old tomato soil?

To sum up

Generally, you want to re-use the same potting soil for your tomatoes until it’s old and breaks down. This means that it will have lost a lot of nutrients.

The one exception to this is if you have a fertilizer that has too much nitrogen for your plants (like Miracle-Gro) then you should dilute the fertilizer with regular potting soil.

Also, make sure that when adding compost or manure to your garden or pots, that you mix it into the dirt rather than putting in on top because you want to distribute the nutrients evenly throughout the soil.

About the author

Greg Volente

Greg Volente holds a Naturalist Certificate from the Morton Arboretum, worked for The Nature Conservancy leading environmental education programs and doing natural areas restoration, and worked in the soil science research & testing lab at Michigan State University. Besides gardening, he's an avid wildflower enthusiast, and loves botanizing, hiking, and backpacking.