How To Grow Tomatoes in Clay Soil: From Seeds to Harvest
Clay is far from being an ideal growing medium for tomatoes and many other plants.
When wet, the clay becomes heavy, sticky, and lumpy, and it is practically impossible to work with. When it dries out, it cracks and becomes very hard, making it again almost impossible to work with.
But, with a few tricks, tomatoes and many other plants can be successfully grown even in clay soil.
Published: January 11, 2023.
Pros And Cons Of Clay As Soil
Every gardener wants to grow fragrant, juicy, and tasty tomatoes, which can be done more or less easily when having good garden soil.
But clay is anything but good garden soil.
Depending on the moisture content, clay ranges from hard, almost like a rock material when dry, to sticky, lumpy material that is completely waterlogged and, as such, can be used for pottery and not for gardening.
Tomatoes have many difficulties when growing in clay, especially heavy clay - since it is waterlogged when wet, with almost no air, it causes root rot easily. Also, the roots have difficulties spreading and growing even in wet clay, preventing the plants from taking in nutrients.
And when the clay is dry, not only that the roots cannot grow again, but the cracking of the soil snaps and destroy the root system.
Probably the only good thing about clay soil is the amount of various minerals, which, if made available to plants, can help the growth.
So, can tomatoes successfully be grown in clay? Yes, they can, but it is not easy and requires some extra work.
How To Grow Tomatoes And Other Plants In Clay
There are several ways of growing tomatoes in clay, including:
- Growing Tomatoes in Holes
- Growing Tomatoes in Raised Beds
- Growing Tomatoes in Containers
- Growing Tomatoes in Garden Patch
Each of these methods requires the user to amend the soil with organic matter, organic fertilizers, and similar materials that will, over time, improve the quality of the soil and turn it into light, well-aerated soil with good drainage.
But it takes time and effort...
Growing Tomatoes in Holes
To work with clay in the garden, one must wait for the ideal moment when the clay is not fully wet but also not completely dry and can be worked with spades, rakes, garden forks, and similar garden tools.
When growing tomatoes in clay, using the holes method, the idea is to dig large holes in the clay, fill them with soil mix suitable for tomatoes, and to plant one single tomato plant in each hole.
Imagine growing tomatoes in pots and containers made of clay soil around the holes filled with good soil, simple as that.
But, not the easiest thing to do.
Dig And Prepare The Holes
So, take a sharp spade and dig a hole 16-18 inches (40-46 cm) wide and long and some 12-14 (30-35 cm) inches deep. If the soil is really hard, feel free to make holes 1-2 inches larger and deeper.
Holes in the row should be spaced ~2-2.5 feet (to clarify: that is 2-2.5 feet (60-75 cm) from the center of one hole to the center of another hole), with the rows spaced enough to provide a gardener enough room to work with the plants comfortably.
Now, place ~2 inches (5 cm) of wood shavings on the bottom of each hole - wood shavings prevent the water from accumulating in clay, preventing root rot. Also, over time (months, even years), wood decomposes, refeeding the plants, keeping the soil slightly acidic, and improving the soil.
- Wood shavings should come from untreated wood - if it is treated with various chemicals, they can kill the tomato plants.
- Don't use walnut wood shavings since they produce over time chemicals that can kill the plants.
- Since wood shavings leech nitrogen (N) from the soil, if You intend to grow some other vegetable in the same holes after tomatoes (crop rotation!), add some extra nitrogen, but not much.
Now, it is time to amend the soil by mixing it with aged manure, organic compost, peat moss/potting soil, and topsoil.
Never use more than 20-25% of the clay soil in the mix - some gardeners don't use it at all, and they fill the holes with nothing but good potting soil mixed with various organic fertilizers, spreading the dug-out soil around the holes.
Also, adding a balanced NPK fertilizer (for example, 15-15-15) can help with growth - tomatoes are strong feeders, just keep the nitrogen in check since too much nitrogen can cause the plants to grow large but weak.
Note: tomatoes like full sun positions with at least 8 hours of sun per day with soil having a pH of 6.0-6.7.
Just in case, making a quick test can reveal actual pH levels and the amount of nutrients in the soil.
Planting The Tomato Seedlings
After the dangers of late spring frost are gone, transplant the tomato seedlings - it is better to have seedlings growing in somewhat larger pots and transplant them a week later than have a late frost destroying all or almost all young tomato plants. Just in case, seedlings should be hardened by exposing them to direct sunlight and lower air temperatures for a week.
Now, in the center of each hole, dig a smaller hole large enough to accept the new plant. When planting a young tomato plant, feel free to place a new plant somewhat deeper - the tomato stem will soon grow new roots.
Personally, it is a good idea to place tomato supports right away - putting tomato supports later can cause some root damage.
So, add a stake or two near your tomato plant (depending on the variety and intended growth type) or use a tomato support cage.
Watering The Tomatoes
After transplanting the seedlings, water them with a few cups of water each, every 2-3 days, depending on the local climate.
When watering established tomatoes, it is better to water them with plenty of water, but not so often, for example, every 3-4 days.
The goal is to keep the soil moist but not waterlogged. This way, particles of clay will not cause the soil to crack, exposing the tomato roots, or even damaging them - this is one of the reasons why some gardeners don't add clay to the soil mix that goes into the holes.
Perhaps the best mulch for tomatoes in clay soil is straw - place a thick layer (4 inches/10 cm will do fine) around the plants during the entire growing season, especially during hot summer days.
Mulch protects the soil from wind and sun and helps it retain moisture, preventing clay from cracking.
Also, as it breaks down into smaller pieces, the straw eventually mixes with the topsoil. And, as the straw decomposes over time, improving the topsoil even further - changes in the topsoil are small, but they accumulate over time.
After the harvest, straw can be used for making organic compost in compost bins, for example.
Fertilizing The Tomatoes
Tomatoes are heavy feeders, and they need plenty of nutrients. That is why it is very important to fill the holes with a mix that contains plenty of organic matter and to use balanced NPK fertilizer, preferably with the gradual release of nutrients.
Depending on the tomato plant size and growing season, it is recommended to add every 6-8 weeks some organic fertilizer (worm castings, organic compost) and just a little bit of NPK fertilizer (1/3 to 1/2 of an ounce (10-15g) per plant).
Note: It is better to fertilize more often but with smaller amounts. If You are unsure about the required amounts, use soil test kits.
By planting different varieties at different times, the tomato harvest season can be rather long.
Always pick fully ripe tomatoes and enjoy them fresh or processed - commercially grown tomatoes can't compete with home grown ones.
Prep For The Next Crop
Clean the topsoil from the tomato plants, weeds, and mulch, and remove the supports, but just in case, mark the position of each hole.
Add some aged manure, peat moss, and organic compost to the complete garden patch and mix it with the topsoil completely. This will improve the clay soil, but still not enough to plant tomatoes and similar plants directly.
Now, it is easier to plant the next crop in the current tomato holes, having crop rotation in mind - that's why their position should be marked.
But, it is much better to dig new holes between the first holes and repeat the process completely, just be sure not to plant tomatoes and similar plants in the same patch, not even in the new holes - better safe than sorry.
After 3-4 seasons, hard and heavy clay soil will be thoroughly mixed with aged manure, peat moss, potting soil, organic compost, and other materials used for amending the soil, and the gardener will be able to use it as any other "almost ideal" garden soil.
Just be sure to test the soil periodically and always add organic fertilizers - again, it takes time and effort, but it is doable.
Growing Tomatoes in Raised Beds
Raised beds are an excellent solution for growing tomatoes in clay soil.
The simplest raised beds are often the best - build a raised bed on top of your clay soil with the bottom open - the best material is untreated planks.
The sides of the garden raised bed should be at least 12-14 inches tall, which will give enough room for the tomato root system to grow.
Now, fill the raised bed with a good potting mix, aged manure, organic compost, worm castings, and other organic fertilizers.
Also, adding some balanced NPK fertilizer with a gradual release of nutrients can help support vigorous growth.
Over time (months and years), the organic matter will slowly seep down into the clay, improving the quality of the soil.
When the danger of late frost is over, plant the tomato seedlings in the raised bed - the distance in the row and among the rows depends on the plant size.
Note: one of the benefits of growing the plants in the raised beds is that practically every plant is easily accessible, allowing the distance between the rows to be the same as the distance between the plants in the row. However, one also must ensure plenty of sun and air for the entire plant in order to prevent certain diseases. In short, don't grow your tomato plants too close.
After adding supports, it is time to water the plants - water the plants with plenty of water, but not so often. This promotes root growth, helps with soil aeration, and improves the health of the roots.
Note: if possible, use a dripping water system.
If possible, add a thick layer of straw mulch.
Now, everything else said for growing the tomatoes in the garden soil patch goes for the raised bed on the clay soil as well.
Growing Tomatoes in Pots, Containers, and Grow Bags
Tomatoes can be easily grown in pots, containers, and even grow bags, just be aware that the soil must be tailored to the tomato's needs - it must be rich in organic matter, 6.0-6.7 pH, well aerated, rich in nutrients, have good drainage, etc.
But, the volume of soil in pots, containers, and grow bags can be limited, forcing the gardener to water and fertilize the plants more often.
On the other hand, pots, containers, and grow bags can be placed on the carts with wheels; whenever there is a danger of frost, they can be wheeled indoors.
That is why some gardeners grow tomatoes directly from the seeds:
- fill the pot, container, or grow bag with a suitable growing medium,
- place 3-5 seeds in the center, ~1 inch apart,
- cover the seed with a thin layer of soil and water gently,
- when the tomatoes are 2-3 inches tall, remove all but the best one.
When the warmer days arrive, but night frosts are still possible, harden the plants by taking them outside on the cart during the day and returning them inside during the night.
When the dangers of frost are over, leave the plants outside, even during the night.
Over time add some support, water regularly, prune when required and fertilize every 4-6 weeks with organic and balanced NPK fertilizers.
When the harvest comes, enjoy the fruits of your labor.
After the harvest, the soil from the pots, containers, and grow bags can be spread over the garden patch, and new soil can be used for the next growth season - personally, with crop rotation, one can replace the soil after 3-4 years.
Growing Tomatoes in Garden Clay Patch
Transforming the heavy clay soil into a light, well-aerated growing medium in a single step (relatively speaking) can be done, but it is not the easiest one.
The process requires clay to be mixed with large amounts of aged manure, peat moss, worm castings, organic compost, some sand, and similar materials, which will improve the quality of soil, providing the gardener with at least 1 foot (~30 cm) of ready-to-use garden soil.
In the fall, spread at least 4-6 inches (10-15 cm) of aged manure and plow it at least 6-8 inches (15-20 cm) deep, mixing it thoroughly with clay. Some sand can be added as well, but not much - sand helps with the drainage, but only when there is plenty of organic matter in the soil; if most of the soil is clay, it is more important to add organic matter than sand.
After that, add 4-6 inches (10-15 cm) of peat moss and plow it again, this time at least 8-10 inches (20-25 cm) deep.
Let everything settle down until late winter/early spring.
In late winter/early spring, when the soil allows, check the soil and, if required, add some more (but not as much as in the fall) peat moss and aged manure and till it into the soil some 8-10 inches deep (20-25 cm).
Also, add some organic compost, worm castings, and NPK fertilizer, and till it into the ground at least 6-8 inches deep (15-20 cm).
Note: growing tomatoes in clay soil using holes or garden raised beds sounds so much easier now, right? ;-)
Now, transplant the tomato seedlings into the garden patch and grow them as usual, just be sure not to let the topsoil dry out completely since it is still rich in clay - mulch the tomatoes.
Few Final Words
Clay is not a good garden soil, it is either very hard for both the plants to grow and the gardener to work with, or it is too wet and, as such, too lumpy and sticky to work with and completely waterlogged for the plants to grow.
But, there are workarounds requiring additional effort, time, and money. But, after a few growing seasons, the clay soil can be transformed into rich, soft, and light garden soil.
Personally, if You have clay soil, raised beds are an excellent way to go, with or without a few additional pots, containers and/or grow bags.
If You wish to use your garden patch directly, growing tomatoes in holes is recommended method - You will successfully grow excellent tomatoes, and over several growing seasons, You will gradually transform the soil completely.
However, if You don't want to wait that long, with enough aged manure, peat moss, organic compost, worm casting, potting soil, and similar materials, even the hardest clay soil can be improved in just a few months...