7 Soil Tips for Better Tomatoes

The sweethearts of the summer garden, tomatoes will grow stronger and plumper with these soil-boosting tips.

by Erica Strauss
PHOTO: Ben McLeod/Flickr

I remember the first tomato I ever liked. I was probably 7 years old and visiting my grandparents’ small home in the rural islands north of Seattle for the weekend. I was wandering the garden while the adults chatted. My grandpa had planted his heirloom beefsteak tomatoes against the west side of a white-painted, concrete-block garage wall.

In that warm, sheltered microclimate, bathed in full-day sun, his tomatoes grew huge and flavorful. I snagged one of the lower tomatoes—it took two hands to hold it—and plopped down in the warm crabgrass. The juice from that tomato ran down to my elbows, and by the time I was done eating, I was covered in sticky, sweet-tart juice.

Every summer I hope to grow tomatoes like the one I pilfered from my grandpa’s garden, and I’m not alone. Tomatoes are the most widely grown garden crop in the United States, and one of the most versatile. Raw or cooked in sandwiches, salads and sauces, the tomato is a delicious flavor of summer.

The best, most flavorful tomatoes start with the right soil. Like all fruit, tomatoes will develop their best flavor when all their nutritional needs are fully met but aren’t coddled with excessive water or fertilizer. Here’s how to prepare your garden soil for the most delicious tomatoes.

1. Grow In Sandy Loam
Most tomato varieties have expansive root systems. Give those roots room to grow by planting your tomatoes in rich, loose soil high in organic matter—sandy loam is ideal. If possible, avoid planting tomatoes in heavy, high-clay soil, which will limit root growth and is slow to warm up in the spring. Clay soils can be improved over time by working in lots of rough-textured organic matter like homemade compost and top dressing regularly with worm-friendly mulch, like straw.

2. Trench Compost for a Fertility Boost
When planting a row of tomatoes, consider trench composting food scraps at the same time. Worms will flock to this underground nutrient buffet, creating a layer of fertility the tomato roots will be eager to utilize.

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To trench compost, simply dig a trench about 12 inches deep and 12 to 15 inches wide where you’ll be planting your tomatoes. Scatter fresh fruit and vegetable kitchen scraps an inch or two deep over the trench. Banana peels, coffee grounds, vegetable trimmings, et cetera, are ideal. Don’t include any animal products, citrus or food scraps that are very slow to break down, such as pineapple tops. Add 2 to 3 inches of soil over the food scraps, then plant out your tomatoes as you normally would.

3. Add Seaweed Powder for a Potassium Boost
Potassium helps flowering and fruiting plants performs to their best, and seaweed powder is a natural source of potassium that’s highly bio-available to tomatoes when used as a foliar feed or side-dressing.

Follow package directions for exact dilution and application rates, as this varies by brand, type of seaweed sourced and fineness of grind. Be very careful about applying naturally harvested seaweed or kelp directly to the garden. Not only is this illegal in many areas, the salt that comes along with fresh seaweed can be detrimental to your garden crops.

4. Check Your pH
Tomatoes prefer a slightly acidic soil pH of 6.0 to 6.5. Outside of this range, soil nutrients are harder for the tomatoes to use. If you know your soil is rich in compost and organic matter but vegetables grow poorly, soil pH may very well be the culprit. Get a good soil test from a university or extension lab. These are reasonable in price—typically less than $20—and will determine exactly what your soil needs. Precise suggestions will be included that will help you modify the pH of your soil if necessary through the addition of garden lime or sulfur.

5. Discourage Weeds With A Living Mulch
Get the most harvest from your soil space by intercropping lettuce or other fast-maturing, low-growing, shade-tolerant greens, such as mesclun blends or arugula, with your tomatoes. Early in the season, while your tomatoes are still small, these greens will act as a living mulch, shading out weeds that would compete with your tomatoes. The lettuces will mature just as the tomatoes are growing large enough to need the extra space.

6. Grow Potassium Bio-Accumulators as Companion Plants
Some plants are particularly good at mining the soil for nutrients and then incorporating that nutrient into their tissues. This is called bio-accumulation. Because tomatoes have a high need for potassium, growing potassium bio-accumulators with tomatoes can help naturally provide this needed nutrient.

Good potassium bio-accumlators include yarrow, comfrey and horsetail. To get the best use out of these companions, chop foliage from the companion periodically throughout the growing season and mulch the tomatoes with this foliage.  As the mulch breaks down it will feed the tomatoes.

7. Bonus Beneficials!
Great soil will grow great tomatoes, but pests can still ruin what should be a great crop. Are tomato hornworms a tomato-destroying problem in your area? The braconid wasp is the organic solution. This beneficial insect is as creepy as it is useful in the garden.

The mature female wasp lays eggs just under the skin of the tomato hornworm and several other garden pests. When the wasp larvae hatch, they feed on their living host animal before tunneling out through the host’s skin to pupate. As you’d imagine, this weakens and eventually kills the host.

Attract braconid wasps to your garden with small-flowered, nectar-rich plants, such as yarrow, dill, fennel, sweet alyssum, chamomile and buckwheat.

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