Put compost or aged manure in the hole, mixed with your garden soil

When you dig a hole to plant a tomato, fill it with a mixture of what you dug out and compost or aged manure. A typical mix is one part compost to two parts native soil. This gives the roots something rich to grow into without burning them with fresh, hot manure. The compost holds moisture and feeds the plant slowly as it breaks down over the season.

If your garden soil is heavy clay or mostly sand, use more compost—up to half the hole. If your soil is already dark and crumbly, one-quarter compost is enough. You are not replacing the soil; you are improving what is already there.

Aged manure works the same way. "Aged" means it has sat for at least six months and turned dark and crumbly. Fresh manure will burn roots and attract flies. If you are not sure whether manure is aged, ask the source or skip it and use compost instead.

Key Takeaways

  • Mix compost or aged manure with your native soil at a ratio of roughly one part compost to two parts soil.
  • Aged manure must be dark and crumbly; fresh manure will damage roots and should not go in the planting hole.
  • Heavy clay soil benefits from more compost—up to half the hole—to improve drainage and root penetration.
  • You can also add a handful of balanced fertilizer (like 5-5-5) to the hole, but compost alone is usually enough for the first month.

Why compost matters more than fertilizer alone

Compost does two jobs at once: it feeds the plant and improves the soil structure. Fertilizer feeds the plant but does nothing for the soil itself. Over time, compost makes your soil hold water better, drain better, and stay warmer in spring.

Tomatoes are heavy feeders, but they do not need a lot of fertilizer in the planting hole. A handful of balanced fertilizer (something labeled 5-5-5 or 10-10-10) mixed into the hole is optional. The compost will release nutrients slowly as the plant grows, and you can feed again in four to six weeks with a side dressing or liquid fertilizer.

How deep to dig and how much to add

Dig a hole about as wide as the root ball and slightly deeper. For a transplant in a 4-inch pot, that is roughly 6 to 8 inches deep and 8 inches across. For a larger transplant, dig proportionally deeper and wider.

Fill the bottom third to half of the hole with your compost-and-soil mix. Set the plant in, then fill around it with the same mix. The top of the root ball should sit at or just slightly above ground level. Bury the stem deeper if it is leggy (tall and thin)—tomatoes will root along a buried stem and develop a stronger base.

What not to put in the hole

Do not use fresh manure, peat moss alone, or pure compost. Fresh manure is too hot and will scald roots. Peat moss holds water so well that it can waterlog the soil around the roots. Pure compost, without any native soil mixed in, creates a pocket that drains differently from the surrounding soil and can trap water.

Do not add lime, bone meal, or wood ash unless a soil test tells you to. Tomatoes prefer slightly acidic soil (pH 6.0 to 6.8), and these amendments raise pH. If you add them without knowing your soil, you may lock up nutrients the plant cannot reach.

Mixing and preparing the hole the day before

If you have time, dig and fill the hole the day before planting. This lets the soil settle and the compost begin to integrate. Water the hole lightly after filling. When you plant the next day, the soil will be moist and settled, and the roots will make better contact with the surrounding earth.

If you are planting the same day, that is fine too. Just water the hole after you plant and water again the next morning. The goal is to eliminate air pockets around the roots and keep the soil consistently moist (not soggy) for the first two weeks.

Soil testing if you are not sure what you have

If you have never grown tomatoes in this spot, or if your plants have struggled in the past, a soil test takes the guesswork out. Your local cooperative extension office (search "[your county] cooperative extension") can test your soil for pH, nutrients, and texture. The test costs $15 to $30 and takes one to two weeks.

A soil test tells you exactly how much compost to add, whether you need lime or sulfur, and what nutrients are missing. It is the most reliable way to know what to put in the hole. Without one, the one-part-compost-to-two-parts-soil rule works for most gardens.

Frequently Asked Questions

Can I use fresh grass clippings or leaves in the planting hole?

Fresh grass clippings will heat up as they decompose and can burn roots. Leaves take months to break down and create air pockets. Use compost or aged manure instead. If you have leaves or clippings, add them to a compost pile now and use the finished compost next year.

What if I do not have compost on hand?

You can plant in native soil alone, though the plant will grow slower and you will need to feed it more often. If you have aged manure, use that. If you have neither, buy a bag of compost from a garden center and mix it in. A single 2-cubic-foot bag is enough for three to four planting holes.

Should I add fertilizer spikes or pellets to the hole?

Fertilizer spikes and slow-release pellets work, but they are not necessary if you are using compost. They can also burn roots if placed too close to the stem. If you use them, follow the package directions and keep them an inch or two away from the root ball.

Is it okay to reuse soil from last year's tomato pot?

Reused potting soil is depleted and may harbor disease. Mix it with fresh compost at a one-to-one ratio, or add it to a compost pile and use it next year. For planting this year, use fresh potting soil or garden soil mixed with compost.