How to Fertilize Tomatoes for a Big Harvest

To grow the juiciest, most flavorful tomatoes, your tomato plants need a steady diet of nitrogen, phosphorus, and potassium, as well as other key nutrients. But you can't just dump fertilizer on tomatoes and expect great results. Your soil likely already contains some nutrients, but could need replenishing. It's best to start with a soil test to get a picture of the nutrients available in your soil and then create a plan for fertilizing your tomato plants. This guide will help you through the process, plus give you tips for choosing the best synthetic and organic tomato fertilizer to use.

Do a Soil Test

A soil test will give you a comprehensive picture of the nutrients available in the unique soil in your garden, whether it's a raised bed or an in-ground planting. In many areas, soil test kits are available from a local Cooperative Extension Service for a small fee. There are several commercial soil testing services too.

While the test kits vary slightly, the process generally includes gathering a representative sample of the soil in your garden and then packaging it and sending it to a lab for testing. The lab will then send you a summary of nutrient levels and the pH reading of the soil. Most soil summaries include suggested amendments for nutrient deficiencies and soil tips for growing specific plants, such as vegetables or turf grass.

Best Tomato Fertilizer to Use

High quality compost—material that is well-decomposed, dark in color, and crumbly—is the best tomato fertilizer to use. Not only does an annual application of compost boost the nutrients available in the soil, but it also improves soil structure. Loose, well-drained soil that is packed with available nutrients paves the way for strong, healthy plants.

Compost is an organic fertilization method, making it especially attractive for food crops like tomatoes. Begin improving soil with compost by spreading a 4-inch layer of material over the planting area in spring or fall. Use a spading fork or shovel to gently mix compost into the planting area. Continue fertilizing with compost yearly by spreading a 1-inch-thick layer of compost over the planting site in spring or fall. Too much compost, like too much of any fertilizer, can damage plants. Don’t apply excessive amounts.

In combination with the native soil, compost often provides all the nutrients most types of tomato plants need to thrive. There might not be a need to apply additional fertilizer. Your plants will tell you what they need. If growth is slow and spindly, plants might be suffering from a lack of nitrogen. Leaves with a pronounced blue-green tint along with poor growth might indicate the plant is lacking phosphorus.

How to Fertilize Tomato Seedlings

Give tomato seedlings a boost with a water-soluble fertilizer that is high in phosphorus. Phosphorus is essential for new tissue development so fast-growing seedlings benefit from ample amounts of this nutrient. Begin fertilizing tomato seedlings weekly when they have two sets of leaves. Continue feeding with the water-soluble fertilizer until seedlings are transplanted outside.

Look for a fertilizer that has a high middle number, which represents phosphorus, in the N-P-K analysis. Commonly available fertilizer analysis that are great for tomato seedlings include 8-32-16 and 12-24-12. Mix the fertilizer with water according to package directions.

Fertilizing Tomatoes in the Garden

Soil rich in compost or decomposing organic matter often has all the nutrients a plant needs. If your garden soil is lacking, add quickly available nutrients by adding fertilizer when the fruit starts to enlarge. An easy way to do this is side-dressing, which is garden-speak for applying dry fertilizer at the base of a plant and scratching it into the top inch or so of soil. Follow product package directions carefully to make sure you use the correct amount.

Here's a simple recipe for side-dressing tomatoes: When fruit begins to develop, spread ½ cup of a 5-10-5 fertilizer around the base of a tomato plant. Gently work the fertilizer into the top inch of soil. Fertilize tomatoes again with a ½ cup of 5-10-5 when the first fruit is harvested.

Fertilizing Tomatoes in Containers

A tomato growing in a pot has limited soil volume to mine for nutrients. Provide nutrients by mixing slow-release fertilizer pellets into the planting hole at planting time according to package directions. The slow-release pellets are helpful but will not supply nutrients the entire growing season. About 6 weeks after planting, fertilize tomato plants with a water-soluble fertilizer at the rate and frequency recommended on the package. A nutrient analysis of 5-10-5 is excellent for tomatoes.