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Best Practices for Fertilizing Tomatoes for a Large Harvest

Take all the guesswork out of fertilizing your tomato plants with these essential tips.

To cultivate the juiciest, most flavorful tomatoes, your plants need a steady supply of nitrogen, phosphorus, potassium, and other essential nutrients.

However, you can’t just apply fertilizer randomly and expect great results. Start by understanding your soil through a soil test. This test will reveal which nutrients are already present and which are needed.

This information lets you develop a fertilizing plan for your tomato plants. This guide will walk you through the process and provide advice on selecting the best synthetic and organic tomato fertilizers.

How to Test Your Soil

A soil test provides a comprehensive overview of the nutrients in your garden’s soil, whether you’re planting in a raised bed or directly in the ground.

In many regions, your local Cooperative Extension Service can provide soil test kits for a small fee. Several commercial soil testing services are also available.

While kits can differ slightly, the general process involves collecting a representative sample of your garden soil, packaging it, and sending it to a lab for analysis.

The lab will then send you a report detailing nutrient levels and the soil’s pH. Most reports also include recommendations for addressing nutrient deficiencies and tips for growing specific plants, such as vegetables or grass.

The Optimal Soil pH for Tomatoes

Tomatoes thrive in slightly acidic soil with a pH between 6.2 and 6.8. A soil test will tell you if your soil’s pH falls outside this range and what steps to take if it does.

Best Tomato Fertilizer to Use

High-quality compost—dark, well-decomposed, and crumbly—is the best fertilizer for tomatoes regardless of your soil type.

Annual applications of compost not only enhance the soil’s nutrient content but also improve its structure. Loose, well-drained soil rich in nutrients promotes strong, healthy tomato growth.

Since compost is an organic fertilizer, it’s particularly beneficial for food crops like tomatoes. In spring or fall, start by spreading a 4-inch layer of compost over the planting area.

Use a spading fork or shovel to mix it into the soil gently. In spring or fall, spread a 1-inch-thick layer to continue annual compost applications.

Don’t Overdo It

Compost combined with native soil often provides all the nutrients tomato plants need. Additional fertilizer might not be necessary. However, too much compost or any fertilizer can harm plants. Avoid excessive application.

How to Fertilize Tomatoes as They Grow

As your tomatoes grow, they may show signs of needing more nutrients. Slow, spindly growth might indicate a nitrogen deficiency. Leaves with a blue-green tint and poor growth could signal a lack of phosphorus.

When the fruit starts to enlarge, adding fertilizer will quickly supply nutrients. Side-dressing, which involves applying dry fertilizer at the base of the plant and mixing it into the top inch of soil, is an effective method. Follow package directions carefully to ensure correct usage.

Side-Dressing Tomatoes

Here’s a simple method for side-dressing tomatoes: When the fruit starts to develop, spread ½ cup of 5-10-5 fertilizer around the base of the plant. Work the fertilizer gently into the top inch of soil. Repeat with another ½ cup of 5-10-5 fertilizer when the first fruit is harvested.

How to Fertilize Tomatoes in Containers

Tomatoes grown in pots have limited soil from which to draw nutrients. Following package directions, mix slow-release fertilizer pellets into the planting hole at planting time.

While helpful, these pellets won’t provide nutrients for the entire season. About six weeks after planting, fertilize with a water-soluble fertilizer as the package recommends. A 5-10-5 nutrient analysis is ideal for tomatoes.