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. First, you need to get to know your soil by conducting a soil test. Once that test tells you what nutrients your soil already has and what needs replenishing, you can 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.
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How to Correctly Fertilize All Your Plants
How to Test Your Soil
A soil test will give you a comprehensive picture of the nutrients available in your garden's unique soil, whether you're planting in a raised bed or in the ground. 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.
The Optimal Soil pH for Tomatoes
Tomatoes do best in soil that's slightly acidic, a pH range of 6.2-6.8. A soil test will tell you if your soil falls outside this range and what to do about it.
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 regardless of the soil you're working with. 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 tomatoes.
Because compost is an organic fertilization method, it's 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.
Don't Overdo It
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. But remember: Too much compost, like too much of any fertilizer, can damage plants. Don’t apply excessive amounts.
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How to Fertilize Tomato Seedlings
Soil rich in compost or decomposing organic matter is usually enough to support tomato plants. But when tomato plants are just starting out, they can sometimes also benefit from a extra phosphorus, which is essential for new tissue development. So you may opt to give tomato seedlings a boost with a water-soluble fertilizer that is high in phosphorus. 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.
If fertilizer is applied too frequently, or too much product is used at one time, your tomato plants may be damaged. The excess fertilizer also can harm the surrounding environment. Always follow package directions for application rate and frequency.
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How to Fertilize Tomatoes as They Grow in the Garden
As your tomatoes grow, they will sometimes indicate that they need additional nutrients. If growth is slow and spindly, for example, 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.
Adding fertilizer when the fruit starts to enlarge will make nutrients quickly available. 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.
Side-Dressing Tomatoes
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.
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How to Fertilize Tomatoes in Containers
A tomato plant 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.
Frequently Asked Questions
How often should I fertilize my tomatoes?
Tomatoes thrive when fertilized every 4 to 6 weeks during the growing season. Harvesting your first tomato signals the last fertilizer application of the season.
When should I add fertilizer to my tomato plants?
Fertilize tomatoes at planting time with a water-soluble fertilizer high in phosphorus. Fertilize again with a 5-10-5 fertilizer after you see the first fruit. Finally, fertilize one last time after harvesting the first fruit.
Will adding coffee grounds to the soil help my tomato plants?
You might think that coffee, which is high in nitrogen and other nutrients, will benefit tomato plants. But research shows that the benefits to plants are marginal, plus, it's easy to go overboard when adding grounds to the soil around the plant. It's best to add coffee grounds to your compost pile rather than directly to soil.
