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Fertilizing Tomatoes: Frequently Asked Questions 

Tomatoes need lots of nutrients. That’s one reason fertilizing tomatoes is a key step in growing healthy plants. 

If you wonder why tomato plants need so many vitamins and minerals and other good stuff in the soil, think about what a tomato plant accomplishes in a very short time. 

• A tomato seed uses nutrients from its carry-along seed packet to sprout and put out its first leaves.
• The tiny seedling works hard to use water and sunshine by extending its stem up to the light and putting out extra sets of leaves. 
• Seedlings are transplanted to bigger pots. The lucky ones are fed a bit of extra nutrients.
• Plants are placed in a nutrient-rich home in your garden, enriched by compost, and go into overdrive. They push up leaves and branches and blossoms with the ultimate goal of creating fruit. 
• The process continues for a few weeks or months as the plants work to produce as many fruits as possible. All those tomatoes will allow plants (and variety) to survive … because fruit produces seeds. 

Tomatoes need lots of nutrients to accomplish all that activity within six months or so. Their growing process is compressed into a short period of time. And it is the reason that fertilizing tomatoes plays a significant role in your tomato plant’s success.

Fertilizing Tomatoes FAQs

Q. How do you know when tomatoes need fertilizer?

The fact is that even the healthiest soil may not have adequate nutrients for tomato plants. Most tomatoes need fertilizer – even if it’s simply an added layer of compost worked into the soil at the beginning of the growing season. Conduct a soil test in your garden to find out what kinds of nutrients to add to your garden to give your tomatoes the best chance to flourish. 

Q. What is the best fertilizer for tomatoes?

Fertilizers are a combination of the three nutrients commonly fed to plants: nitrogen, phosphorus, and potassium (referred to as the “N-P-K ratio”). The three numbers on a fertilizer package represent the proportion of three nutrients. 

• Nitrogen encourages leaf growth (rather than blossoming or fruit development.) A higher nitrogen number indicates a fertilizer best used at the beginning of the growing season, when plants are working hard to get established by putting out foliage. A lower nitrogen number (with higher phosphorus and potassium numbers) can benefit tomatoes in mid- to late season when blossoming and fruiting is at its peak.
• Phosphorus, the second number in the N-P-K ratio, encourages flowering, and therefore fruiting – clearly a good benefit for tomato plants.
• Potassium (the “K” in the ratio) encourages fruit development. A The strong third number in fertilizer indicates that it will help tomato plants produce healthy, quality fruit.

You can use various tomato fertilizers at different points in the season. Or you can keep things simple with a fertilizer especially formulated for tomatoes, like Miracle-Gro Tomatoes (18-18-21), a crystal formula mixed with water and applied through a hose or sprayer … and Tomato Tone (4-7-10 or 3-4-6), a granular organic fertilizer that is worked into the soil. 

Q. Can I make homemade tomato fertilizer?

Yes! You can make a natural fertilizer for tomatoes from readily available ingredients you find in your own home. Start with a gallon of compost. Then mix in ingredients that provide nitrogen, phosphorus, and potassium – as many or as few of these as you like – in much smaller increments.

• Tea or coffee grounds are rich in phosphorus, potassium, and small amounts of nitrogen
• Wood ashes are a good source of potassium
• Egg shells are a good source of calcium
• Alfalfa meal (leaves or pellets) is a complete NPK fertilizer
• Bone meal contains a 4-12-0 ratio – a good source of phosphorus, particularly during the flowering stage
• Rabbit droppings are rich in nitrogen and phosphorus (be sure to compost them first)
• Cottonseed meal is rich in nitrogen, phosphorus, and potassium, in various ratios of 6-2-1, 6-2-2, or 6-0-4 – best for young seedlings because of its high nitrogen content
• Fish emulsion has a 4-1-1 ratio – also suitable for young tomato seedlings
• Blackstrap molasses typically contains a 1-0-5 NPK ratio – good for the last stage of the season when fruiting is strong
• Seaweed contains trace elements which are useful to tomatoes all season long

Q. What’s a good tomato fertilizer schedule?

When you plant tomatoes, add a handful of tomato fertilizer or bone meal to the planting hole.

Then keep a careful eye on newly-planted seedlings for the first two weeks. Transplanted seedlings with yellowed leaves at the base need to be fed again. 

Apply tomato fertilizer once fruit has formed. Some gardeners look for their first tomatoes to be golf ball size as a signal to begin the season’s systematic feeding program.

Then, apply tomato fertilizer every 3 weeks until frost. 

Q. When should I stop fertilizing tomatoes?

At frost! Tomatoes keep growing throughout the season and continue to need nutrients to produce healthy blossoms and fruit.

Q. How can I know whether or not I’m over fertilizing tomatoes?

When you over-fertilize young tomato plants they grow tall and spindly, with plenty of leaves but few blossoms. Reason? Too much nitrogen, which encourages foliage development.

But you can also recognize that you’re over fertilizing tomatoes mid-season when …

• Tomatoes remain mostly green
• Tomatoes turn red from the bottom up but do not fully ripen
• Inner fruit contains a hard core center, either brown or white, and is stringy and tough
• The plant’s roots get burned and the plant is stunted

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By Kathy Widenhouse, award-winning writer and owner of Tomato Dirt, a leading online source for growing tomatoes and using them.

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