Thursday, October 8, 2009

Fertilizing Tomatoes

First off, a word with South Florida ...

Listen, I get it. I like summer as much as the next guy, and the subtropical thing you've got going on is wonderful. But this heat? In October? I'm trying to transplant tomatoes, and when it's hitting 93 degrees every day, it just makes things hard for me. So can we cool down a little bit? Just a bit? Thanks.

Back to business. The transplants are doing pretty well at this point, even with the brutal heat. I'm watering every morning, and I've noticed that leaf miners have already discovered the lower leaves of a few plants.

So, let's talk about fertilizing tomatoes. I don't want to complicate this issue, because it's actually not that complicated. I've tried a lot of different things over the years—I've used Epsom salt to deliver a shot of magnesium, sprinkled powdered milk on the ground for calcium, made my own compost and used bagged compost, and used both organic and synthetic fertilizers. The truth is, you can grow excellent tomatoes any number of ways, with any number of fertilizer programs and products. Some of this depends on you.

Here are some basic approaches to fertilizer:

1. When you plant, you can dress the soil with a band of granular fertilizer at planting time. I didn't do this because I added organic fertilizer amendments straight to my soil (the compost, bone meal and blood meal). But if you're using a sterile soil mix based on sphagnum or coconut coir, you'll want to fertilize at planting time.

2. Like all vegetable fertilizers, tomato fertilizer should have a higher second number (phosphorus) than first number (nitrogen). For example, the label might read 6-8-8 or 3-4-6. This represents the percentage by weight of the three major macronutrients all plants need to grow: nitrogen, phosphorous and potassium. Nitrogen encourages leaf growth at the expense of fruit; phosphorus encourages strong fruit and flowers; and potassium strengthens the plant and contributes to fruit growth. For tomatoes, beware of fertilizers that contain too much nitrogen. Nitrogen contributes to a condition called blossom end rot, which you don't want.

3. Start a regular fertilizer program two to four weeks after transplanting. You can use an organic granular fertilizer (many people like Espoma's Tomato Tone, which is organic), or you can you use any balanced vegetable fertilizer. With granular fertilizer, side dress according to label directions. Follow label directions for a liquid, hose-end fertilizer. In general, you can expect to fertilize every other week at full strength or weekly at half strength.

4. Starter solutions can help at transplant time to get the tomatoes off to a quick start. Fertilome makes a product containing hormones that stimulate root growth, or you can use SuperThrive or a plant starter solution at transplant time. These products contain vitamin B-1, which encourages early root growth. This isn't necessary, but I've had good results in the past with these products.

5. Tomatoes need proper amounts of calcium and magnesium to bloom well. Calcium deficiencies result in blossom end rot, which will destroy your fruit. Dolomite (which I've used as a soil amendment) supplies both calcium and magnesium, and bone meal also supplies calcium. However--and this is important--your watering habits are critical when it comes to maintaining the right levels of calcium in the plant. Even if you have adequate calcium in the soil and in the plant itself, improper watering can still result in a calcium deficiency in the fruit. I'll talk more about this when I get to watering, but for now keep your watering absolutely consistent. Chances are, if you've amended your container mix with dolomite and you're feeding with a balanced vegetable fertilizer, your plant will have access to plenty of calcium and magnesium.

6. Foliar sprays containing micronutrients and calcium chloride can be used early in the growing season. Personally, I don't use foliar sprays for tomatoes, so I don't have any experience with it, but I know that commercial farms use these products. I would love to hear from someone who had experience with either ...

7. Finally, if you're really worried about optimal nutrient levels, have your soil professionally tested. This is really only necessary for in-ground tomatoes. Test results will indicate the levels of all macronutrients, as well as pH and other factors necessary for growth. Follow the lab's recommendations regarding supplementation.

It feels like there should be more, right? But fertilizing and feeding tomatoes only has to be as complicated as you want it to be. I guarantee you can grow wonderful, tasty tomatoes with a bag of standard granular vegetable fertilizer ...


  1. John,

    I have read a few other pieces of advice & wonder if you recommend these steps.

    One piece of advice is to crumble egg shells into your tomato soil through the growing season as a calcium additive. The second was to cover the soil with grass clippings, being careful to keep them away from the tomato stalk.

    What do you think?

  2. Bonnie,

    I've heard of the egg shell thing ... like you said, people use it as a calcium supplement. My concern would be the rate of break-down for the shells. I couldn't find any solid research on how quickly powdered egg shells break down in soil, so it's hard to say for sure how accessible that calcium would be. But this is one of those things that has been around for years and years and years, and I'm always surprised at how often the "old wives tales" turn out to have a solid grain of truth in them. Personally, though, if I was going to use egg shells, I think I would compost them first in my compost pile to make sure they were available.

    As for grass clippings, they are often used as a mulch and natural fertilizer. According to the University of Missouri, grass clippings are about a 4:2:1 natural fertilizer. This would be great for landscape plants, but for tomatoes, I'd be a little concerned about the relatively high nitrogen. Then again, you'd probably be using them in such small quantities that the main benefit would be moisture retention in the soil. So my advice is that if you want to use grass clippings, I'd make doubly sure to use a good vegetable fertilizer high in phosphorus and potassium to offset the extra nitrogen.

    Or I would add them with the eggshells to my compost pile :)

  3. I really like your blog very informative. This will help me a lot as I have started container gardening. I am a pepper farmer but I also grow vegetables in my back yard for the house.

  4. Alexia,

    Glad you like it! I'm on hiatus right now because we don't grow tomatoes in the summer, but I'm already starting to look forward to the season.

  5. Alexia,

    and p.s., I just checked out your pepper site. Very cool!

  6. I'm so glad I found a South Florida tomato gardening blog. Last weekend I transplanted by first tomato seedling, a green zebra, to an 18 gallon container on my balcony. I used the organic slow release fertilizer I had on hand, Dynamite Organic Fertilizer 10-2-3. According to your post, a very bad mix. I'm afraid of getting blossom end rot & no tomatoes. Would using Espoma Organic Traditions Garden Lime help? If it does, how would I apply it? I just got a bag from Home Depot.

    When planting the seedling I deep planted it & added 4 eggshells to the hole & container soil. Also I've been watering it every day.

    I know I'm super late but I didn't know better when I bought the seedling at the nursery or started seeds in my living room. I'm just keeping my fingers crossed.

    Thanks for your help!