Tomato Fertilizer: What to Feed Tomatoes and When

Start with a soil test

Every source here begins the same way. Clemson Cooperative Extension calls a soil test always the best way to find out what a crop needs, and University of Minnesota Extension and UF/IFAS Gardening Solutions both start with one. Clemson says the report gives your soil’s pH, its nutrient levels and how much fertilizer and lime to add, and that following it prevents both shortages and the problems of too much: excessive leafy growth, delayed ripening, salt burn and wasted money. It also says nitrogen is not tested, because it moves through the soil too quickly; the report recommends it from the crop instead.

• pH. Clemson keeps tomato soil at pH 6.0 to 6.5, Penn State Extension at 6.2 to 6.8, UF/IFAS at 6.2 to 6.5 and Oregon State University Extension grows vegetables at 6.0 to 7.0.
• Timing. Clemson tests well ahead, at least three months before planting, because lime takes a few months to change the pH. Penn State says that if the season has already started, make a note to test this fall.
• Where. Your county extension office takes samples. Clemson mixes 8 to 10 or more small samples, from the surface to 6 inches deep, into one composite for the vegetable garden.

The numbers on the bag

A fertilizer label always lists three numbers in the same order: nitrogen, phosphorus and potassium, or N-P-K. Oregon State explains that a bag marked 5-15-10 is 5% nitrogen, 15% phosphorus (as phosphate) and 10% potassium (as potash), so 10 pounds of it holds half a pound of nitrogen, 1½ pounds of phosphate and 1 pound of potash. A fourth number, where there is one, is sulfur.

• Nitrogen (N) drives leafy growth. Oregon State says plants use the most just before and during rapid growth, and that rain and irrigation carry most unused nitrogen away, so it usually has to be added each year.
• Phosphorus (P) helps seedlings grow, especially in cool, wet spring weather, Oregon State says. Minnesota adds it only where a soil test calls for it, because many Minnesota soils have enough.
• Potassium (K) is important for disease resistance, according to Oregon State.

What ratio for tomatoes? Without a soil test, Penn State works in 2 to 3 pounds of a complete fertilizer such as 10-10-10 per 100 square feet before planting, and UF/IFAS feeds with a 6-8-8 or similar. Clemson puts all the phosphorus and potassium a soil test calls for, and part of the nitrogen, into the soil before planting, then side-dresses with calcium nitrate (15.5-0-0), which supplies nitrogen alone. Minnesota’s side-dressing choices are all nitrogen-heavy too, such as 46-0-0, 27-3-3 or 10-3-1. Oregon State adds that fertilizers labeled for a particular crop are generally an expensive alternative to a standard blend at the right rate.

More is not better. Minnesota says too much nitrogen gives bushy, leafy plants that are slow to bear fruit, and Penn State says a high-nitrogen fertilizer delays flowering and fruiting.

When to feed: planting, flowering, fruiting

• Before or at planting. Clemson broadcasts the preplant fertilizer and works it about 6 inches into the soil, so the nutrients are there when the transplants go in. Illinois Extension applies a starter fertilizer at transplanting, and Penn State waters new plants in with a diluted liquid starter fertilizer high in phosphorus, for root growth.
• While the plants grow and flower. Clemson side-dresses 1 pound of calcium nitrate per 100 square feet (30 feet of row) three to four weeks after planting, and splits it into two feedings three to four weeks apart on sandy soil, where it washes through faster. That is the dated column in the table below.
• Once fruit sets. Minnesota feeds alongside the row when the first fruits start to enlarge, and Illinois side-dresses when the first tomatoes reach the size of a golf ball, then again 3 and 6 weeks later. UF/IFAS fertilizes at planting and then regularly through the season.

The sources disagree on one point, and we print it rather than choose. Oregon State’s general vegetable advice for Oregon adds nitrogen to plants that turn pale and slow about four to five weeks after planting, and says not to give that extra nitrogen to peas or tomatoes. The eastern and southern sources above all side-dress tomatoes. Your soil test, and how your plants look, are the tiebreakers.

How to apply it

• Side-dress, do not dump. Clemson places side-dressed fertilizer 4 to 6 inches from the plants to avoid burning the roots. Minnesota spreads it in a 6-inch band alongside the row and scratches it into the surface.
• Keep it off the leaves. Illinois says not to get fertilizer on the leaves, and Clemson waters overhead after broadcasting to wash the granules off.
• Water it in. Oregon State says the nitrogen in chemical fertilizers dissolves and reaches the roots with water, so water the garden if no rain falls within a day.
• Per plant. Illinois gives its side-dressing as about 1 tablespoon of ammonium nitrate per plant, or 3 pounds of 10-10-10 per 100 feet of row where ammonium nitrate is not sold. Oregon State counts one large plant, such as a tomato, as about 5 square feet, so a rate given per 100 square feet divides by 20.
• No weed-and-feed. Minnesota says not to use any fertilizer that contains a weed killer, which may kill vegetable plants. For any product of that kind, follow the label and your state’s rules.

Liquid or granular, fish, compost and manure

Liquid or granular. UF/IFAS says either works for tomatoes, preferably in controlled-release form. Granular fertilizer goes into the soil before planting and alongside the row later; a liquid is how Penn State gives its starter feeding at planting. Oregon State says plants can use chemical fertilizers as soon as they are applied.

Organic or chemical. Oregon State says excellent gardens are grown either way, and that plants take up the same form of each nutrient from both. The difference is speed: soil organisms have to break organic sources down before plants can use them. So if you feed mostly with organic materials, it adds a small amount of a quicker nitrogen source early in the season, and names liquid fish, blood meal or a chemical fertilizer. Oregon State gives blood meal at 1.5 to 2 pounds per 100 square feet. None of the sources we read gives a fish fertilizer rate for tomatoes, so follow the label.

Compost and manure. Minnesota improves the soil with composted manure or other compost in spring or fall, and says that depending on how much organic matter you add, you may not need more fertilizer. Oregon State says organic matter improves soil structure as well as feeding it, and is used up over time, so add it regularly. Both say not to use fresh manure: Minnesota because it may carry harmful bacteria and weeds, and Oregon State because it can damage plants. Oregon State composts manure before using it in a vegetable garden, and digs any fresh manure in at least 90 days before harvest.

Homemade fertilizers: what the evidence says

Kitchen scraps on the tomato bed are a long tradition, and University of Illinois Extension notes that there is some research behind some of these claims. Here is what two extension services found when they looked.

• Eggshells. The idea is sound on its face: blossom end rot, the dark, leathery patch on the bottom of the fruit, is a shortage of calcium in the fruit. But Minnesota says that shortage almost never comes from soil short of calcium, since most soils have plenty. It comes from water: calcium reaches the fruit only with water, so uneven moisture and damaged roots cut it off. Minnesota and Illinois both say eggshells break down too slowly to help (Illinois says one to five years) and suggest putting them in the compost. Minnesota adds that they do no harm.
• Coffee grounds. Minnesota says used grounds contain carbon, nitrogen and other compounds that feed soil life and make an excellent addition to compost, but they have not been shown to lower soil pH. Illinois warns that a thick layer can stop water soaking in.
• Epsom salt. Epsom salt is magnesium sulfate, and Minnesota says it will not prevent blossom end rot, which is about calcium, and that too much magnesium can keep calcium out of the plant and make it worse. It finds no evidence that Epsom salt makes tomatoes or peppers more productive unless the soil is short of magnesium, which is most likely on sandy, acid soils, and says to use it only if a soil test shows a shortage. Sprayed on leaves, it can scorch them.

What does prevent blossom end rot, according to Minnesota, Illinois and Clemson: keep the soil evenly moist, mulch, do not cultivate close to the plants, and fertilize to a soil test. Illinois says soil applications of calcium seldom help. If your homemade routine has worked for you, these sources would add one thing: a soil test, so you know which part of it is doing the work.

Peppers, the short version

Peppers are fed much like tomatoes, slightly more lightly. Clemson calls their needs moderate and starts, again, with a soil test and a pH of 6.0 to 6.5. It works the preplant fertilizer into the soil, then side-dresses half a pound of calcium nitrate per 100 square feet (30 feet of row) three to four weeks after planting and the same again three to four weeks later, 4 to 6 inches from the plants. Minnesota adds phosphorus and potassium only as a soil test says, and warns, as for tomatoes, that too much nitrogen gives bushy, leafy plants that are slow to bear. Clemson reads a lot of foliage with few blooms and fruit as a sign of too much nitrogen.

Peppers go out later than tomatoes, so their feeding starts later too: see the pepper column in the table above, or the full pepper planting guide. For tomato varieties, planting depth and staking, see the tomato planting guide.

Common mistakes

• Feeding without a soil test. Clemson says it risks both shortages and too much, and wastes money.
• Too much nitrogen. Minnesota and Penn State say it gives leafy plants that are slow to fruit.
• Phosphorus by habit. Minnesota uses a low- or no-phosphorus fertilizer unless a soil test asks for it.
• Fertilizer against the stem or on the leaves. Clemson keeps it 4 to 6 inches away; Illinois keeps it off the leaves.
• Fresh manure. Minnesota and Oregon State compost it first.
• Blaming the soil for blossom end rot. Minnesota says it is usually a water problem, not a calcium one.

Where the sources run out

We found no Texas, Louisiana, Alabama or Mississippi source on feeding tomatoes that we read for this page; the Southern advice here is Clemson’s and UF/IFAS’s. No source we read gives a rate for fish fertilizer on tomatoes, and none ties a feeding to a frost date: the side-dressing dates in the table count Clemson’s weeks-after-planting rule from our set-out dates. For the rest of the season, see when to start seeds indoors by zone and how to plan next year’s garden.