How to Fertilize Tomatoes Without Overfeeding

Take the soil sample before you add compost, lime or fertilizer, and follow the lab's directions for depth and mixing. The report gives the pH, the phosphorus and potassium already in the bed, and a fertilizer recommendation for the crop. Nitrogen is the exception, because standard tests do not measure it. It changes too fast with leaching, loss to the air, soil organisms and plant uptake, so the nitrogen figure comes from what the crop needs and from past data.

Read the phosphorus and potassium lines before you choose a product. Extra of either on soil that already has plenty will not benefit roots and may harm beneficial soil microbes, so a bed with high test values calls for a nitrogen-only fertilizer. In Minnesota, where many soils have enough phosphorus, the extension advice is a low- or no-phosphorus fertilizer unless the report specifically recommends more.

The same report gives the pH. Minnesota calls a pH of 5.5 to 7 ideal, while Clemson, writing for South Carolina, aims for 6.0 to 6.5 and tills lime in at least 3 months before planting, according to the report's recommendation.

On the bag, the three numbers are the percent by weight of nitrogen, phosphate and potash. A 20-10-20 holds twice the nitrogen of a 10-10-10, so half as much of it delivers the same nitrogen. Start the arithmetic with nitrogen and then phosphate, because an excess of either can harm the environment.

To turn a nitrogen rate into product, divide it by the bag's nitrogen percentage written as a decimal. At 0.3 pound (4.8 ounces) of nitrogen per 100 square feet, a 46-0-0 urea works out to 4.8 ÷ 0.46, a little over 10 ounces. The nitrogen recommendation on a soil test report takes the place of both figures.

A seedling draws its early energy from nutrients stored in the seed, and the two extension guides for soilless mix differ on when feeding begins.

A seedling whose stem stretches and leans over is short of bright light directly overhead, so the light is the first thing to check before the fertilizer.

In the garden, the preplant fertilizer is spread evenly and tilled in before the transplants go in, so the nutrients are available to them. Clemson works it in to about 6 inches, while Maryland mixes dry fertilizers into the top 2 to 4 inches so the pellets sit closer to the roots and are less likely to be lost to wind or heavy rain. A starter fertilizer is also recommended for tomato transplants.

The side-dress goes on when the first fruits appear, and Minnesota waits until the fruits start to enlarge. Too much nitrogen before then leads to bushy, leafy plants that are slow to bear fruit.

Organic matter releases nutrients slowly during the growing season, and it may not supply enough in early spring or when fruits start to form.

Clemson, writing for South Carolina, counts from planting instead, with one side-dress three to four weeks after planting, split into two applications three to four weeks apart on sandier soil to reduce losses to leaching.

If the flowers fall off instead of setting fruit, look at the thermometer before the fertilizer. Alabama Extension reports that tomato flowers often abort when day temperatures exceed 85°F and night temperatures exceed 72°F, and a week or less of that heat should not cause much trouble. Temperatures below 55°F can also inhibit fruit set. The same page names three more causes of blossom drop.

A bush-type tomato sets a fixed number of fruits that ripen within four to six weeks, while a vining tomato keeps flowering and ripening fruit until cold weather stops it. Additional fertilizer may be needed beyond the first side-dress, depending on plant growth, fruit load and soil fertility.

The first-fruit rate per 100 feet of row is in the table below. Clemson, writing for South Carolina, uses 1 pound of calcium nitrate (15.5-0-0) per 100 square feet, which is about 30 feet of row. Granules piled next to a stem or left on the foliage can burn the plant's tissues, so the band sits off to the side of the row.

Once fruit is forming, uneven watering shows up on the fruit. When soil moisture swings during fruit growth, blossom-end rot can develop, and 1 inch of rain or irrigation a week is ideal.

An inch wets a sandy soil about 10 inches deep and a heavy clay about 6, so a trowel shows whether the water reached the roots. If only an inch or two is wet, the water should keep running. The deep versus frequent watering guide covers that trade-off.

An organic mulch such as woodchips, compost or leaf mold holds water in the soil by reducing evaporation, and it shades the soil to keep the root zone cooler.

A pot holds far less soil than a bed and gets watered far more often, so nutrients leave through the drainage holes faster. A mix with fertilizer added feeds a container plant for about 8 to 10 weeks, according to Clemson, and after that a water-soluble fertilizer at the label rate every 2 to 3 weeks takes over.

The Chicago Botanic Garden suggests the same 2 to 3 weeks even when the mix contains slow-release fertilizer, especially once the plants begin to flower.

Leaf color works as the cue between feedings, because rich, dark green leaves that start to fade are a good sign the plant needs a boost. Water until it runs out of the drainage holes, and never let the mix stay soggy.

Pale leaves, slow growth and low yield are the signs of a nutrient shortage, but four other problems produce the same signs:

An established plant that still shows the signs once those are ruled out may need feeding.

For the bushy plant, look at nitrogen first, because too much of it gives lush leaf growth at the expense of fruit and may make the plant more vulnerable to pest or disease problems. The soil test and a written record of every application show whether the total went past the recommendation.

Salt buildup in a pot reduces growth and can inhibit the plant's uptake of water, and the crust shows around the soil line, the drainage hole or the outside of a clay pot. Watering from the top until water runs out the bottom washes excess salts out, and doing it before you fertilize keeps the wash from carrying away the fertilizer you just added.

In extreme heat, plant processes slow down and added fertilizer promotes growth the plant cannot support, which adds stress. Fertilizing too early in spring is discouraged for a similar reason, since cool soil limits nutrient release from organic matter.