Tomatoes are heavy feeders that generally need quite a bit of fertilizer to get them through the season. The tricky part is figuring out which fertilizer is best, how much to use, and when to apply it. Don’t worry though; fertilizing tomatoes isn’t as involved as it may seem.
Your tomatoes may not need fertilizer at all if you dig a generous amount of well-rotted manure into the top 8 to 12 inches (20-30 cm.) of soil at planting time. Manure is nutrient-rich but tends to be low in phosphorus, so balance things out by adding a little bone meal along with the poop.
A Word about Fertilizer Ratios
If your soil is poor or if you don’t want to bother with manure, you can use a good quality fertilizer. Pay attention to the three numbers on the package, which indicate the levels of nitrogen (N), phosphorus (P), and potassium (K).
Some growers prefer to use a high-phosphorus fertilizer, indicated by a larger middle number. You can also keep things simple with a fertilizer especially formulated for tomatoes – usually with a ratio like 3-4-6 or 4-7-10.
Most importantly, don’t over-fertilize. Too little fertilizer is always better than too much.
The Hows and Whens
Unless you amend the soil with manure at planting time, you’ll want to boost the plants with a little fertilizer to get them off to a good start. Dig the hole, than place the fertilizer, in the amount recommended by the manufacturer, in the bottom. Cover the fertilizer with clean dirt to keep the fertilizer from direct contact with the roots.
Thereafter, side-dress your tomatoes with fertilizer every three to four weeks throughout the season.
To side-dress tomatoes, use the tip of your hoe to make a shallow furrow around the plant, at least 5 to 6 inches (12-15 cm.) from the stem.
Sprinkle the fertilizer in the furrow at a rate of 1 to 1 ½ tablespoons (15-21 ml.) for each plant. Cover the fertilizer with soil and mulch.
Be extremely careful not to get fertilizer on the leaves or stem. Water well immediately after feeding to carry the fertilizer evenly down to the roots.
A Note about Water-Soluble Fertilizer
You can also use a water-soluble fertilizer, which is often easier to apply, especially for tomatoes grown in containers. Just mix the fertilizer in the watering can at a rate of about 1 tablespoon (15 ml) per gallon (4 L). Apply every one to two weeks throughout the season.
If you’re using it in the garden, mix about 2 tablespoons (30 ml) per gallon (4 L).
You should avoid applying it during very hot or very dry conditions. Note that 4 L will cover approximately 4 square feet of your garden or the surface of ten 8’’ pots.