Fertilization of Tomatoes

Tomatoes have a low nitrogen requirement. Under high nitrogen conditions, vines grow excessively large at the expense of fruit production. More correctly stated, tomatoes are a fussy nitrogen feeder. On soils low in organic matter, tomatoes typically run out of nitrogen in mid-summer, reducing yields and predisposing the plants to Early Blight.

At transplanting, apply one to three applications (depending on soil organic content) of a water-soluble, “plant starter” fertilizer. This includes any of the water-soluble products like MiracleGro, Peters, RapidGro, Schults, etc. Transplants would have been “hardened off” (growth slowed) in the greenhouse. Water-soluble fertilizers stimulate renewed growth.

If the weather turns cold late spring after tomatoes are out (that is a week with daytime temperatures below 55ºF), use water-soluble fertilizers to stimulate growth when warm temperatures return. A week with daytime temperatures below 55 ºF stunts tomato growth, reducing yields.

Mid-summer –On low organic matter soils, tomatoes typically run out of nitrogen in mid to late summer. Yellowing of the foliage, starting with lower leaves, is the typical symptom of nitrogen stress. Low nitrogen in the plant allows Early Blight disease to spread like wildfire.  Keeping nitrogen levels up in mid to late summer is a primary means of Early Blight control and significantly improves yields.

Fertilize tomatoes lightly as the first fruits reach two-inches in diameter. Water-soluble fertilizers (such as MiracleGro, RapidGro, and Peters) used according to label directions make a good summer fertilizer supplement. Make applications every two to four weeks, depending on soil organic content. If using a dry granular fertilizer, apply 21-0-0 (ammonium sulfate) at the rate of one level tablespoon per plant. Sprinkle the granular fertilizer in a wide circle 12 to 20 inches out from the plant, and water in. Dry granular fertilizers can easily kill tomatoes if over-applied.