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Plant Expert Cautions Against Over-Fertilizing Tomatoes
Kansas Ag Connection - 06/29/2020

With each passing day, Ward Upham knows that many Kansas gardeners are building anticipation for one of the most exciting times of summer: Tomato harvest.

But first, he says, there's a little more work to do.

"In order to yield well, tomatoes need to be side-dressed with a nitrogen fertilizer three times during the season," said Upham, a horticulture specialist with K-State Research and Extension.

"The first side-dressing should go down one to two weeks before the first tomato ripens," he added. For those who may have planted early and are hoping for tomatoes on the Fourth of July, that is coming quickly.

The most common nitrogen-only fertilizers for tomatoes include nitrate of soda, urea, ammonium sulfate and blood meal. Upham suggests the following rates for each fertilizer:

- Nitrate of soda (16-0-0). Apply 2/3 pounds (or 1 1/2 cups) fertilizer per 30 feet of row.

- Urea (46-0-0). Apply four ounces (1/2 cup) fertilizer per 30 feet of row.

- Ammonium sulfate (21-0-0). Apply one-half pound (1 cup) fertilizer per 30 feet of row.

- Blood meal (12-1.5-6). Apply 14 ounces (1.75 cups) fertilizer per 30 feet of row.

The second side-dressing should be applied two weeks after the first tomato ripens, and the third should be one month after the second side-dressing, Upham said.

"If you can't find the recommended nitrogen-only fertilizers, you can use a lawn fertilizer that is about 30 percent nitrogen (the first number of the three) and apply it at a rate of one-third pound (3/4 cup) per 30 feet of row," Upham said. "Do not use a fertilizer that contains a weed killer or weed preventer."

Many gardeners also are gearing up to bring in the year's first harvest of new potatoes.

"New potatoes are immature and should be about the size of walnuts right now," Upham said. "To check if they're ready, pull soil away from the base of the plants to see if the tubers are the desired size. If they are, dig entire plants and allow the skins of the exposed tubers to dry for several hours before gathering."

Upham said the young potatoes are very tender and prone to the skin 'slipping' if they are not allowed to dry.

"Even then," he said, "these immature potatoes will not store well. Red-skinned varieties are often preferred because they are the earliest to produce."

Upham and his colleagues in K-State's Department of Horticulture and Natural Resources produce a weekly Horticulture Newsletter with tips for keeping yards and gardens healthy and beautiful year-round. The newsletter is available to view online or can be delivered by email each week.

Interested persons can also send their yard and garden-related questions to Upham at

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