A basic soil test, in-season tissue analysis and drip irrigation may improve nutrient efficiency, yield, and quality in commercial tomato production.

Knowing what the crop needs at specific times and applying the proper amount, at the proper rate also decreases likelihood of leaching and damage to the environment, say vegetable fertility experts.

Lynn Brandenberger, Oklahoma State University Cooperative Extension food crops specialist, says a soil test is the first step.

“Tomatoes will produce in soils that are a little more acidic than is recommended for other vegetable crops,” he says, noting recommendations typically call for a soil pH of 6.5 to 6.8 for vegetable crops. “Tomatoes can take a little more acidity, a range of 5.8 to 6.8. Growers might get away with growing tomatoes in soil that’s too acid for something like spinach.”

Tomatoes typically need a lot of potassium. Recommendations call for up to 225 pounds per acre. Phosphorus requirement is about 150 pounds per acre and nitrogen application rate would be 100 pounds per acre.

“We recommend applying the phosphorus and potassium before transplanting, along with a little nitrogen” Brandenberger says. The rest of the nitrogen would be side-dressed during the season. He says about one-fourth of the total nitrogen need would go down before transplant. “The plants need very little nitrogen early on.”

Another 25 pounds of nitrogen would go out about three weeks later with two similar applications made three weeks apart.

If tomato producers irrigate with subsurface drip systems, Brandenberger says nitrogen application “is another ball game. With drip irrigation, producers would spoon feed the nitrogen (and perhaps some potassium). They would still apply phosphorus before transplanting and about one-fourth the recommended nitrogen rate.”

He said growers need a basic formula to get started but then will “tweak fertility to suit conditions.”

Tissue analysis may be a good investment to insure tomato plants receive adequate nutrition during critical growth periods.

“Fertility is extremely important for tomatoes,” says Texas A&M AgriLife Extension vegetable specialist Joe Masabni at College Station. “In fact, inadequate fertility is one of the three primary mistakes made in tomato production.” Poor soil preparation and inadequate water are the other two, he says.

The tomato will tell producers that it needs nutrients. “Plants and fruit will exhibit symptoms of nutrient deficiency quickly,” Masabni says. With too little potassium, for instance, the fruit displays what is known as “yellow shoulder” or uneven ripening.

“That can be a big problem, especially in greenhouse production,” Masabni says. Plants in greenhouses tend to grow faster than plants grown outside and growth may outpace nutrient availability. “Plants may get a bit behind.”

Various sources indicate that potassium is essential for tomato production, one of three essential nutrients required by all plants for best growth. Potassium deficiency may result in weak plant growth and poor flowering, which will result in a poor harvest. Problems may continue to the table as well with poor flavor.

Another symptom of potassium deficiency is a less vigorous plant. Potassium strengthens the plants against adverse effect of droughts, chills, and frosts. It also helps the plant absorb and retain water and increases the plant’s resistance to disease.

Calcium deficiency may result in blossom end rot. “Iron deficiency is also a concern,” Masabni says. “Symptoms may be unique to tomato plants because the leaf turns pale and also may show interveinal chlorosis.”

He says growers ideally should use in-season leaf analyses to determine nutrient needs. “They can err on the safe side by knowing what and when the plant needs certain minerals. Also, producers must consider more than just nitrogen, phosphorus and potassium. Don’t ignore micro-nutrients.”

He agrees that a soil test is the first step. “Then get leaf analyses during various stages of plant growth. Know what the plant needs and when it needs it. Commercial operations may rely on leaf analysis throughout the growing season,” Masabni says, to evaluate in-season plant needs.

A University of Florida fact sheets offers guidelines on fertilizing vegetable plants, including tomatoes, through a drip irrigation system. The report details significant advantages for injecting part of the nitrogen and potassium requirement through the drip system. Tomato fruit quality may improve with nitrogen and potassium applied through drip irrigation, compared to applying all fertilizer preplant.

It’s also more efficient, according to the report, which states: “Fertilizer can be prescriptively applied during the season in amounts the crop needs and at particular times when the nutrients are needed. (Drip irrigation) increases efficiency of fertilizer applications and should result in small, controlled amounts of fertilizer applied throughout the season in contrast to large amounts placed within or on the bed under the plastic mulch at the beginning of the season.”

Prescription fertilizer applications also reduce potential for leaching and groundwater pollution.
Some fertilizer will be applied preplant, including all the phosphorus and up to 40 percent of nitrogen and potassium. “In areas with low nitrogen and potassium levels, growers may broadcast or band starter fertilizer below the bed, 2 inches to 4 inches below the bed surface and to the side of the plant.”

A slow-release fertilizer may be a good option for a starter, followed by 60 percent to 80 percent nitrogen and potassium requirements applied in increments during the cropping season.

Several sources of nitrogen and potassium are available for injecting through drip irrigation systems. Nitrogen sources include ammonium nitrate, calcium nitrate, various nitrogen solutions and potassium nitrate.

Potassium sources for drip irrigation include potassium nitrate, potassium sulfate and potassium chloride.

In some cases, commercial liquid fertilizers are a mixture of nitrogen and potassium.