It requires major changes in water management, but applying water-soluble potassium through a drip irrigation system offers tomato growers a number of money-saving benefits over other fertilization methods.

TOMATO HARVEST depends on a sound nutrition program from transplant through the growing season.

For example, such fertigation can reduce nutrient application rates while using significantly less water than applying a fertilizer with overhead sprinklers. Compared to broadcasting or sidedressing potassium, fertigation results in more efficient use of the nutrient through more accurate, more uniform application and improved nutrient uptake by the tomato plant.

Also, with fertigation, applying fertilizer requires much less time and labor and eliminates any soil compaction caused by fertilizer machinery. Plus, fertigation offers much more flexibility in the timing and rate of nutrient applications through the season.

Potassium plays a key role in improving yield, quality and shelf life of tomatoes. In fact, this crop requires more potassium than any other nutrient. While uptake depends on availability of potassium in the soil, a 50-ton-per-acre crop of processing tomatoes typically can absorb from about 300 to 400 pounds of potassium per acre.

Without enough potassium, tomatoes can suffer from such disorders as stunted growth, uneven ripening, yellow shoulder, graywall and soft, puffy and irregularly-shaped fruits that are low in acidity and more susceptible to ripening diseases.

Fertigation on poor soils

Potassium fertilization of tomatoes is particularly challenging in poor soil conditions, like those which face Florida growers. They use raised beds to produce round, Roma and grape tomatoes for the fresh market on soils naturally low in potassium. These soils range from coarse “ball bearing” sands” and fine “sugar” sands in many areas to pulverized limestone in others.

High average temperatures and abundant annual rainfall has left many agricultural soils with extremely low levels of organic matter. That, plus the porous nature of these sands has left Florida’s soils with very low water holding and cation exchange capacity, reports Eric Simonne. Formerly the statewide Extension water and nutrient management specialist for vegetables with the University of Florida, he is now a district Extension director.

“Low native fertility and low cation exchange capacity dictate that growers use high rates of fertilizer to supply all the essential nutrients that their crops require,” he says. “This makes leaching a distinct possibility, especially under high rainfall or poor irrigation management.

“Although potassium doesn’t pose a threat to water quality, loading of surface water with nitrogen and phosphorus leached from these soils can adversely affect nearby environmentally sensitive areas.”

Except for fields with perched water tables, where growers use subsurface irrigation for growing tomatoes, Florida farmers irrigate their tomato beds with drip systems. For these growers with sandy soils testing very low in potassium (Mehlich-1 extractraction), Simonne recommends broadcasting about 25 percent of the total amount of K20 required for the season in the bed area. The remaining K20, is applied through the drip system daily or twice a week at rates ranging from 1½ pounds per acre per day when plants are small to 3 pounds per day for fully-grown plants.

As he points out, a fertilization plan is no better than the irrigation plan. “If you put down the correct amount of nutrient and then wash it away with too much water, you’ve provided your crop the wrong amount of fertilizer,” he says.

The California approach

In California, where most agricultural soils in California contain sufficient potassium and micronutrients to produce a tomato crop, growers of processing tomatoes can also benefit from fertigation using drip systems, notes agronomist Tim Hartz, Extension specialist, University of California.

Because of higher yield expectations and a more limited root zone from which to draw, potassium fertilization using drip irrigation is likely to increase yields within fields with soil test levels of exchangeable potassium up to 200 ppm (based on ammonium acetate extraction) or even 300 ppm, he reports. That compares to conventionally-irrigated fields where potassium fertilization is unlikely to improve yields with exchangeable potassium levels greater than 130 ppm.

“On the positive side, drip irrigation provides a way to effectively deliver potassium to the root zone, minimizing the soil potassium fixation that can limit the effectiveness of conventional preplant or sidedress potassium applications,” Hartz says.

With drip irrigation, multiple applications throughout the growing season typically replace the traditional sidedress application. Research has shown that fertigation more often than once a week is usually unnecessary, he notes.

Nutrient uptake by tomato plants is slow until fruit set begins, Hartz explains. Then, it increases much more rapidly until the fruit begin to ripen. During that time, nutrient uptake remains relatively constant at about 6 pounds of potassium per acre per day. “That’s when fertigation management is critical,” Hartz says. “Nutrient uptake slows in the final four to five weeks before harvest, with the fruit drawing nutrients out of the vine. In most cases, it is unnecessary to apply fertilizer during this final period.

Where potassium fertilization is appropriate, fertigation will be most effective during fruit set, he notes. “Even in soil of limited potassium availability, tomato plants can usually take up enough of the nutrient to support early vine growth,” Harts says. “But, when fruit set begins, crop uptake quickly exceeds the soil supply. Maintaining the developing fruit, stresses the vine and later-setting fruit are aborted. Concentrating potassium fertigation during fruit set minimizes this vine stress and maximizes fruit set.”

Because potassium levels in fruit is typically 200 to 250 pounds per acre (equivalent to 240 to 300 pounds of K2O), application rates lower than that ends up mining the potassium in the soil. However, the first 100 pounds of K2O per acre would probably achieve most of the potential yield benefit, Hartz explains. After that, the payoff from additional fertigation declines.

At the same time, preventing or reducing yellow shoulder often requires a much higher level of soil potassium fertility than needed to maximize yield. “However, since growers are not usually compensated for reducing yellow shoulder, potassium fertigation management is usually directed toward yield maximization,” Hartz says. “Contrary to a widely held belief, fruit soluble solids concentration is unlikely to be affected by potassium fertigation.