Whether of the home-grown or commercial variety, tomato season will soon be upon us. To bring the wonderful, delicate fruit to harvest in a few months will require ample sunlight, water and proper management – including a balanced fertilizer program.

Many commercial tomato producers have adopted a relatively new technology, which involves the use of black plastic and a number of non-plastic components to help growers increase yields and harvest an earlier, less disease-free crop.

Currently, 95 percent of commercial tomato production is grown using plasticulture. Producers cover raised beds with black plastic, which, in turn, reduces weed pressure, leaching of fertilizers, particularly on light, sandy soils and soil and wind erosion of their fields. The plastic also helps provide better management of certain insect pests.

Because the rows are covered with plastic, the farmer can’t do lay-by or side-dressing like he would in a garden or in conventional row crops. Any additional fertilizer applications must be done through the drip tubing in the irrigation water.

Craig Anderson
CRAIG ANDERSON points to tomatoes on display at a University of Arkansas Field Day.

“What I like to recommend is half the nitrogen, all phosphorus and half the potash goes into the beds when they’re formed and sealed with plastic,” says Craig Anderson, Extension horticulture specialist for vegetables with the University of Arkansas. “The remaining nitrogen and potassium will be provided to the plants through the drip irrigation lines.”

In order to maintain good distribution of water and nutrients, care should be taken with the layout and design of the drip irrigation system, he says. To prevent uneven growth, producers must ensure even flow rates down the row.

One advantage of providing fertilizer through the drip-irrigation lines is more control over nutrient levels. Amounts can easily be tailored to the plants’ needs making the fertilizing process much more efficient.

A grower must also adjust fertilizer applications to the cultivars being grown. “They aren’t all the same – some have more fertilizer demands, others less,” says Anderson. “One formula for all doesn’t really work.”

Potassium is an easily leachable element and “we can apply it in small enough quantities through drip lines that it isn’t an issue,” he notes, adding that growers can also apply magnesium sulfate/Epsom salt through the lines.

Things have been “fairly even keel” for the Arkansas tomato crop over the last couple of years, says Andersen. The largest impact on the state’s growers has been the price of inputs. (Arkansas grows from 1,200 to 1,500 acres of fresh tomatoes annually.)

Because of input prices “they’re paying much more attention to details and how to wisely and efficiently use everything from fertilizer to drip tape. Economic conditions are forcing growers to be really good managers.”

Besides N, P and K, tomatoes also require calcium, which puts a premium on proper soil pH. The latter is critical for the prevention of blossom-end rot, one of the major disorders seen in Mid-South tomato production.

“Generally, we recommend soil pH be above 6.2 to ensure adequate amounts of calcium in the soil,” says Andersen. Micro-elements in the soil aren’t usually a problem although we find that too low a soil pH will lead to increased availability of manganese and aluminum, which are toxic to tomato plants. That’s a problem we see in low pH soils.”

For home gardens, all the above principals hold true. However , there are differences, cautions Andersen.

A typical 10X10 home garden plot needs one pound of a complete fertilizer; something like 13-13-13 or 10-20-10.

“The best way for a homeowner to measure that – something that’s handy and accurate since most people don’t carry scales out to the garden – is to use a good, old one-pint mayonnaise jar. That holds a pound of fertilizer.”

That one pound often doesn’t appear adequate for a 10X10 plot. However, “when you look at how tomatoes require nutrients, we’re trying to fertilize them on a nutrient demand curve. That means when they start off as young plants, the demand for nutrients is rather low. As they set fruit and that fruit is sizing up, the demand increases dramatically and that’s when more fertilizer is added. Then, after the fruit has sized up, the demand drops off and we quit adding fertilizer. So there’s a lull, a peak and another lull.”

As for forms of nitrogen the plant requires, “if you have about 50/50 in both ammonia and nitrate forms that usually works out best. If you go 100 percent nitrate, the fertilizer is available to the plant immediately and provides a quick burst of growth. But it disappears very quickly and that’s why I prefer the mix.”

Many growers like to use chicken litter or other barnyard manures. Andersen says those can contain up to 5 percent N, P, and K: 5-5-5.

“It may not be as much as that, but can be. Growers must realize that if more than 10 pounds of manure is added per 100 square feet, it’s probably to the point where over-fertilizing has occurred. Over-fertilizing with nitrogen leads to vegetative growth and reduces the yield of fruit dramatically. Whether it’s tomatoes, squash or (a similar crop) too much nitrogen suppresses flower and fruit formation.”

Andersen recommends when planting tomato transplants a starter solution be used. A formula for that: one tablespoon of a complete soluble fertile, something high in phosphorus, in one gallon of water. Apply one cup per plant. The phosphorus in the mix stimulates root growth and prevents transplant shock.

Commercially, the same is done but with a bit higher phosphorus and a bit lower nitrogen.

For the tomato gardener, Extension suggests having soil tests done ahead of time and following resulting recommendations. Normally that means “1.5 tablespoons of a complete fertilizer per standard salad tomato or cherry tomato plant in about six square feet of space.

“Again, that doesn’t sound like a lot of fertilizer. However, that amount will keep the plants from going vegetative and it follows the nutrient demand curve for fertilizer utilization.”

At the time fruit begins to set on the plants and has increased from the size of a marble to a quarter, come back with another tablespoon of fertilizer per plant as a lay-by or side-dressing. Depending on how sandy the soil is, or how much watering is being done, that will be repeated every two to three weeks.

Then, after the fruit has sized up and is maturing, stop fertilizing. There should be enough residual fertilizer in the soil to provide the plants’ needs.

An indicator of over-fertilization will be on the vine’s flowering branch. “If it goes from flowers and, at the very tip, reverts back to leaves, that’s an indicator of over-fertilization of nitrogen and it’s time to back off.”

A critical thing with tomato production is to maintain an even moisture level in the soil – not too wet, not too dry. The best tomato soils are well-drained and well-aerated.