Irrigation Plays Key Role in Determining Yields and Quality

By Lisa Lieberman
From The Tomato Magazine, October 2003 5

W hen it comes to growing tomatoes, the single most important factor that affects yield and fruit quality is irrigation. Growers many have a certain margin for error when applying pesticides or fertilizers, but in improperly irrigated fields, yield losses can reach up to 25 percent of the entire crop, according to Tim Hartz, an extension specialist with the University of California at Davis.

“Improper irrigation is the number one cause of crop yield losses in tomatoes,” Hartz says.

Every year, tomato growers perform a difficult tightrope walk act by trying to balance their watering regiments so that their plants get enough, but not too much, water, Hartz says. Growers don’t want to overwater their plants because of the risk of exposing them to Phytophthora root rot. But, if they don’t get enough water, the plants will become stressed and underproduce.

Not Always Easy
While this may sound simple, it’s not always easy to tell how much water plants are getting, the researcher points out.

“Growers assume that when they put water on the fields, that the water is going to go down a couple of feet,” Hartz says. “If they go and probe their fields, sometimes they’ll find that the water didn’t get very far down because of the soil structure. And even though the grower’s irrigation schedule is correct, the plants aren’t getting enough water.”

When they irrigate, growers need to take a number of factors into account as well as the plants’ basic water requirements, Hartz says. The most important factors are soil type and how well different soil types absorb water.

Growers tend to assume that water will penetrate sandier soils more easily than clay soils. What determines water penetration has more to do with how much organic matter the soil has and the amount of magnesium and calcium in the soil, the researcher says. Soils low in organic matter, or those which have more magnesium than calcium in the top layers, can have a difficult time with water intake. Water tends to well up at the surface and create a seal, making it difficult for further water applications to penetrate the soil.

To make sure that his soil gets good water penetration, Jeff Dolan, field manager for the DiMare Co., Newman, Calif., spoon-feeds his tomato plants with calcium every time he irrigates. The trick to good irrigation practices, aside from making sure there’s enough calcium in the top layers of the soil, is to check the fields regularly by hand to see if they need water, Dolan points out.

Check the Entire Field
“We have machines that monitor soil moisture, but the problem with them is that they’re just monitoring one spot in the field. You need to check different areas of the field, including the headlands, the drain ends and the middle,” Dolan says.

He or his crewmembers monitor most of their fields daily, checking soil moisture levels by hand. By keeping daily tabs on their fields’ irrigation needs, it’s easier to avoid one of the most common pitfalls growers fall into, which is waiting too long between irrigations.

“When they wait too long between irrigations, they also tend to overirrigate,” Dolan warns.

Too much water at once can smother the plants by cutting off the roots’ oxygen supply, the researcher says. Although plants sustaining damage to their primary roots earlier in the season may grow secondary roots to compensate for their initial losses, the plants never quite recover fully enough to be able to produce optimum yield.

While growers need to be careful to avoid overirrigating their plants in mid- to late season, they should remember that early in the season most plants need plenty of water. That helps avoid stressing the plants while they are setting fruit.

“It’s important to begin the season with a full soil moisture profile. If you stress the plants before the end of fruit set, then you risk sacrificing yield,” Hartz warns.

Although furrow irrigation is a common practice among west side growers in Fresno County, Dolan uses sprinkler irrigation in the beginning of the season as his transplants are moved to the fields.

When the transplants are tiny and the water has to move 30 inches across the furrows to reach the plants, most are not getting enough water fast enough, Dolan warns.

“Using furrow irrigation, it takes too long to get water to the transplants,” he adds. “We can get the same amount of water to the plants with sprinkler irrigation in six to 12 hours as we can in 48 hours with furrow irrigation.”

Once the plants have taken off and are actively growing it becomes more difficult to irrigate sufficiently without over irrigating and inducing Phytophthora, he reminds.

“It’s a fine line later in the season between getting decent water penetration in the soil and finding all your plants dead because of root rot from overwatering,” Hartz notes.

With irrigation, growers also need to remember that water intake tends to become slower as the season progresses. Successive irrigations wash out deepened furrows, flattening them and making them less able to absorb water.

“Irrigation water may also push calcium away form surface soils downward,” he explains. “This means that as you go through the season, you’re going to get less water infiltration for the same amounts of water you apply to the fields.”

These fluctuations in irrigation conditions make it even more critical for farmers to monitor their fields on a daily basis if they are to properly determine moisture requirements, Dolan notes.