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Probing peppers’ water needs

Middle East meets North America

March 13, 2008  By Marcia Wood

Crisp, crunchy bell peppers in a rainbow of bright colours add texture,
flavour, and pizzazz to new and traditional foods in both the Middle
East and North America.

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Water scientist Naem Mazahrih (left), engineer Nedal Katbeh-Bader (kneeling) and ARS research leader Tom Trout (holding the clipboard) use a pressure chamber to check pepper plants’ water status while soil scientist Ron Seligman (in the background) checks soil-water content with a neutron probe. Photo courtesy of USDA-ARS
In the San Joaquin Valley of California, visiting soil scientist Ron Seligman collects data underground inside a lysimeter while visiting engineer Nedal Katbeh-Bader (left) and visiting soil and water scientist Naem Mazahrih place equipment to compare accuracy of soil-water sensors. Photo courtesy of USDA-ARS

Middle East meets North America

Crisp, crunchy bell peppers in a rainbow of bright colours add texture,
flavour, and pizzazz to new and traditional foods in both the Middle
East and North America.


But the competition between growers and city dwellers for clean, fresh water can be intense in both areas of the world.

How much water does a bell pepper need?
The accelerating need for good-quality water has increased the thirst for knowledge of how to grow premium produce – like delicious bell peppers – with the least possible amount of water.

“There’s very little information about pepper plants’ water use here on the west side,” says James E. Ayars, an agricultural engineer based at the San Joaquin Valley Agricultural Sciences Center at Parlier, California.

Applying too little water “can stress the plants, which can lower their resistance to attack by insects or diseases,” notes soil scientist Steven Evett. Applying too much water is not only wasteful but also “poses a risk that the excess water will seep into the underground water supply, perhaps bringing farm chemicals, salts, and toxic elements with it,” he points out.

Research on how to use the water more efficiently is, he says, vital to the future success of farms and orchards faced with declining water supplies.

Pinpointing peppers’ precise water needs is a focal point of a lively collaboration between Ayars, Evett, and Middle Eastern scientists Ron Seligman from Rehovet, Israel; Naem T. A. Mazahrih of Ajloun, Jordan; and Nedal A.Q. Katbeh-Bader of Hebron in the Palestinian Authority.
Agricultural engineer Thomas J. Trout, now in Colorado, participated in the
pepper research while based at the Parlier laboratory.

Soil scientist Ron Seligman observes a data acquisition and control system used in a greenhouse study of nutrient and water uptake by pepper plants.

Array of irrigation options explored
The visiting scientists worked with Ayars and Trout to track plants’ water use at sites that had one of three common types of irrigation systems:
•    furrow, in which water flows down channels between crop rows;
•    surface drip, in which water is delivered to plants a drop at a time via a network of flexible black tubing;
•    subsurface drip, in which the tubing is buried beneath the surface and the water is delivered directly to plant roots, where it’s most needed.

Peppers received one of four different amounts of irrigation water from the furrows or tubes. These amounts ranged from replacing some, all, or more than all of the water that plants took up from the soil.

Scientists monitored the pepper plants’ water use by measuring the change in weight of a research lysimeter – a large, in-ground, soil-filled box with pepper plants growing on top of it and a huge scale beneath it.

The lysimeter data is a first step toward determining what’s known as a “crop
coefficient” specifically for bell peppers. The region’s farm advisors, irrigation
specialists, and growers – who may not have the benefit of their own research lysimeter – can use the figure to calculate how much water their pepper crop used in the previous several days.

“From there,” says Evett, “you do some pretty straightforward adding and subtracting and use your answer to decide how much of that water you want to replenish, if any, and when you should do that.”

Evett’s Bushland laboratory and Texas A&M University make crop-water-use information available every day of the year for major crops grown in the Panhandle. “It’s invaluable for irrigation scheduling,” he says.

Soil probes ‘pepper’ the plots
Bell pepper plants were also the crop of choice for an experiment that pinpointed the accuracy of an array of different soil-moisture sensors. Scientists, irrigation consultants, and growers lower these devices through vertical metal or plastic pipes access tubes to get a real-time, underground reading of the amount of moisture in the soil profile. It’s one way to determine how much water is available to slake plants’ thirst.

If probes are “sufficiently accurate and reliable, they could be an alternative to lysimeters for tracking a crop’s water use,” says Evett. He designed and led the soil-sensor study, a segment of a five-year investigation.

In all, the scientists evaluated five new kinds of sensors at 36 sites throughout the amply-drip-irrigated pepper plots.

The scientists monitored sensor readings for the entire growing season. The study spotlighted the worrisome variability in water-content estimates from the newer sensors.

“We also showed how many sensors of the same type you would need to get enough readings for a reasonably accurate soil-moisture estimate,” explains Evett. The numbers were so large that use of the newer sensors proved too costly for crop-water-use studies.

Plans call for the venture – started in 2003 – to continue through at least 2008, as scientists on both continents continue to learn from each other and apply what they’ve learned to other crops.

Marcia Wood is a member of the U.S. Department of Agriculture’s Agricultural Research Service’s information staff.

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