Tomato and pepper growers face one major irrigation issue across Florida—how to most efficiently deliver water and nutrients to plants’ roots in widely diverse soil types.
From the deep soils of north Florida to the sandy ridge in the central part of the state and the challenging uncoated sands of Immokalee, growers are embracing new technology and using soil moisture sensors to meet that need.
“The challenge becomes if you go out and over-irrigate, you tend to leach the nutrients from the growing beds below the surface,” says Bob Hochmuth, a Suwannee-based multi-county Extension agent. “Sensors are important helping us determine when the farmer should turn on the irrigation and how long it should be run.”
Hochmuth, with the Institute for Food and Agricultural Science, has watched as sensors have evolved during the past few decades. When drip irrigation was fine tuned 15 years ago, tensiometers were popular.
They’ve given way to granular matrix sensors, such as The Watermark, which measure soil water tension through electrical resistance readings.
More recently, many growers have chosen the time domain reflectrometry, or TDR, probe. A portable TDR is available for about $600. It uses two prongs buried in the soil to measure moisture near the surface, in the root zone and beneath it. The prongs are wired to a controller that can start or end an irrigation set.
“That one is going to give the best information about what’s happening in the entire soil profile,” Hochmuth says of a TDR probe. “The technology in the soil sensor area is greatly improved in the last five years. The utilization at the farmer’s level has made that quantum leap.”
Soil variability plays a role
Growers in Immokalee face this issue regularly. Although some use the FAWN weather network as a predictive model with evapo-transmitters, most of those who farm the area’s collective 5,000-plus acres of peppers and 22,000 acres of tomatoes opt for the TDR.
South Florida’s sandy soil particles are uncoated and don’t bind well with phosphorus. As a result, phosphorus may easily leach into environmentally sensitive areas if too much water is applied, says Gene McAvoy, a regional vegetable agent and Hendry County Extension director in Immokalee.
“Our soil has gone from drought to flood in 10 to 14 days; there’s no buffering capacity,” he says. “We’re constantly moving water, taking it off the land or putting it back on.”
McAvoy’s research focus is to develop nutrient profiles so over-watering and over-fertilization don’t occur. Too much nitrogen can open the door for diseases, such as bacterial spot and viruses spread by whitefly.
Proper water and nutrient management, when used in conjunction with best management practices, may mean higher vegetable yields.
“If you go back 20 years, someone got 45,000 pounds of tomatoes and thought they were done,” he says. “Now they’re getting that on the first pick.”
Heavy rains in August and September increase disease pressure in Manatee County’s 12,000 acres of tomatoes. Crystal Snodgrass, the Extension agent based out of IFAS’ Palmetto office, keeps tabs on it.
“There’s more fertilizer leaching with the additional water, and it’s also not good for the plants themselves,” she says. “It creates the right conditions for bacteria.”
Focus on root-zone irrigation
As the irrigation and technology specialist for Six L’s, Randy Moss remains aware of such issues across its 6,000 drip-irrigated acres spread from Collier to Manatee counties.
Six L’s grows some peppers, but its mainstays are romas, cherries, grape and heirloom tomatoes.
High-tech soil moisture sensors help fine-tune water management
Soil moisture sensors, coupled with dataloggers, let you grab a minute-by-minute picture of soil moisture depletion. You can then base your irrigation on that data.
With many sandy fields that don’t retain water, Six-L’s has used drip irrigation and various soil moisture sensors since 1985. Moss relies on tensiometers as a safety measure to prevent over-watering and opts for Netafirm’s Growpoint to wirelessly relay data to his office.
“We irrigate to the root zone and try to focus on that,” Moss says. “There’s no more work for me to irrigate five cycles a day versus one. It’s a good tradeoff—I’ve always believed automation pays for itself.”
Moss’ challenge is that maintaining a stable moisture level is economically possible only through the use of TDRs, such as the Growpoint and occasional use of a handheld Hydrosense across 100 irrigation zones at one Naples farm.
“The technology does exist to automatically control your systems with a probe,” he says. “With these large systems, you have to do what’s best for all of [the acres]. It’s just budget restraints.”
Balancing cost with efficiency
As the lead researcher examining automated irrigation systems in the growth of tomatoes and green peppers, IFAS’ Michael Dukes is acutely aware of the cost of completely automated systems.
Dukes, whose one project through 2010 evaluates on-demand soil moisture irrigation controllers for vegetable production, says data collection via soil moisture sensors shows promise.
The tensiometer that had been recommended for decades has for many proven to take too much maintenance and human input. Attaining a constant soil moisture level, albeit one that varies from field to field, is the challenge that Dukes sees.
Using a soil moisture sensor that costs from $600 to $1,000 is part of balancing a higher price with increased efficiency, experts say.
“All of these technologies add cost,” Dukes says. “I think we’re trying to educate growers they can use these tools to more effectively manage water.”
Contact Vicky Boyd at vlboyd@att.net or (209) 571-0414.
Soil moisture sensors offer real-time information
Soil moisture sensor manufacturers offer a wide array of instruments to meet your needs.
Some still rely on evapotranspiration meters to determine how much soil moisture is lost through evapotranspiration and how much must be replaced.
But most manufacturers have turned to time domain reflectrometry, or TDR, probes. These send an electronic signal between two probes that measure moisture within the top 12 inches of soil, says Bob Hochmuth, a multi-county vegetable Extension agent in Live Oak.
Although Hochmuth says the TDR 100 from Spectrum Technologies (specmeters.com and about $695) is the most widely used around Suwannee, the newest sensors show a specificity that tensiometers lack.
“The thing about tensiometer readings is they are a checkpoint of when to irrigate,” says Rafael Muñoz-Carpena, a Gainesville-based researcher with the Institute of Food and Agricultural Science’s agricultural and biological engineering department. “There is no real translation into what is the real moisture content. That’s always a tradeoff between simplicity, reliability and cost.”
Tensiometers report in centibars as an expression of moisture energy or potential. Growers try to keep moisture at or below field capacity.
Those who use soil moisture sensors have a distinct advantage, says Muñoz-Carpena. Their use can help avoid overwatering, which with Florida’s shallow aquifer, helps prevent chemical and nutrient leeching below the rootzone into groundwater.
“If we apply them in the right way, they will maintain moisture conditions in the soil for plant productivity and also minimize losses,” he says. “Another nice thing about soil moisture sensors is you’re no longer relying on forecasting of weather or things that happen above the root zone.”
