Mar 19, 2015

Drip irrigation, fertigation keep strawberries on track

Using proper drip irrigation and fertigation techniques can be akin to pursuing “a business of Goldilocks,” according to a University of Georgia Extension horticulturalist.

Erick Smith, speaking to strawberry growers at the 2015 Southeast Regional Fruit & Vegetable Conference in Savannah, Georgia, said, “It is a matter of getting the right amount of water and maintaining the right amount, dependent on the weather parameters you’re in, so you can make it just right. It’s getting to a Goldilocks of (how much water) per zone.

“One of the things to think about regarding irrigation is it’s really a chemical application,” Smith said. “You’re adding the most important nutrients for the plant – hydrogen and nitrogen. Maintaining water at the appropriate levels in the soil is very important. If you have too much water in the soils, the roots can’t breathe. Too little water and the roots are starving.”

Smith said the shallow nature of strawberry roots makes them sensitive to water stress.

“The amount of water applied is critical,” he said. “Too much leads to nutrient leaching, disease and yield reduction. Too little equals drought stress and loss of production.”

Smith said studies have shown that in as little as five days following a soaking rain, strawberries can require irrigation – especially in sandy soils.

“Supplemental irrigation/fertigation can increase yields from 40 percent to 60 percent,” he said.

Reviewing the two systems of production for irrigating strawberries, Smith discussed matted-row, overhead irrigation as well as annual hill – drip under plastic mulch.

“Drip is the preferred method because it outpaces disease and brings good fruit quality,” Smith said. “You can reuse that drip line. Drip tape breaks down rather easily. Critters like to chew on it. I’ve seen lots of holes in there.”

With the annual bed/hill system, Southeast growers use a seven-month production process, with bed preparation in the fall and installation of mulch and drip line, or tape. Irrigation is placed on the soil surface under the mulch, or buried 1 to 2 inches in the bed. Strawberries are planted through holes in the mulch and watered in with overhead irrigation immediately.

Smith urged growers to understand the characteristics of the bed system itself.

“It needs to be right on top of the ground or 1 or 2 inches under the soil,” he said. “Make holes in the plastic, figure out your spacing. When you first set the plants down, get a supplemental, overhead system. You can’t handle it with drip alone. You shouldn’t take down your overhead. If you need it for frost control in the spring, you will have it available. With drip irrigation systems, prepare to get the water out and keep the pipes from freezing.

“You need to utilize the overhead for a few days until the plants are established,” Smith said. “Then you can switch to drip.”

With a spring crop, water usage needs to be at least 0.2 inches per day, or 5,431 gallons per acre a day.

“Drip is 80 to 85 percent efficient,” he said. “The pumping capacity should be able to deliver 6,390 to 6,789 gallons per acre a day.

To achieve this, growers have two commonly used emitter products: thin-walled drip tape (good for a single-season use) and thick-walled drip line (multiple season use).

Growers should shoot for a water pumping capacity of at least 115 to 120 percent.

“At 6,400 to 6,800 gallons per acre per day, you will have the ability to put the right amount of water out,” Smith said.

Backflow, filtering needs

Backflow prevention is essential – and required in most areas.

“It prevents reverse flow of water to the source,” Smith said. “It protects water sources from chemigation practices. It’s mandatory and regulated. States, counties and districts may have specific codes. You should consult your county agent or regulatory agency before construction to determine appropriate rules.”

Common backflow prevention devices include: air gap, atmospheric vacuum breakers (AVB), pressure-type vacuum breakers (PVB) and double-check valves.

Flow meters (propeller, doppler and magnetic) also are important in monitoring water consumption. Smith said accurate flow meters can efficiently manage nutrient, pesticide and fungicide applications. They also can help determine pump outlet flow and watering time on the crop.

Proper filtration systems minimize emitter clogs. Manufacturers of drip lines will not guarantee products without filtration, Smith said. Pressure gauges are used to indicate the need for line cleaning.

“Filters are a good idea,” Smith said. “If you put anything into the system, filter it before it goes out to the holes. Any time you lose water going to those plants you’re losing potential production, which means you’re losing money.”

Drip tape requires a specific water pressure, said Smith, who advocates for 10 psi. Pressure higher than 15 psi can cause ruptures in the line.

“You need to maintain optimum pressure to ensure appropriate distribution, regardless of pressure flow fluctuations at the pump head,” he said.

Some chemigation/fertigation equipment choices suggested by Smith included the Venturi Bypass (Mazzei), which provides negative pressure (suction). Various Venturi sizes are available to accommodate the area being irrigated.

Positive displacement is another type of system that provides a reciprocating action, is accurate and does not require a bypass.

When using fertilization and fertigation approaches, Smith suggested that at the pre-plant stage it’s important to get a soil sample, and consider applying 33 percent of the required nitrogen for the crop at that point, based on the soil analysis. Also, at the stage beds are being assembled, apply phosphorous and micronutrients.

With fertigation – the application of plant nutrients through the drip line – Smith said custom blends are available but are dependent upon a nutrient analysis and the soil type.

Schedule fertigation to meet soil moisture needs and crop water demand. Two suggested timings by Smith were a six-day and a two-day approach.

“You want to know how long it takes to get the water out there, the soil type, how quickly the nutrients are going to pass through,” Smith said. “Know the relationship of how to deliver water to the system over time.”

Soil monitoring essential

Smith views soil moisture monitoring as a crucial best management practice that can help minimize nutrient leaching and avoid excess irrigation.

Types of monitoring devices include tensiometers, which measure soil moisture tension; and neutron probes, which Smith suggests are not suitable for short-term cropping such as hill-system strawberry.

“They are expensive, require operator training, licenses and regulatory inspections,” Smith said.

Irrigation scheduling is dependent on soil moisture content and varies with the time of year, rainfall and temperature conditions, Smith said.

That makes use of tensiometers a recommended approach, according to Smith.

They can be placed 4 to 6 inches in an actively growing root zone, placed offset to the drip tubing, while maintaining 10 to 20 centibars.

“Close monitoring should be done in sandy soils, in particular,” Smith said.

The system calibration should entail three factors: flow rate, injection rate and system pass-through time.

Flow rate is determined by the flow meter – gallons per minute or hour. The injection rate is the volume of material injected per unit of time.

“Use a known volume, put in the suction line of the injector and time how long it takes that volume to enter into the system.”

The system pass-through involves a calculation of flow rate and liner feet of tubing, Smith said.

He said other common water-use monitoring systems involve use of local and regional weather station data and mobile device apps that are available in many areas of the country, including the Southeast states, which have developed specific applications for strawberry growers.

– Gary Pullano