Florida citrus growers have been using variable-rate application technology to help spray their groves for several decades. But the newest enhancement to the process enables them to also apply fertilizer more efficiently, resulting in material and cost savings.

The core of the system is a sensor mounted on the application rig—typically at the front of the tractor—and a computer controller, says Arnold Schumann, associate professor at the University of Florida’s Citrus Research and Education Center at Lake Alfred.

The sensor has two functions.

It determines the presence of trees, so the system doesn’t fertilize areas where nothing is growing. And it measures the size of the trees and adjusts the amount of fertilizer applied based on tree size.

“It basically does a very targeted application of fertilizer, rather than applying it whether there’s a tree or not,” Schumann says.

The delivery mechanism is a dual-chain spreader capable of applying fertilizer independently on the left and right sides between two rows of trees.

There are various types of sensors available, including laser, ultrasonic and infrared optical sensors.

“The most reliable and cost-effective ones are infrared optical sensors,” Schumann says.

How sensors work

When the sensor detects the presence and height of the tree, it feeds that information into a controller, which essentially is a dedicated, custom-built computer that interprets information from the sensor and determines the fertilizer application rate, Schumann says.

The spreaders have been around since the 1990s and are used for a variety of crops, including peaches, apples, avocados or anything that is grown in rows, he says.

Spreaders were first used in conjunction with sensors about the year 2000.

The latest embodiment of the system, which Schumann developed with Kevin Hostler, senior laboratory technician at Lake Alfred, over an 18-month project period senses the speed of the tractor as it applies fertilizer and adjusts for it with the help of a global positioning system.

The system, called the CC-Eye-8000 Tree Sense Control System, available from Chemical Containers Inc. of Lake Wales, automatically applies more or less fertilizer as needed or turns off the application when the tractor stops.

The previous system, with no speed reference, was plagued by poor synchronization, causing the wrong tree to be fertilized or applying fertilizer after the tractor had passed the tree.

“You need to have very good synchronization so that the tree is detected and the fertilizer is released at exactly the correct time,” Schumann says. “You can only do that with a good source of speed measurement and a fast computer.”

Cost to retrofit a fertilizer spreader using three sensors per side is about $13,000, Schumann says. Four sensors per side would cost about $15,000.

Schumann’s newest controller replaces physical switches with a color touch-screen liquid crystal display using software-driven switches that can set the applicator to spray or fertilize only certain sized trees, such as resets.

That’s especially important during the spraying process, since some pesticides are labeled only for non-bearing trees.

Good results

Florida growers have reported good results with variable-rate systems.

Hunt Bros. Services in Lake Wales has been using variable-rate technology to spread fertilizer for three years, says W. Deeley Hunt, president.

Some growers have used the system longer, but Hunt waited, he says, “to make sure I felt comfortable with it and the design was working properly.”

“We’re thrilled,” he says, because the system reduces costs of fertilization by 13 percent to 25 percent per grove and reduces groundwater contamination.

The company has about 2,500 acres of round oranges and other citrus in the area.

Hunt Bros. already has recouped the cost of the system.

“It’s well worth the investment,” Hunt says.

Lykes Citrus Management of Lake Placid saves about 15 percent per application using the system on its 16,000 acres of round oranges, says John Gose, manager of special projects. The company saves 20 percent using it for spray applications.

Map-based system

Although nearly all of the Florida growers who have the system use it to detect the size and presence of trees when applying fertilizer, a handful also use it to apply fertilizer based on the nutrient content of the soil.

The control system can be set up to be guided by maps based on an analysis of the soil or leaves.

Using the system for this purpose is time consuming and costly, Schumann says, but it can be worth the effort if a grower has soil that is unusually weak or sandy.

Lykes takes it a step further.

The company first collects electro-conductivity measurements, which indicate differences in soil types. All of the data is referenced with GPS coordinates.

Lykes then makes maps showing zones with similar soil types and collects samples from each zone. The soil sample results and georeferenced map coordinates are entered into a laptop computer connected to a controller that allots the proper amount of soil amendments.

“It’s been a valuable tool,” Gose says.

Systems requiring a map-reading-type controller are available from TeeJet Technologies of Carol Stream, Ill., and Raven Industries Inc. of Sioux Falls, S.D.