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Precision agriculture techniques are everyday tools in row crop farming, but they have been slower to catch on in tree fruits.

In 1994, Quincy, Washington, orchardist Jim Fleming tried out a small variable-rate fertilizer applicator that had been developed by Cenex to fit between tree rows.

Fleming recalled that it worked well. After using a full-size machine to apply a pre-plant treatment to the ground where he was planting a new block, he used the smaller machine for a couple of years after the trees were planted.  His soil is very variable with some areas of caliche in the upper soil layer and some deeper down.

“I thought we had real uniform growth of the trees,” he said, recalling that the nursery trees were unsorted and were not very uniform at planting. “I thought they did real well.”

The sticking point was the cost of the soil samples that were needed in order to calculate how much fertilizer the machine should apply in the different areas, he said. A soil sample had to be taken for every 100 square feet of ground.

“I’m a hundred percent in favor if they could get it down to where the soil samples weren’t so expensive,” he said.

Jim Nelson, who was a fieldman with Cenex at the time and helped develop the small machine, said several orchardists tried the machine in the 1990s, but it was 20 years before its time. Many orchards still had trees planted at low densities and some were using rill irrigation. Growers were happy to use practices their fathers and grandfathers used.

An effort to introduce yield monitoring by weighing bins in the orchard also failed to catch on.

Now, if the variable-rate applicator was available, Nelson thinks growers would be interested, particularly as there are alternatives to taking lots of soil samples.

“It really does work and you got very uniform growth from the trees,” he said. “It was phenomenal. It was just before its time. I think the interest in precision ag in tree fruit has completely turned around. In row crops, there’s nothing that can’t be monitored or done.”

Also in 1994, Dr. Tim Righetti, then a horticulturist with Oregon State University in Hood River, was exploring ways to ensure that trees in an orchard got the appropriate amount of fertilizer.

His studies had shown that 75 percent of the trees could be overfertilized while the other 25 percent had too little fertilizer.

His approach included surveying the orchard from the ground and with aerial photographs to produce a nitrogen value for each tree. With that information, a fertilizer application could be tailored to meet the need of each tree. He envisioned that the trees would be barcoded and a computer on the fertilizer applicator would read the barcode.

Clark Seavert, former superintendent at the Mid-Columbia Research and Extension Center, said Righetti’s system was never adopted commercially.

Mapped

Dr. David Brown, soil scientist with Washington State University, said it’s no longer necessary to intensively sample the soil. Typically, before a preplant fertilizer treatment, the variability of the soil would be assessed using an electromagnetic-induction instrument, which measures soil conductivity (see “Precision ag tools”). This can indicate properties such as soil depth, soil texture, and some soil minerals. A near IR probe could also be used.

Once the ground is mapped, based on those factors, zones that have similar soil types can be identified. Then, it’s only necessary to take one sample to represent areas that have similar soil types.

Once the trees are in the ground, fertilizer needs can be inferred from how the crop is growing, which can be assessed through remote sensing.

Karen Lewis, Washington State University extension specialist, said adoption of new technologies in tree fruits is often lower than anticipated. “And it’s based on the fact that the execution is one more thing they have to do.”

Growers often think they don’t need monitoring technologies, for example, because they can walk or drive up and down their rows and check on tree growth with their own eyes.

“Every time we’ve surveyed growers on adoption of technology, regardless of what it is, the same thing always comes up,” she said. “They want to see it’s been proven to be reliable and dependable in the field. They want to know it’s not a toy—that it contributes to their bottom line and improves fruit quality. What they want is improved fruit quality day in, day out. So somebody has to prove, very robustly, that the technology does that.”

Bruce Allen, president of Columbia Reach Pack in Yakima, Washington, said he sees benefits in growers having more tools to help them make better decisions. However, even now, growers are making informed decisions based on fairly accurate assessments of their orchards.

“Most growers have been around for a long time, so they have lots and lots of experience,” he said. “You may not be sitting there going down a list of experiential factors, but it’s there in your subconscious—those trees look a little weaker or look a little strong; fruit size looks a little small; the thinning job looks a little excessive—that’s based on a huge amount of experience.” •