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Reasons to fertilize your crop may range from indications that the plant is deficient, you want yield or quality that you once had, or you want performance that other areas in the field have, says an Oregon State University agronomist. But when making nutrient management changes, grape and tree fruit growers should always confer with those who deal with the end product, be it the winemaker or packer.

It can be difficult to distinguish nutrient deficiencies when looking at plants, said Don Horneck, extension agronomist from OSU’s Hermiston Agricultural Research and Extension Center. "Deficiencies can be tough to diagnosis, because they look similar to each other," he said. "Almost all deficiencies result in poor growth, poor fruit set, chlorosis, poor cluster development, and poor yields. Yellowing of the leaves and leaf burn often occur from too little or too much."

The key to identifying which nutrient may be involved is to look at new growth versus old growth, Horneck said during the Washington Association of Wine Grape Growers annual meeting. New growth means you have a certain set of nutrients—ones that are not mobile in the plant. Deficiencies of iron, boron, and sulfur tend to show up in new growth, whereas nitrogen, potassium, and phosphorus tend to show up in old growth parts.

He suggests that growers pay attention to nutrients that are commonly found to be deficient in their crops and that can have a profound effect if they are missing from a plant.

Molybdenum—Though a micronutrient that’s needed in minute quantities, molybdenum is a key component to nitrate reductase, helping convert nitrate to ammonia. Grapevines in low pH soils with elevated nitrate levels that are growing poorly could have a molybdenum deficiency, he said.

Boron—Boron is "absolutely" critical for flowering and fruit set, Horneck said, adding that it’s a very common deficiency in many fruits and vegetables, including grapes. Boron is mobile in the soil. "If you have poor fruit set, but the rest of the plant looks okay, you should look at boron first," he said. "In grapes, it can show up as a deficiency in the cluster without ever showing up as a foliar symptom. But it can also be toxic, so don’t overdo it. It has a reputation for being the easiest nutrient to go from deficiency to toxicity."

Studies of soil and foliar boron treatments on Thompson Seedless grapes in California showed that the most efficient uptake of boron was from a postharvest fall foliar spray. The next most efficient uptake was from a foliar spray made at bloom; the least efficient treatment for uptake was from a dormant soil application or prebloom foliar treatment.

Iron—Iron is another common deficiency, but one that is difficult to correct because it is related to high pH soils. Horneck said that iron deficiencies can be variety and site specific, but commonly occurs in patches as a result of high soil pH that is compounded with too much soil moisture. In apples and grapes, symptoms are yellow leaves with bright green veins.

Potassium—A macronutrient that is in high demand by the plant, potassium deficiency can be a problem in sandy soils, particularly in Washington State’s Columbia Basin. Leaves showing signs of leaf rubbing or salt burn (yellow edges or brown or bronze spots) could be an indication of deficiency.

Know your soil

Soils in the Pacific Northwest can be extremely diverse, he said, so it pays to know what you have. Many problems and issues are caused by or are related to the soil—drainage, surface sealing, compaction, alkalinity, productivity, aspect, and slope. Soil tests are necessary to learn the soil pH and levels of macronutrients like nitrogen, phosphorus, and potassium.

"The big one to worry about is soil pH," Horneck said. In eastern Washington, low pH is not usually an issue, with most soils around 8.0. Low pH is easily cured with lime applications.

But high soil pH—above 8.3—is much more difficult to deal with and can cause iron deficiency. "With high pH, you need to change your soil and fertilize," he said, adding that it can be expensive to battle. Last year, soil acidification costs increased from $100 per acre to $800 per acre. "It’s expensive and not an easy or quick fix. You have to do it again and again, and again and again. And your grandkids will have to do it again, and again, and again."

Analysis trends

One of the biggest mistakes growers make regarding their annual plant tissue tests is to read the analysis and throw it away. "It has more value than that," he said. While the tissue test is a snapshot at a certain point in time, if taken at the same time during the growing season and compared against past seasons, it is an important diagnostic tool that can show trends.

"Knowing the trends from year to year is extremely important," Horneck reiterates. But unfortunately, he said that few soil testing laboratories provide charts to their customers showing the nutrient trends over time.

He encourages growers to ask their labs for this service of charting nutrient levels over several seasons. "It’s not that hard for them to do," he adds.

Horneck’s presentation was part of a fertilizer session at the annual meeting of the Washington Association of Wine Grape Growers.