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Dennis Smith, horticulturist with G.S. Long Company, Inc., in Yakima, Washington, has been doing soil and leaf sampling for fruit growers for over 20 years and has seen the importance of nutrition change as the industry has moved from widely spaced trees to high-density trellised plantings.

“We’ve got to get production coming as soon as possible to return revenues on very high investments, and we’re looking at 100 bins per acre on some varieties to keep profitable,” he said. “Our nutritional standards are much more robust than they were 20 years ago.

Smith said standards for soil sampling that were developed for row crops don’t apply to a perennial crop like apples, and so the industry has been developing its own research on a practical level in an attempt to make orchards economically sustainable.

He typically tests soil in a high-­production orchard every three to five years, unless there are indications of a problem. He looks at soil samples the way a doctor would do a complete blood count for a patient having a physical checkup, just to see if the soil is healthy overall or not.

The soil pH level is one of the most important indicators, because that influences the availability of nutrients to the tree. Typically, orchards in established growing regions, where the ground has been farmed for a long time, have more acidic soils due to the prolonged use of fertilizers. Orchards in newer districts, such as the Columbia Basin, tend to have more alkaline soils, though they can ­acidify quickly when in fruit production.

He also looks at the calcium, magnesium, potassium, and sodium levels and at the balance of the nutrients. Calcium is particularly important for some varieties, such as the bitter-pit–prone Honeycrisp, and is also important for overall tree health, Smith said.

While soil tests give a general report on the potential health of the orchard, leaf samples taken during the growing season can help diagnose specific problems. In some orchards, Smith takes leaf samples each year to track trends.

“Each one of our different kinds of tests is going to give us a different snapshot of the tree health,” he said. “If the grower is not economically viable, he will not be in business, and in order to be economically viable, he has to have the highest production possible, which means a very robust production system. We have less leeway to make mistakes.”

Gary Johnson with Wilbur-Ellis Company in Wenatchee, Washington, said his company has moved away from doing tissue samples and more commonly does saturated-soil tests for its clients. With many thousands of test results in its database, it has established parameters in terms of nutrient levels and ratios and how to correlate those to fruit quality.

The saturated-soil test was first used in other parts of the country where farmers depend on rain for moisture, rather than irrigation, Johnson said. Rainwater is almost as pure as distilled water, but irrigation water can contain significant amounts of minerals, depending on the source, and can carry those minerals into the tree.

The saturated-soil test shows what nutrients are available in the soil’s water solution for uptake by the tree. One soil testing company has described the standard soil test as an indicator of the soil’s “savings account” and the saturated test as a measure of its “checking account.”  Both show nutrients that are available, but the checking account nutrients are more easily accessible.

“What we’ve done in the last ten years is taken that test and applied it to irrigation water here,” Johnson said.

The mineral content of irrigation water used in Washington varies greatly from region to region. In the Wenatchee Valley, the water comes primarily from snowmelt from the Cascades and is very pure. Water from Lake Chelan has a very light mineral content. Well water has higher mineral content, though the amounts vary at different times of year, Johnson said. For some locations and varieties, tests are done several times a season.