Relying only on the results of soil testing to determine the needs of fruit trees for potassium and other nutrients, won’t give the full picture of your orchard’s nutrient status. Keeping a close, accurate eye on potassium levels becomes even more important as production increases.

“If you’re trying to maximize yields and returns, keep in mind that the tree’s demand for potassium increases proportionally,” says Eric Hansen, extension horticulturist, Michigan State University. “The more intensive your management, such as high-density planting, the more important it is to know if you’re supplying enough potassium and other nutrients to meet the tree’s needs.

“For example, increasing nitrogen fertilizer rates increases vegetative growth which reduces potassium concentration in the tree. Whether that results in a deficiency of potassium depends on what the potassium levels were before adding the nitrogen.”

A soil test won’t provide the best picture of a tree’s the nutrient status. In fact, he says, fruit tree growers tend to over use it.

Hansen and other tree fruit experts recommend spending the extra money for a leaf analysis to determine which and how much of various fertilizers you may need to supply. A standards soil test to measure soil pH and levels of phosphorus, potassium, calcium and magnesium, usually costs about $5 to $8. A leaf tissue analysis, which measures all the nutrients absorbed by a tree, may cost around $25. However, this cost can be reduced by measuring just the levels of selected nutrients, Hansen adds.

One problem with soil testing is getting samples that adequately represent soil conditions in an orchard, he notes. Trees roots can grow very deep, making it difficult to collect good samples. Also, many growers tend to take soil samples from under the tree canopy, while roots grow laterally beyond this area.

Unlike the case with perennial crops, results of orchard soil tests often don’t correlate with the actual amount of nutrients in the tree. “Soil tests may show low potassium levels in an orchard, while the leaf analysis indicates the tree has ample amounts,” he says. “By contrast, we may see just the opposite results in another orchard.”

Hansen recommends doing a leaf analysis every two years or so in young orchards and every three or four years in those with bearing trees.

Still, soil tests have their place in fruit tree orchards. As David Lockwood, Extension fruit specialist for the University of Tennessee and the University of Georgia, points out, the soil test measures pH. If it gets too far out of line, pH can increase or decrease the availability of potassium and other nutrients. “Leaf analysis tells you what nutrients are in the trees while the soil test can help explain the cause of any nutritional problems,” he says.

Prior to planting an orchard, he recommends testing the soil at two levels, the upper 8 inches and from 8 inches below the surface to a 16-inch depth, to measure pH and to determine the need for adding and incorporating any phosphorus, potassium and lime. Once the trees are in production, he says, leaf analysis can help keep potassium at proper levels

When it comes to producing a high-quality crop – fruit trees use large amounts of potassium – often requiring more of this macro nutrient than nitrogen. The annual potassium demand by an apple orchard in Washington, for example, can range from 125 to 175 pounds per acre. That’s equivalent to 150 to 210 pounds of K2O.

Fruit trees use all this potassium to support a wide range of functions – from photosynthesis and formation of protein and starch to transporting water and nutrients within the tree and regulating more than 60 enzyme systems that control crop quality. In addition to contributing to the yield, color and flavor of fruit, potassium improves a tree’s ability to resist disease, withstand winter stress, tolerate drought and crop pests and make the most of nitrogen and other nutrients.

The fruit, itself, is also a major user of potassium. Typically, an apple crop permanently removes about half or more of the tree’s total annual requirement each year. Cornell University researchers report that in a study of Gala apple trees, the fruit at harvest contained more potassium than any other nutrient. The total potassium in fruit at fruit harvest accounted for just over 71 percent of its total amount in new growth (shoots, leaves and fruit).

Sulfate of potash (SOP) or potassium sulfate is an excellent source of potassium for high-value crops, like tree fruits, because it typically contains less than 1 percent chlorides. In fact, SOP has the lowest salt index of any conventional potash fertilizer. As a result, it allows higher rates of potassium to be applied without risking salt burn.

Potassium sulfate is a commonly used fertilizer in California fruit tree orchards, reports Scott Johnson, University of California extension specialist. “Peach trees are quite sensitive to chloride,” he says. “So, we get pretty nervous about using chloride fertilizers in these orchards.”