The best way to meet the potassium needs of a walnut tree depends to a large extent on where it’s growing. In fact, in California, where nearly all commercial walnut trees in the United States are grown, soil potassium deficiencies occur only in limited areas. They include some of the alluvial soils in the southern Sacramento Valley as well as some of sandy soils in the San Joaquin Valley.

Still, it’s a good idea for all walnut growers to check their orchards for adequate potassium, says Janine Hasey, University of California Cooperative Extension farm advisor for Sutter County. “In some cases, the soils may be lacking the nutrient,” she says. “In others, soil potassium levels are high enough but it’s unavailable to the tree, because the nutrient is being held tightly by the soil. Also, a low level of potassium in the leaves might indicate a problem with the tree’s root system that prevents it from taking up the nutrient.”

Like other plants, walnut trees require large amounts of potassium, which they take up through their roots from potassium released into the soil solution by soil particles near the roots. However, providing the trees enough potassium to meet their needs is complicated by several factors. The nutrient moves very little in the soil and its concentration in the soil solution is very low. Also, certain clay minerals can hold onto potassium very tightly. In fact, the surface of certain clay minerals may contain 100 times more potassium than is in the soil solution. Sandy soils, on the other hand, cannot adsorb large amounts of potassium. With fine-textured soils, the problem is getting an adequate amount of the nutrient deep enough in the soil to meet the tree’s needs. That requires chiseling or trenching.

Symptoms of potassium deficiency begin appearing in early to midsummer when leaves of walnut trees turn pale. Later, the edges of leaflets fold upward and curl, exposing a grayish underside. Most of the affected leaves are along the middle of the shoots. Leaf size, shoot growth and, eventually, nut size is reduced.

Hasey recommends checking potassium levels using a leaf analysis of samples taken in July. That’s when potassium levels tend to stabilize, she explains. A reading below 0.9 percent is considered deficient, while a value over 1.2 percent indicates the tree is receiving an adequate amount of the nutrient.

“Take samples in areas of your orchard where the majority of trees look similar,” she advises. “Sample poorly-growing trees separately.”

Although it costs more, many walnut growers use potassium sulfate for correcting potassium deficiencies to avoid possible toxicity problems when using potassium chloride, Hasey reports.

She does not recommend using a blended fertilizer, like 16-16-16, to supply potassium for walnut trees. “Because a blend won’t provide enough potassium in a concentrated form, it will get tied up by the soil and won’t be available for the tree,” she says.

The amount of supplemental potassium required can vary depending on the type of soil and the individual orchards’ needs. Normally, if walnut trees require additional potassium, it is applied after harvest before fall rains begin, Hasey notes. Correcting a chronic deficiency in heavy clay or silt-loam soils calls for massive amounts of potassium sulfate or other potassium fertilizer – about 1,700 to 2,000 pounds per acre, she says.

“You need to apply very high rates of potassium in a concentrated area so that it will penetrate deep enough into the soil to correct this type of deficiency,” Hasey says. “You may not see the affects until the second season. But this treatment will remain effective for about 4 to 5 years after application.”

To correct a deficiency in sandy soils, she recommends applying a potassium fertilizer at the rate of 400 pounds per acre annually.

For orchards where leaf analysis reveals borderline potassium levels (.9 percent to 1.2 percent), Hasey recommends applying a maintenance rate of 400 to 500 pounds of potassium per acre once a year as needed to reach an adequate level.

Potassium should be banded or drilled on either side of the tree, rather than broadcast, in the fall, she notes. It can also be supplied through the irrigation system during the growing season.

“Previous research here in northern California found that four years after potassium was broadcast, it had moved less than 6 inches into the soil,” Hasey says. “However, in that same period, potassium applied in a band on the soil surface had moved more than 2 feet into the root zone.”