Adapted from: “Potassium Nutrition – Maintaining Optimum Levels by Replacing What You’ve Lost” by Katherine Jarvis-Shean. Originally published in Sacramento Valley Prune News, Fall 2014.

Fall is an excellent time to manage the potassium (K) needs of prune trees. Assessment of your trees’ nutrient needs should take into account July leaf sample results, visual symptoms, and the amount of fruit harvested this year.

Deficiency
Potassium deficient trees will have a pale upper canopy in early to mid‐summer, weak growth of new leaves and shoots, and possibly reduced fruit size and quality. Leaves will turn pale, burn on the margins and drop early. Weak canopy growth can encourage sunburn, making trees vulnerable to Cytospora canker. After harvest, K deficient trees often show bare upper shoots and branches, as K deficient leaves are removed by the shaker.

If you see symptoms of deficiency in leaf color or tree growth, you’re probably already losing yield potential. Leaf sampling can give an earlier warning of deficiencies. July leaf levels of 1.3‐2.0% K are considered adequate. Leaves under 1% K are deficient.

Maintaining and Replacing
Around 70% of the K used by a heavily cropping, mature prune tree is removed at harvest in the fruit. Thus, when planning K management, it is important to consider the size of the crop just harvested. If your leaf samples showed adequate K and symptoms of deficiency were not observed, focus on maintaining adequate K levels by replacing what was removed. If you had low yield this year, you may dial back your K application. If you had an above average harvest, an above average K application would be appropriate.

The table below considers possible yields per acre and breaks down how much K is removed in the crop (column 2), how much potassium sulfate would be needed to replace that K if every bit applied were taken up by the tree (column 3), and how much potassium sulfate actually needs to be applied estimating that only about 50% of what is applied will be taken up (column 4). Thus column four lists how much is reasonable to apply to replace what was taken off in the previous harvest.

Table 1. Potassium application necessary based on harvested yield. Table by R. Buchner, UCCE Tehama.

Application
Potassium ions are positively charged (K+) and are thus adsorbed to the negatively charged surface of soil particles, much like opposite poles on a magnet are attracted to each other. Since K adsorbs to clay particles and prunes are often planted on heavy (high clay) textured soils, sufficient material must be concentrated in small areas to overwhelm the soil’s ability to hold K and keep some K in soil solution for uptake by the trees. For this reason, K needs to be banded or shanked, not broadcast, and always applied to the same area year after year. Banding should be 4 to 5 feet from the tree row. Calcium ions (Ca++) have a two plus charge and can replace K+ on the negatively charged clay particles making more K available to tree roots in the soil solution. Gypsum (CaSO4) banded at a rate of 1000 to 4000 pounds per acre in the same location as previous potassium bands may improve K availability.

Potassium is commonly applied as potassium sulfate or potassium chloride. Potassium sulfate (0‐0‐50; sulfate of potash) applied in the fall or winter should be banded in non‐tilled orchards with solid set or micro‐sprinklers, or shanked in where orchards are cultivated or flood irrigated. Orchards on well‐drained soil can use potassium chloride (0‐0‐60; muriate of potash) at about the same rate, provided ample rain occurs to leach chloride out of the root zone (at least 10 inches of rainfall by 6 weeks before bud break). Applying after leaf‐drop will help avoid chloride uptake by the tree. Potassium chloride should not be used on young or weak trees, or in orchards with fluctuating water tables, hardpans, stratified soils or any other restriction that would keep the chloride from moving down out of the root zone with excess soil water.