Becky Wheeler-Dykes, UCCE Orchard Systems & Weed Ecology Advisor, Glenn, Tehama, and Colusa Counties
Franz Niederholzer, UCCE Orchard Systems Advisor, Colusa, Sutter and Yuba Counties

A prune grower’s potassium (K) program can be one of the most critical components of an orchard management plan. Potassium deficiency in a single season can negatively affect tree health for years to come. In addition to the direct effects of K deficiency, such as defoliation and reduced crop quality, secondary symptoms like increased disease susceptibility and canker infection can be costly for several seasons.

Potassium basics

Unlike nitrate, K does not leach out of most soils. Potassium cations (K⁺) are held on the soil exchange phase, allowing growers to build a soil ‘K bank’ of plant available K. However, the flipside is that K added to soil can be fixed as nonexchangeable K in certain soil minerals and be unavailable for plant use at least in the short term. K fixing soils are generally associated with weathered granitic minerals in the eastern San Joaquin Valley and Sacramento Valley.

Potassium at the root surface is readily absorbed into plants. It is movement to the root surface that limits plant access to otherwise available soil K. There are three paths: root interception, diffusion, and/or mass flow by which potassium can arrive at root surfaces. Researchers report that most K arrives at root surfaces by diffusion. Diffusion is aided by high local soil K levels, adequate soil moisture and root growth. Early in the growing season, fall banded soil K, good soil moisture levels and good root growth allow potassium to reach roots and meet tree K demand. Later in the season there is the possibility, especially under heavy cropload where root growth is limited, that less K reaches the root surface than is needed by the plant. When this happens, and since fruit K demand is the priority for the plant, leaf K may be remobilized and moved to the fruit to meet the needs of the crop.  Unfortunately, this can lead to K deficiency in the leaves and a cascade of trouble from the start of K deficiency to increasing leaf yellowing with further reduction of leaf K, to leaf drop, sunburn of shoots and limbs, and eventual infection by bark cankers, especially cytospora. Maintaining adequate plant K through the season is important for good fruit production and tree health.

Potassium fertilizer programs historically centered on a heavy rate (400-700 lb/acre 0-0-50 SOP) of fall applied, dry fertilizer banded on the soil down the tree row.  Supplemental foliar applications of potassium nitrate helps maintain good leaf K levels through the season. With the widespread adoption of micro-irrigation, many growers have moved to in-season K fertigation as their main fertilizer program or as a supplement to fall banded K fertilizer. Many growers prefer the fall banding plus in-season supplements as the fall application provides a foundation for later programs at an easier time for labor management.

How much Potassium to apply

Cropload is the major determining factor setting prune K need. Natural conditioned dried prunes contain about 1% potassium or 25 lbs K₂O (potash) in a dry ton of prunes. So, a 3 dry ton/acre crop will remove 75 lbs K₂O, equivalent to 150 lbs 0-0-50 SOP fertilizer. A larger crop removes more K and generally needs more K fertilizer to avoid deficiency. A smaller crop removes less K. In the fall, when deciding how much potassium to put on the soil, no accurate information exists about the crop size the next spring. Applying 400-700 lbs SOP as a maintenance rate in the fall before the crop is set risks spending hundreds of dollars per acre on fertilizer on a crop that may not set. In a case like this, the excess K may not be lost, but it isn’t a good investment in the current crop. Research will start this season to see if banding of dry SOP fertilizer is effective in delivering adequate K to prune trees in the spring and not just the fall.

To make sure that a fall, banded K application is delivering enough K to the trees the next year, take an April or May leaf sample and compare the K leaf levels from the lab analysis with the standards for July leaf samples*. Since leaf K levels generally decrease over time, the closer the spring leaf sample result are to the critical levels, the greater the risk of K deficiency in July. If spring leaf K levels are approaching July critical levels, the orchard should benefit from adding additional fertilizer K by foliar spray or fertigation.

Other considerations

Don’t forget to monitor crop load after fruit set. If crop load is heavier than expected, additional K (foliar or fertigated) may need to be applied to avoid drawing down leaf levels to deficient. Use the prune fruit thinning calculator tool to help decide whether and how much to thin this season.

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*July prune leaf K standards:
Deficient: <1.0% K
Adequate: >1.3% K
Excessive: >2.0% K (not damaging, but not beneficial to the crop)