Potassium deficiency can occasionally be found in California peach and nectarine orchards, especially in sites that have been leveled and the top soil has been removed. Symptoms include pale green or yellow leaves that develop in late summer. Often the leaves are rolled. Fruit size is greatly reduced by K deficiency. Leaf levels of 1.0% or lower in mid summer leaf samples indicate deficiency, but it is advisable to maintain levels above 1.5% to prevent problems with fruit size. A survey of commercial orchards indicates some are in this range. Correction can easily be achieved by soil applications of potassium sulfate. With low volume irrigation systems, applying fertilizer through the irrigation line (fertigation) is especially effective.
Nutrition Chapter in UC Stone Fruit Manual