Mature Prune Trees in a Commercial Orchard Did Not Absorb a Measurable Amount of Potassium Analog Applied to the Soil Beneath an Adjacent Tree

Can a soil applied, tree-specific nutrient application be exclusively accessed by the target tree?  As a case study to address this question, rubidiun (Rb) sulfate (100 gms) and potassium (K) sulfate (1.5 kg) were both banded on 1.35 m of the soil surface equidistant from the trunk along the tree row of four mature, 15-year-old ‘Improved French’ prune (Prunus domestica) trees in a commercial orchard near Live Oak, CA, in December 2009.  This application rate and method is common commercial potassium fertilizer practice in flood-irrigated prune orchards in the Sacramento Valley of California.  The orchard layout was a square with a tree spacing of 6.1 m across the row and 5.5 m down the row.  Trees treated with Rb/K were at least 18 m apart.  The Rb was applied as a potassium analog.  It was assumed that where Rb appeared in tree tissues potassium from the same application was also present.  Leaf analysis in Summer 2010 revealed no significant (P = 0.05) difference in Rb concentration on a dry weight basis between bearing (12 ppm Rb⁺) and non-bearing (11 ppm Rb⁺) spur leaves.  On August 19, 2011, just prior to harvest, 30 non-bearing spur leaves were sampled from each tree treated with Rb/K as well as from each tree for a distance of 3–5 trees down and across (perpendicularly) the tree row from each of the treatment trees.  Leaf Rb concentration on a dry weight basis was determined for each tree.  Leaf Rb concentration was significantly greater for treated (13 ppm Rb) compared to untreated (6 ppm Rb) trees both down (P = 0.005) and across the tree row (P = 0.0006).  Leaf Rb levels from untreated trees were not significantly different.  We conclude that tree-specific potassium fertilizer applications in this specific orchard would not be accessed by roots of the adjacent trees.