See the results of an oak mulch study on the impact of using the mulch to improve soil and tree health in the era of citrus greening.

There have been a number of studies into the effects of a natural compound in oak trees to combat citrus greening after observations were made that citrus trees growing underneath the dripline of oak trees had little to no symptoms of citrus greening when compared with trees not growing under oak trees. According to a Citrus Industry article, a 2019 study by UF/IFAS postdoctoral associate Marco Pitino (now at AgroSource, Inc., according to the article) found “that liquid extracts from the leaves of oak trees reduced the bacterial titer in HLB-affected sweet orange trees grown under greenhouse conditions.” This led to an oak mulch study with USDA Agricultural Research Service (ARS) scientist Robert Shatters in September 2019 to see if the results of Pitino’s study could be replicated with the use of oak mulch. See the results of the study below.

Update on the Oak Mulch Study

According to the article, the oak mulch was made from laurel oak tree branches and trunks growing on the property of the UF/IFAS Indian River Research and Education Center. The mulch was then applied three inches deep around the dripline of 4-year-old Valencia orange trees grafted on US-812. The study was run three times on a plot of four citrus trees per plot, for a total of 24 trees in the experiment.

Results of the oak mulch study included:

  • “Higher amounts of phosphorus were measured in mulched soils. Additionally, at different times of the year, mulched plots had higher potassium, magnesium and nitrate compared to non-mulched plots. These macronutrient increases varied throughout the year with some months seeing no difference between mulched and non-mulched plots. Other months saw as high as 26% increases in the mulched plots.”
  • “When soil sensors were used, soil moisture was at times 10% higher in mulched plots compared to non-mulched plots. Higher soil moisture may reduce potential water-stress events on HLB-affected trees while also promoting a better environment for the biological community.”
  • “Mulched plots had an abundance of earthworms and fungi, but these organisms were not observed in non-mulched plots,” and “higher bacterial diversity was observed in the mulched plots.”
  • “Oak mulch applications did not improve overall tree health, even after two and a half years and three mulch applications.”
  • “No differences in leaf bacterial titer were observed between the control and mulched plots.”

The conclusion of the study was that “oak mulch applications increased soil nutrients, moisture and biological diversity,” but it was not found “to be an effect treatment for HLB-affected trees. However, “Adding organic amendments to increase soil quality is likely to take many years to manifest into increased tree growth and yield,” and that “For those growing on nutrient-poor and biologically weak soils, mulch application may be an effective option for soil improvement.”

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