An Update on Oak Mulch to Increase Soil Health

By Lukas M. Hallman, John M. Santiago and Lorenzo Rossi

At first glance, one may not see how oak trees relate to citrus production. But in the early 2010s, growers and scientists reported seeing little to no HLB symptoms on citrus growing underneath the dripline of oak trees compared to citrus trees nearby but not directly under oak trees.

These observations from many parts of the state raised both eyebrows and questions. Why do citrus trees under oak trees have less symptoms? Researchers at both the U.S. Department of Agriculture (USDA) and the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) hypothesized that a natural compound in oak trees could be the reason for reduced HLB symptoms of nearby citrus trees. This hypothesis was tested in 2019 by UF/IFAS postdoctoral associate Marco Pitino (now at AgroSource, Inc.). His research revealed that liquid extracts from the leaves of oak trees reduced the bacterial titer in HLB-affected sweet orange trees grown under greenhouse conditions.

The results of the liquid extract experiment led to interest in using chipped oak trees to improve soil and tree health. If a compound is present in oak leaves, it is reasonable to assume it could also exist in the woody portions of oak trees. At that time, some growers were also successfully experimenting with the use of oak mulch as a soil amendment. To help clarify what these mulch applications were doing to both soil health and tree health, an oak mulch study in collaboration with USDA Agricultural Research Service (ARS) scientist Robert Shatters was initiated in September 2019.

The oak mulch was sourced from the hardwood branches and trunks of laurel oak trees naturally grown on UF/IFAS Indian River Research and Education Center property. These woody parts were ground into chips similar to the consistency of bagged mulch that homeowners use for their gardens. Three inches of oak mulch were applied around the dripline of 4-year-old Valencia orange trees grafted on US-812 (Figure 1).

The experiment was set up in a randomized complete block design at the USDA ARS Picos Farm in Fort Pierce, Florida. Applications occurred once a year in the month of September. The experiment was replicated three times, and each plot has four citrus trees for a total of 24 trees.

NUTRIENTS
Within three months of mulch applications, 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 (Figure 2).

It is likely that oak mulch is leading to a greater retention of soil nutrients. This pattern is often observed when organic amendments are added to soils. To further confirm this, nutrient analysis of the mulch itself is currently underway. From this data, the nutrient contribution of the mulch itself versus the ability to retain nutrients already in the soil should be made clear.

SOIL MOISTURE
Differences in soil moisture were evident even before any sensors were placed in the ground. The mulched soils feel much better in the hand compared to sandy soils. When soil sensors were used, soil moisture was at times 10% higher in mulched plots compared to non-mulched plots (Figure 3). Higher soil moisture may reduce potential water-stress events on HLB-affected trees while also promoting a better environment for the biological community.

BIOLOGICAL COMMUNITY
Impacts on the biological community were clearly observed. Mulched plots had an abundance of earthworms and fungi, but these organisms were not observed in non-mulched plots (Figure 4). Both earthworms and fungi are known to assist in the breakdown of organic components and increase overall soil quality.

On the microbial level, higher bacterial diversity was observed in the mulched plots compared to non-mulched plots. A healthier microbiological community can assist in the breakdown and recycling of nutrients, suppress pathogens and create an overall better environment for plant roots.

SOIL pH AND CEC
Soil pH and cation exchange capacity (CEC) were also measured. Both control and mulched soils had a pH between 5.5 and 6.5 throughout most of the study. Cation exchange capacity did not change between mulched plots and non-mulched plots. These results were not unexpected as both pH and CEC of native soils can take years to change.

TREE HEALTH AND GROWTH
Oak mulch applications did not improve overall tree health, even after two and a half years and three mulch applications. Although higher amounts of macronutrients were generally seen in the soils of mulched plots, higher root and leaf nutrients were not observed. Root density analysis did not reveal any differences between mulched and non-mulched plots. No differences in leaf bacterial titer were observed between the control and mulched plots.

CONCLUSIONS
While this project did not find oak mulch to be an effect treatment for HLB-affected trees, it did find that oak mulch applications increased soil nutrients, moisture and biological diversity. Adding organic amendments to increase soil quality is likely to take many years to manifest into increased tree growth and yield. For those growing on nutrient-poor and biologically weak soils, mulch application may be an effective option for soil improvement.

Although hardwood oak mulch was tested in this experiment, mulch made from leaves and other tree species are likely to result in similar soil health benefits.

The oak mulch study will continue until August 2022.

Acknowledgments: The authors aregrateful to Southern Region Sustainable Agriculture Research and Education for funding the graduate student project #GS20-225, “Deploying Oak Mulch to Contain and Suppress HLB Disease in Citrus.” Additionally, this work was supported by the UF/IFAS Krezdorn Memorial Fund. Thank you to Brian Scully and Robert Shatters (USDA-ARS) for their generosity and collaboration, and for the use of their USDA ARS Picos experimental grove in Fort Pierce. Thank you to Steve Mayo (USDA ARS) for his grove management, Randy Burton (UF/IFAS) for procuring the oak mulch and John-Paul Fox (UF/IFAS) for assistance in treatment application and data collection.

Lukas M. Hallman and John M. Santiago are graduate research assistants, and Lorenzo Rossi is an assistant professor — all at the UF/IFAS Indian River Research and Education Center in Fort Pierce.

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