Oak Mulch Can Improve Florida Flatwoods Soil

Tacy CalliesSoil Improvement, Tip of the Week

Freshly applied mulch with decomposed mulch layers

By Lorenzo Rossi

With no cure for HLB available, searching for alternative mitigation strategies is an urgent priority for a sustainable citrus industry. Interest has grown regarding the use of organic amendments to improve the fertility of Florida native soils. Although hardwood mulch applications have been demonstrated to improve both tree health and soil fertility in other fruit industries, little research is available on the impact of hardwood mulch on tree health and soil fertility in the citrus industry. 

Researchers from the University of Florida Institute of Food and Agricultural Sciences Indian River Research and Education Center (UF/IFAS IRREC) and U.S. Department of Agriculture Agricultural Research Services (USDA ARS) conducted a three-year field study to assess the effects of hardwood oak mulch on HLB-affected citrus trees and soil characteristics. The mulch was sourced from the hardwood branches of oak trees. Three inches of oak mulch were applied once a year over the course of three years to 4-year-old Valencia sweet orange trees grafted on US-812 rootstock. The control trees had no mulch applications.

Natural grove soil from control treatments (left) and mulched soil (right) after 2.5 years

Throughout the study, plots treated with oak mulch had higher soil moisture, soil phosphorus and soil potassium levels. Additionally, mulched plots had a greater abundance of earthworms and fungi, as well as higher microbial diversity compared to control plots. However, mulch applications did not improve tree growth or health, even after three years.

While oak mulch applications may be an effective option for improving soil fertility and microbiome diversity in the short term, improvements in HLB-affected tree health using mulch may take additional years to manifest.

Although oak mulch was used in this study, it is likely that mulch sourced from other hardwood tree species may lead to similar benefits. Mulch can be readily found at landscape and land-clearing companies and most municipalities and can be applied once or twice per year directly underneath citrus trees creating a layer of 2 to 3 inches.

Lorenzo Rossi is an assistant professor at the UF/IFAS IRREC in Fort Pierce.