Integration of Organic Amendments and Weed Management to Improve Young Citrus Tree Growth Under HLB-Endemic Conditions

Florida citrus production has declined by over 90% since the bacterial disease huanglongbing (HLB) was found in the state. In the absence of an effective cure, growers are adopting more frequent fertilization and irrigation practices to improve tree health and prolong the life span of their orchards. However, Florida’s soils under citrus production are sandy, with little organic matter, a low water holding capacity, and a low cation exchange capacity (CEC), rendering them prone to nutrient leaching. Organic amendments can be used to improve soil health and the environment for citrus roots, but may promote a higher incidence of weeds competing with trees for water and nutrients. A large field trial was established in a commercial citrus orchard in southwest Florida to evaluate the effects of organic amendments and weed management on young tree growth. The organic amendment treatments were as follows: (1) plant-based compost, (2) humic acid, and (3) a non-amended control. The weed management (herbicide) treatments were (1) glyphosate, (2) glufosinate, (3) flumioxazin, and (4) a maintenance herbicide control. Trees were planted in August 2019, and treatments began in 2021. Tree growth and physiological variables and soil physicochemical properties were evaluated during the two-year study. Compost-amended plots had a higher volumetric water content throughout the experiment, and soil nutrient content, organic matter, CEC, and pH were higher after two years of application. Humic acid amendments were less effective in altering these soil properties. Compost’s effects on tree and fibrous root physiology were moderate, and tree growth, fruit yield and fruit quality were not affected by either organic amendment. In contrast, the use of post-emergent herbicides (glyphosate and glufosinate) improved tree growth and nutrient uptake. The results suggest that in Florida, the use of organic amendments needs to be integrated with weed management to prevent resource competition. In the short term, these practices did not improve the productivity of the trees in the current Florida production environment.