Compost-amended plots (left) had more weeds than those with no compost.
A new study published in Scientific Reports examines whether repeated compost applications could improve the health and productivity of young citrus trees growing under endemic huanglongbing (HLB) conditions in Florida. Gabriel Pugina, Caroline Tardivo, Brittney Monus, Emma Dawson, Sarah Strauss and Ute Albrecht of the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Southwest Florida Research and Education Center conducted the research.
Researchers evaluated young Valencia orange trees grafted on four commonly used rootstocks (US-802, US-812, US-897 and X-639) and grown in a commercial grove in Southwest Florida. Compost was applied twice a year for four years to determine whether improving soil health could help trees better cope with HLB.
SIGNIFICANT SOIL IMPROVEMENT
The good news is that compost significantly improved soil conditions. Trees receiving compost had higher soil organic matter, better moisture retention, improved nutrient availability and greater nutrient-holding capacity. Compost-treated soils also contained more potassium, calcium, magnesium and boron.
The improved soil environment stimulated root activity, leading to higher root respiration and more efficient fine-root growth. Researchers also found that compost changed the microbial communities living around citrus roots and altered root metabolism in ways that suggested improved nutrient use and reduced stress.
LACK OF OTHER BENEFITS
However, those below-ground benefits did not translate into better tree growth or higher yields.After four years, compost-treated trees were no larger than untreated trees, and fruit production was essentially unchanged. Compost also did not reduce HLB bacterial levels in leaves or roots. Researchers concluded that while compost improves soil and root health, those gains are not enough to overcome the severe physiological limitations imposed by HLB.
ROOTSTOCK EFFECTS
The study found that rootstock selection had a much greater impact on tree performance than compost. Rootstocks influenced canopy size, root characteristics, nutrient uptake and yield. Among the rootstocks tested, US-897 produced the highest yields, although it also experienced greater fruit drop than some of the other rootstocks.
COMPOST AND WEED GROWTH
One interesting finding was that compost increased early-season weed growth, which may have competed with young trees for water and nutrients. Researchers suggest this competition could have offset some of the benefits gained from improved soil conditions.
GROWER TAKEAWAYS
For citrus growers, the takeaway is clear: Compost can be a useful tool for improving soil health, water retention and root function in HLB-affected groves. However, compost alone should not be expected to increase production under severe HLB pressure.
The study reinforces the importance of selecting productive, HLB-tolerant rootstocks as one of the most effective management decisions available today. Combining strong rootstocks with sound soil-management practices may provide the best path toward maintaining grove productivity in Florida’s HLB era.
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