By Ute Albrecht, Gabriel Pugina, Antonio Castellano-Hinojosa and Sarah Strauss
Root health is important for tree growth and directly affects a tree’s ability to cope with adverse biotic and abiotic stresses. Most citrus production in Florida occurs on natural infertile sands with very little organic matter and a low cation exchange capacity (CEC), resulting in minimal amounts of soluble nutrients to be retained.
The poor nutrient retention is problematic for tree health, especially during the early phase of establishment when rapid development of the tree canopy is essential. This situation is exacerbated when trees become weakened by HLB.
SOIL ORGANIC MATTER
Increasing soil organic matter can improve the soil physicochemical properties including nutrient availability and nutrient and water retention. This, in turn, can increase root production and root health.
Greater organic matter can also increase soil microbial diversity and enhance microbial activity, providing additional benefits to the roots which are interacting directly or indirectly with the microbes. Some of the beneficial effects caused by soil microbes are improved cycling of nitrogen and other nutrients, reduction in the incidence of soil-borne and possibly other diseases, and physiological effects on root growth and metabolic activity.
COMPOST CONSIDERATIONS
One relatively simple way to improve soil organic matter is through the application of compost. Compost is a mixture of various decaying organic substances manufactured through the controlled aerobic, biological decomposition of biodegradable materials like plant matter.
The compost should have undergone mesophilic (medium) temperatures (68 to 113 to degrees) and thermophilic (high) temperatures (113 to 252 degrees). This reduces the viability of pathogens and weeds and stabilizes the carbon, so it is beneficial to plant growth.
Compost should only be obtained from reputable sources or vendors who provide certified information on its nutritional, physical and biological properties. This is especially important for ensuring that the material is free of contaminants like pathogens and viable seeds. See compostingcouncil.org for detailed information on compost manufacturing, certification, regulation and more.
FIELD TRIAL
In 2019, researchers at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) started a field trial on 20 acres of commercial citrus groves in Southwest Florida. The objectives were to determine whether regular applications of compost can improve soil and root health and, in turn, improve growth and productivity of the grafted scion.
Because root structure and root physiology are at least partially determined by the rootstock cultivar, four different rootstocks were included in this trial: US-812, US-897, US-802 and X-639. The scion was standard Valencia.
Compost has been applied twice annually (May and November) at a rate of 5 tons per acre per application. The compost was tilled under the soil in the first application. All other applications have occurred by broadcast spreading. Compost applications are being compared against an untreated control (no compost).
The experiment was established in a split-plot design with compost as the main plot and rootstock cultivar as the subplot. Each compost/no-compost plot is composed of two rows of 100 trees per row (one bed with 200 trees). Rootstocks are arranged in linear subplots of 50 trees per cultivar. Eight replications are included in this trial for a total of 3,200 trees.
RESEARCH FINDINGS
After more than two years of treatments, the compost applications induced significant changes in the soil and leaf nutrients. The CEC was higher in the compost-amended soils compared to the no-compost control as were the concentrations of many of the macronutrients and micronutrients. The soil nutrients that increased were nitrate, potassium, magnesium, calcium and boron. Also notable was the decrease in copper and the increase of the pH caused by the compost.
Compared to the soil, researchers found fewer and different effects for the leaves. In the leaves, only potassium and magnesium increased in response to compost applications. Boron and manganese decreased.
Another interesting observation was that the weed pressure was noticeably higher when compost was applied to the soil. Weed management practices may therefore need to be modified when using compost in citrus production, at least in groves with heavy weed pressure.
So far, no significant effect on tree growth caused by the compost applications has been measured. There were also no differences in the fruit yield between compost and control treatments. However, this was the first harvest, and the trees were not even three years old. Any potential differences may not manifest until trees reach maturity.
Researchers did observe differences among rootstock cultivars. Regardless of the soil amendment, US-802 is currently the most vigorous rootstock of the four included in this field trial, and US-897 produced more fruit (15 pounds per tree) than the other rootstocks (8.5 to 10 pounds per tree).
Fine root characteristics and the abundance, diversity and functionality of the rhizosphere bacterial community are also being monitored. So far, no effect of the compost treatments on the fine root length and root metabolic activity has been noticed.
However, some trends in the root/soil microbial community have been observed, and the responses to the compost appear to be specific to the rootstocks. In particular, there was a greater diversity, abundance and different composition of bacteria near the roots of US-812 and US-897 when compost was applied compared to the other two rootstocks.
There were some bacterial species that increased in abundance near the roots for all rootstocks when compost was applied, and there were other bacterial taxa that only increased near the roots of US-812 and US-897. The different abundances of some bacteria in the rhizosphere of US-812 and US-897 were positively linked to variations in specific root nutrient concentrations (e.g., root magnesium, manganese and zinc). How and whether these different bacteria might impact root health, tree growth and/or productivity are still being investigated.
CONCLUSIONS
The data collected during less than three years of experimentation show that compost amendments can improve soil characteristics, increase nutrient availability and alter the root/soil microbial community. However, these changes have not yet affected tree performance.
Tree growth, productivity and the rhizosphere community were, however, influenced by the rootstock.
The increased weed pressure resulting from the compost amendments may require changes in weed management.
Acknowledgment: This project is supported with funds from the Citrus Research and Development Foundation.
Ute Albrecht (ualbrecht@ufl.edu) and Sarah Strauss are assistant professors, Gabriel Pugina is a master’s student, and Antonio Castellano-Hinojosa is a postdoctoral research associate — all at the UF/IFAS Southwest Florida Research and Education Center in Immokalee.
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