Skip to main content
ARS Home » Southeast Area » Fort Pierce, Florida » U.S. Horticultural Research Laboratory » Citrus and Other Subtropical Products Research » Research » Publications at this Location » Publication #333035

Title: Effect of molasses in anaerobic soil disinfestation: Focus on the soil microbiome

item Hong, Jason
item DI GIOIA, FRANCESCO - University Of Florida
item OZORES-HAMPTON, MONICA - Desiderio Finamore Veterinary Research Institute (FEPAGRO)
item Rosskopf, Erin

Submitted to: Proceedings of Methyl Bromide Alternatives Conference
Publication Type: Proceedings
Publication Acceptance Date: 9/2/2016
Publication Date: N/A
Citation: N/A

Interpretive Summary: Anaerobic soil disinfestation (ASD) is biological based non-chemical pre-plant technique to manage plant pathogen and weed population. ASD is made up of three steps soil amendments working into the soil, tarping the soil, and then saturating it. The soil remains covered for three weeks, during this time environment under the plastic becomes anaerobic. In Florida, the standard method for applying ASD includes using composted broiler litter, 22 Mg ha-1, and molasses, 13.9 m3 ha-1 as the soil amendments. For this experiment, various rates of molasses, 0-4 times the standard, were applied to the soil to determine the effect that molasses concentration had on the soil microbiome. Each trial compared various variables including: pH, temperature, soil moisture, and reduction potential of the soil; bacterial and weed populations; and plant vigor. Minute changes in the microbiome were observed in the no molasses treatment. The microbiome continuously changed in the standard molasses concentration, and it had similar plant vigor rating to the no molasses treatment. The microbiome in the became static in the increased molasses rates by day 7. Soil pH in the no molasses treatment had minute fluctuation in the soil pH, while throughout the treatment the pH for the molasses treatments dropped. Treatments with molasses above the standard concentration remained acidic.

Technical Abstract: Growers in Florida have begun to apply molasses to increase the soil microbial biodiversity. Previous reports have indicated that a greater microbial diversity could cause disease suppressive soils. Furthermore, it has been shown that under moderate temperature, 15-20° C, an increase of carbon could be more effective in for creating a disease suppressive soil. The current standard for applying ASD in Florida consists of incorporating composted broiler litter (CBL), 22 Mg ha-1, and molasses, 13.9 m3 ha-1 as the soil amendments, covering the soil with a polyethylene film , and saturating the soil through irrigation. The objective of this study was to compare various rates of molasses, 0-4 times the standard rate (treatment 2): treatment 1 = 0, 2 = 13.9, 3 = 27.7, 4 = 41.6, and 5 = 55.5 m3 ha-1. This study compared the effect of the molasses on the soil microbiome and factors that are associated with the microbiome including, soil pH, moisture, and cumulative redox potential (Eh). In conjunction with the microbial data, tomato plant vigor and weed count were also measured. The experiment was performed fall/winter of 2015 in greenhouse conditions at University of Florida, IFAS SWFREC (Immokalee, FL). Soil was amended with CBL and stored in 10 L plastic pots. Various rates of molasses were added in accordance to the treatments. Soil was then saturated with 5 cm of water and covered with Vaporsafe Totally Impermeable Film. Three-week post ASD treatment, the film was removed and a tomato seedling, cv Ridge Runner (Syngenta), per pot were transplanted. The experiment was conducted in a randomized complete block design, with four replications. Five pots made up an experimental unit. Soil cores for each replication of each treatment were homogenized. Samples were taken at 1, 3, 7, 14, and 21 days post ASD treatment (dpt). and was frozen at -20° C. DNA was extracted using a modified protocol of the MO BIO PowerSoil® DNA Isolation Kit. Length heterogeneity-PCR (LH-PCR) was used to determine bacterial population. LH-PCR was performed as a normal PCR, using the V2 and V3 primers, except the V2 primer was tagged with fluorescent tag (6-fluorescein amidite) on the 5’ region. Detected amplicons were interpreted by Genemapper v5.0 (Applied Biosystems, Foster City, CA, USA) and analyzed by PRIMER v6 and PERMANOVA+ software packages. Shifts in the microbiome correlated to molasses rate, changes in soil pH, and reduction potential. During the treatment, pH dropped for all of the treatments except for treatment 1, and treatments 3-5 remained acidic at 21 dpt. All of the treatments with molasses went anaerobic. From day 1 to day 3 the bacterial communities, as detected by LH-PCR, shifted except for treatment 1. Bacterial diversity decreased for the treatments 3-5 at 7 dpt, and these communities were static. The relative abundance of population 435 was 438 were greater in treatments 3-5 than treatments 1 and 2. These populations correspond to members of the Bacteroidetes and Proteobacteria phyla respectfully. Plant phytotoxicity and mortality were observed in treatments 4 and 5. Regarding leaf, stem, and total dry plant biomass, treatments 1 and 2 were similar, while these variables were inversely proportional to the molasses rate. In conclusion, CBL alone did not change the soil pH and had little effect on the soil microbiome over the 21-day treatment. Furthermore, molasses applied greater than 13.9 m3 ha-1 can select for bacterial communities that lower the pH and can cause phytotoxic environments.