Anaerobic Soil Disinfestation: Carbon Rate Effects on Soil pH, Temperature, Redox Potential, and Tomato Plant Growth

Anaerobic soil disinfestation (ASD) is a non-chemical soil disinfestation technique increasingly proposed for the control of soil-borne pathogens, plant parasitic-nematodes, and weeds in different crops. The technique consists of amending the soil with a readily available carbon (C) source, tarping the soil with impermeable mulch, and irrigating the soil to saturation to create anaerobic conditions that promote the growth of facultative anaerobic microorganisms, which produce organic compounds that are toxic or suppressive to soil-borne pathogens, nematodes, and weeds. The level of anaerobiosis achieved and the efficacy of the technique seems to be influenced by several factors, among which are the C-source and its application rate. Being locally available, molasses (Agricultural Carbon Source, TerraFeed LLC) has been proposed as a C-source in Florida. However, limited information is available on its optimal application rate. Therefore, a greenhouse pot study was conducted to evaluate the effect of increasing molasses (C) rates on soil pH, temperature, redox potential (Eh), and tomato (Solanum lycopersicum L.) plant growth. Black 10-L plastic pots were filled with sandy soil amended with composted poultry manure (22 Mg·ha^-1), and molasses was applied at the rate of 0 (control), 13,900, 27,700, 41,600 and 55,500 L·ha^-1. Pots were saturated with 5 cm of water and tarped with a 0.03-mm black/white TIF mulch. Treatments were arranged according to a randomized complete block design with four replications and five pots per experimental unit. After three-week treatment, tomato seedlings (cv Ridge Runner) were transplanted in each pot to evaluate the plant growth response. At the end of the three-week treatment period, soil pH decreased with increasing molasses rate, ranging from 8.4 in the control to 5.1 with the highest molasses rate. Cumulative-Eh was not influenced by molasses rates (54,000 mV hr), except for the control that showed significantly lower cumulative-Eh (11,000 mV hr). During the three-week treatment period, soil temperature was on average 25.8 °C and it was not influenced by molasses rate. Plant phytotoxicity, and 10% and 35% of plant mortality was observed with the highest molasses rates, respectively. Leaf, stem and total plant dry biomass were significantly influenced by the molasses rate. Total plant dry biomass was similar at 0 and 13,900 L·ha^-1 of molasses, and consistently declined with increasing molasses rate. It is concluded that molasses may be an effective C source to induce reducing conditions; however, application rates above 13,900 L·ha^-1 may be phytotoxic for the crop.