Effects of soil steaming on weed seed viability

A study aimed at evaluating the efficacy against weeds of soil steam application have been carried out both in the field and laboratory in Italy in 2008. Field trials were conducted in two sites using a self-propelled machine equipped with a multi- injector steam distribution system. It consisted of 96 injectors arranged in a 2x2.5 m2 plate. In each injector four holes ensured steam deliver at a single depth of about 18 cm. Steam applications of 6, 8, and 10 minutes duration were compared on plots with area ranging from 5 (site 1) to 25 m2 (site 2). Temperature data at several depths up to 16 cm were collected at 30 sec intervals for about 2 hrs, starting from beginning of steam application, by means of appropriate multi-channel temperature probes, connected to a datalogger. Effects on weed seeds were estimated from emergences recorded both in the field and from soil samples (0-15 cm layer) collected before and 12 hrs after the treatment. In the laboratory, 15-min steam treatment was performed on a soil volume of about 40x40x35 cm by using either superficial application (A) or sub-soil injection at 12 cm depth (B). The two systems of steam application were compared in terms of temperature dynamics in the soil and effects on germinability of Echinochloa crus-galli, Setaria viridis, Solanum nigrum, Galinsoga ciliata, Chenopodium album e Portulaca oleracea seeds kept buried in bags at 2, 7, 13 and 19 cm during the treatment. Main weeds found in the field trial were G. ciliata and P. oleracea in both sites. Good levels of heating were already obtained after 6 min of steam supply in the entire considered slab of soil, with a mean temperature of about 60°C measured one hour after the end of the treatment. In comparison to untreated plots, emergences recorded in the field were reduced by 89% to 95% (65% to 70% in terms of biomass) and by 100% in site 1 and 2, respectively, with no correlation with the treatment duration. Emergences from soil samples were reduced by 75%, 84%, and 93% at 6, 8, and 10 min duration, respectively. In the laboratory trial, steam application with system A resulted in a large non-linear temperature gradient from surface (about 90 °C) to the deepest layer (about 40 °C) that prevented germination in seeds buried up to 7 cm; at 13 cm germinability was reduced by 50%, at 19 cm germinability was generally not affected, excepting seeds of E. crus-galli, which were stimulated. System B reduced germinability of all species by at least 80% in seeds buried at up to 13 cm; from surface to 13 cm-depth, loss of germinability was inversely correlated to the distance from the injector and apparently unrelated to depth of burial. This is likely to be attributable to the homogeneous heating due to the upward steam flow occurring in the bulk of soil close to the injector. The results of this study pointed out that soil steaming is an effective method to reduce weed seed germination. As observed for E. crus-galli, sub-lethal temperatures, which can occur on the margins of the treated areas, can contribute to break seed dormancy, thus increasing the overall emergences.