Three plastic films [low-density polyethylene (LDPE) plastic films, 100 mm thick, produced by Nortène Plásticos Ltda] were evaluated in their adequacy for soil solarization:. a) LDPE with UV light stabilizer additive, based on hindered amine; b) LDPE with half load of the same additive, c) without additive, and d) a control treatment without plastic mulch. Two experiments were set up from January to February 2000, in Mogi das Cruzes and Piracicaba, São Paulo State, Brazil. After solarization, a lettuce crop was grown in both experiments. Chemical analyses were performed in soil samples and in the harvested lettuce heads of all plots. Weed infestation and the fresh weight of the harvested lettuce heads were assessed. In Piracicaba the soils were analyzed for microbiological properties and the viability of Pythium aphanidermatum was evaluated. In Mogi das Cruzes incidence of lettuce drop, caused by Sclerotinia minor, was assessed. The mean soil temperature under the different plastic films, at 10 cm soil depth, was similar during the treatment, between 44 and 41ºC in the solarized soils of Piracicaba and Mogi das Cruzes, respectively and of 33.9 and 30.2ºC in the uncovered soils from both places. An increase in plant growth of about 49% in Piracicaba and 24% in Mogi das Cruzes was detected, regardless of the kind of plastic applied. A great reduction in weed infestation was observed in all solarized plots of the two experiments, for all plastic films. The microbial analysis of the soils showed no changes in CO2 evolution, biomass C, metabolic quotient, and the number of bacteria. The number of fungi was smaller in the treatments with plastic films containing UV stabilizer, whereas the viability of P. aphanidermatum was reduced in all solarized treatments. Increases in pH, V%, and in the concentrations of NH4+ (190%), Mn (94.6%) and Mg2+ (18%) were observed in the solarized soils. Lettuce plants from the solarized plots showed higher levels of Zn (43%), Mg2+ (12%) and K+ (4%). In Mogi das Cruzes solarization caused an increase in the concentrations of Mn (236%) and Cu (18%) in the soil and in the lettuce plants collected in these plots (increases of 99% in Mn and 27% in Cu levels). Reduction in lettuce drop incidence from 27.7% for control plots to less than 1% in the solarized soils with all plastic films was observed in Mogi das Cruzes. The plastic film without additive tended to splinter at the end of the field trials, after 60 and 90 days of exposure to the environment and was considered inadequate to use for solarization. There was no difference between the plastic films in relation to all analyzed variables.
Lactuca sativa L.; Sclerotinia minor; Pythium aphanidermatum; weed plants
