Effects of fumigation on soil nitrogen, plant nitrogen and root disease incidence in wheat at Wongan Hills, Western Australia

Abstract

Fumigation of three Western Australian wheat field soils with a mixture of chloropicrin and methyl bromide (2:l) caused changes in soil nutrients and microflora which resulted in increased vegetative growth at all sites, leading to increased grain yields on the two light soils (33 and 49% respectively) but not on the heavy soil, probably because of dry conditions during the grain filling period. Soil fumigation increased concentrations of mineral nitrogen in soil early in the season. Ammonium nitrogen in fumigated soil 49 days after treatment ranged from 18.2 to 26.5 ppm, compared with 1.4-2.2 ppm in untreated soil, while nitrate nitrogen ranged from 0.3 to 2.1 ppm and 1.5 to 7.9 ppm respectively. Much of the growth increases appear to be due to this nitrogen and to a lesser extent the reduction of Gaeumannomyces graminis var. tritici. The possibility that minor pathogens like Fasarium avenaceum, Microdochium bolleyi and weakly pathogenic forms of Rhizoctonia spp. contributed to yield reduction is considered to be unlikely.