CHLORIDE AND LIMING EFFECTS ON SOIL-NITROGEN FORM AND TAKE-ALL OF WHEAT

Little is known about how Cl functions in suppressing take-all root rot [Gaeumannomyces graminis] of wheat (T. aestivum L.). A field experiment was conducted to determine the effects of liming and Cl application on ammonium (NH4+-N) and nitrate (NO3--N) concentrations in soil as they correlate with development of the disease. ''Hill 81'' soft white winter wheat was grown on a limed (pH 6.6) and unlimed (pH 5.5) Aquultic Argixeroll fertilized with factorial combination of 30 kg N ha-1 as NH4NO3, (NH4)2SO4, or NH4Cl banded at planting and 140 kg N ha-1 as Ca(NO3)2, (NH4)2SO4, or NH4Cl topdressed in the spring. Soil was sampled to 10 and 20 cm depths and analyzed for NH4+-N, NO3--N, Cl and pH at weekly intervals for 10 wk following spring application of N. Take-all was assessed at 0, 6 and 11 wk after spring fertilization. Take-all severity, NH4+-N and NO3--N concentration, NH4+-N:NO3--N ratios and grain yield were all influenced by a highly significant interaction between liming and spring-topdressed Cl. Limiting hastened disappearance of NH4+-N and increased appearance of NO3--N in unlimed soil but not in limed soil. A "critical" NH4+-N:NO3--N ratio for take-all suppression of 3:1 was estimated from data in the literature. Take-all severity was negatively correlated (r2 = 0.84) with the length of time the NH4+-N:NO3--N ratio remained above the estimated critical ratio. Chloride increased grain yield from 3.5 to 4.7 Mg ha-1 on the unlimed soil but had no effect on yield on the limed soil. Results suggest that Cl may reduce take-all severity and increase grain yield on moderately acid soils in western Oregon [USA] by reducing nitrification and maintaining a favorable NH4+-N:NO3--N ratio for disease suppression.