Changes in the Bioavailability of Ionizable Herbicides in Volcanic Soils Due to Soil Acidification by Urea as Fertilizer

The application of urea as a nitrogen fertilizer and herbicide is a common practice in agricultural systems. However, there is no background information on how the soil acidification caused by urea might affect the herbicide bioavailability in volcanic soils. The persistence study was conducted under microcosm incubation conditions in two Andisol soils amended with a field equivalent nitrogen dose of 200 kg N ha-1 and double dose of 400 kg N ha-1. Clopyralid, fluroxypyr, picloram, and triclopyr, acidic ionizable herbicides, were applied at the field equivalent dose. Adsorption studies were also carried out on both soils at pH 4, 5, and 6. Clopyralid and picloram showed the greatest increase in half-life in the range of 20-80%. The application of twice the dose of urea resulted in minor changes. A higher adsorption implies a higher persistence of the herbicides. This was more evident for the Piedras Negras soil (PNS). The conclusion of this work is that soil acidification by urea increases the persistence of ionizable herbicides in Andisol soils and that this effect depends on the acidity of the herbicide and the physico-chemical characteristics of the soil, which are among the most determining factors.