Simple mathematical model proposition of herbicide persistence in the soil

The present study proposes a simple model of persistence of herbicides in s oil based on a series of modifications to the model developed by Walker & B arnes. The model proposed simulates the daily degradation of a herbicide in soil, through three submodels: a) soil temperature submodel, b) soil moist ure content submodel and, c) degradation of the product submodel. A theoret ical description of the modifications introduced to the model and a detail of the software program in language BASIC are given. An independent validat ion of each one of the modified submodels was done and it concludes that al l of them improve the efficiency of prediction of the original model. The v alidation of the degradation submodel was made using field obtained informa tion in two different soils from the herbicides metsulfuron-metil and trias ulfuron. Finally it is possible to conclude that the proposed model would b e efficient in the simulation of the persistence of these sulfonylureas in the soil, using a kinetic first degree model for metsulfuron-mehtyl and one of second degree for triasulfuron.