For a model at high water fluxes, the inclusion of first-xder adsorptiondesorption rate constants leads to slower leaching of pesticide and increased spread than predicted using instantaneous equilibrium. The initial condition of the substance near the soil surface is also found to be important.A model with simultaneous fast and slow adsorptiondesorption equilibria describes and explains published experimental data comparatively well. Under a pattern of intermittent rainfall, the effect of adsorptiondesorption kinetics on pesticide movement is comparatively small.

No abstract

This paper on food energy production and consumption illustrates to what extent agriculture in the Netherlands is dependent on fossil fuel. It can be seen from tabulated data on energy balance in agricultural systems that energy input for secondary production (e.g. meat, milk and eggs) exceeds energy output by approx. 60 X 1012 kcal/yr (70.1 vs. 10.2 X 1012 kcal/yr); this large difference, however, is mainly due to use of fodder (43 X 1012 kcal/yr) rather than use of fossil fuel. Possible ways to diminish the fossil energy input of the modern farm are discussed. (Abstract retrieved from CAB Abstracts by CABI’s permission)

Three models describing adsorption-desorption kinetics of pesticides in soil, that could be incorporated into computer programs on pesticide movement in soil, were discussed, The first model involved single first-order rate equations for adsorption and desorption. Results from an analytical and a numerical solution for local equilibration were compared. Concentration-time relationships for the solution and absorbed phases were calculated for different rate constants, initial conditions, and partition ratios at equilibrium. The second model described simultaneous adsorption-desorption equilibration with two mechanisms, both with their own rate constants. After a comparatively fast equilibration with the first mechanism, there was a gradual increase in extent of overall-adsorption, accompanied with a shift to greater amounts adsorbed by the second mechanism. With the thrid model, adsorption equilibration occurred by diffusion into a stagnant region. With diffusion distances ranging from 0.1 to 4.0 cm, the time needed for approach to adsorption equilibrium varied from about 0.25 days to about one year. Some of the possibilities of these models were discussed considering published experimental results.

Summary: In a column experiment with dry aggregated soils, the effect of aggregate size on leaching of metribuzin from the upper soil layer (2.5 cm) was studied. The penetration of the herbicide was less than predicted by the equilibrium retardation factor. The penetration was less for large than for smalt aggregates due to kinetic effects induced by the aggregates. These kinetic effects overruled the effect of adsorption. Although the mean herbicide penetration was limited, part of the herbicide moved freely with the water as the herbicide was found throughout the column and in the leachate.