Geoscience Reference
In-Depth Information
Fig. 12.40 Distribution of a moisture, b napropamide, and c bromacil 24 h after application of
the herbicide solution to Bet-Dagan soil, at 4 L/h from a point source (Gerstl et al.
1979a
)
Differences in the leaching of bromacil and napropamide through the undis-
turbed soil columns were observed; in particular, there is a striking effect of initial
soil water content on herbicide transport (Fig.
12.41
). Pre-wetted soil was found to
retain more herbicide than initially dry soil, and the highest concentration of
herbicide was consistently found in the uppermost soil layer. Napropamide has a
high-adsorption coefficient, and this, in general, should be the dominant process.
However, because adsorption equilibrium is not attained instantaneously and water
flow is rapid especially in the dry soil, leaching also occurs. This is a typical case
of preferential flow. Discontinuous leaching of the pre-wetted soil permits more of
both herbicides to be retained in the surface layer than continuous leaching, pre-
sumably because diffusion of the herbicides into aggregates occurs during the
interval between leachings. The results of White et al. (
1986
) clearly show that, for
