Agriculture Reference
In-Depth Information
Box 7.9
Persistence of Chemicals On- and Off-site
A chemical's persistence at the point of application in a vineyard depends
mainly on its
• volatility (hence loss as a vapor) and solubility (hence loss in runoff and
drainage water),
• sorption by soil mineral and organic matter, and
• decomposition by chemical (including photochemical) and/or microbiological
processes.
A chemical that has mobility and is not easily degraded may become widely
disseminated in the environment, as did the organochlorine insecticides such as
dieldrin and DDT when heavily used in the 1950s and 1960s. On the other hand,
the bipyridyl herbicides paraquat and diquat are immobile and very persistent at
the site of application because of strong adsorption by soil clay minerals.
The sorption of many organic pesticides and herbicides is governed by the
soil's organic matter content and the affinity of the compound for organic matter,
expressed by its
K
OC
value. The smaller the
K
OC
value, the lower the sorption of
the compound and the greater its potential mobility. As table B7.9.1 shows, there
is an approximate inverse correlation between the water solubility of herbicides
used in Australian vineyards and their
K
OC
values.
Because soil microorganisms can adapt to consume almost any added C
compound as a substrate for energy and growth, microbial decomposition is a
most important process determining pesticide persistence. The process generally
follows first-order kinetics (section 2.3.5.1). Persistence is expressed in terms of a
chemical's
half-life
(equation 2.2). After one half-life has elapsed, the concentration
is reduced to one-half of its initial value, to one-quarter after two half-lives, and so
on. The half-lives of nonpersistent chemicals are a few days, whereas those of
persistent chemicals are measured in months or even years.
Table B7.9.1
Properties of Herbicides Used in Australian Vineyards
Proprietary Name of Herbicide
and Chemical Name
Solubility in Water
K
OC
(In Parentheses)
(mg/L)
(L/g Soil)
Goal (oxyflurofen)
0.12
14,125
Treflan (trifluralin)
0.22
13,300
Ronstar (oxadiazon)
1.0
4,365
Simazine
6.2
138
Solicam (norflurazin)
28
690
Source:
Data from Williams (2000)
cial insects, indigenous flora and fauna, especially mammals and fish—and its
per-
sistence
, that is, how long it remains active in the environment. Factors that de-
termine a pesticide's persistence are discussed in box 7.9.
Waste Management and Reuse
The composting of pomace is discussed in section 5.7. More and more wineries
are using this approach to recover nutrients from the pressed grapes that would
7.6.2
