Environmental Engineering Reference
In-Depth Information
may be subject to wind drift and, in the case of some herbicides, may
cause significant damage to nearby trees or other desirable plants. The
potential for drift is reduced if pesticides are applied during periods of
calm weather, preferably in the morning with correctly calibrated pesti-
cide sprayers. Sprayers should operate under proper tank pressure and
have nozzles of the correct size. If drift is likely, tank pressure can be
reduced to make spray droplets larger and more resistant to drift.
Once applied, pesticides may be subject to run-off across the turf
area. If rain is expected, pesticide application should be delayed because
run-off of pesticides can cause harm to plants in the adjacent landscape,
unnecessary contamination of the natural environment and loss of
pesticide.
Some pesticides can be adsorbed or bound to soil particles or thatch.
Application rates of these pesticides may have to be increased when they
are applied to soils containing large proportions of clay or organic matter.
Some pesticides are volatile. Especially at high temperatures they can
easily change from a liquid to a gaseous form and be lost to the atmos-
phere. The vapours can then travel downwind, causing damage to sen-
sitive plants located nearby. Watering them into the turf after application
can reduce losses from volatilization of soil-applied pesticides.
Pesticides that are not readily adsorbed to soil particles may leach
downward, especially in sandy soils. Leaching may carry pesticides
down to the water table, where they can contaminate wells, ponds,
streams, etc.
Once released to the environment, pesticides often undergo chemical
transformations. For example, some foliage-applied pesticides are subject
to photodecomposition (breakdown in sunlight). Because little can be
done to prevent the process or slow it down, photodecomposition is
usually taken into account when application rates are developed.
Virtually all types of pesticides applied to the soil undergo microbial
degradation. Organic molecules of pesticide are used by microbes as a
food source and degraded to relatively simple inorganic compounds
such as carbon dioxide and other compounds naturally occurring in
biological systems. Microbial degradation is usually much faster in the
tropics than in cooler climates. As a result, the duration of residual
pesticide activity is shorter, disappearance from the natural environment
is quicker, and the potential for leaching to the groundwater is less.
Pesticides can be absorbed by the plants to which they are applied,
and in the case of systemic pesticides, they can then be translocated to
other parts of the plant. Translocation can permit control of insects or
diseases otherwise protected from contact with the pesticide, e.g. those
inside plant tissues. Stressed plants, however, may not be able to absorb
enough pesticide to control the pest, so only actively growing plants
should be sprayed.
