3.2 Unintended spreading on or through the soil
As a result of regular spraying, pesticides inevitably will come
into contact with the soil. This is also the case when pesticides are emptied or
spilled on the field, in watercourses or in other places. Such direct discharges
can cause particularly high and acute pesticide concentrations. What follows
below is an outline of how pesticides can be spread to surrounding areas by
surface water, or by being transported by the water through the soil, or by the
root systems of plants, before ultimately being released into the soil.
a) Surface leaching: Plant protection agents generally have the
same leaching pattern as nutrients and soil particles. The leaching pattern can
vary from year to year, from place to place, and depends on weather conditions,
topography and the nature of the soil.
Sorption to soil particles. The degree of the pesticide's
sorption to soil particles depends both on the pesticide and the nature of the
soil. Pesticides with strong sorption to soil particles are especially likely to
be transported by surface water. This usually occurs immediately after heavy
rainfall, and can result in pesticidic ponds in low-lying areas, or in polluted
brooks and rivers.
Soil preparation: The extent of soil preparation usually has an
effect on the surface transport of pesticides. Less soil preparation reduces the
loss of pesticides with strong sorption capacity, which mainly are transported
by soil particles. However, the extent of soil preparation has no influence on
the loss of pesticides with weak sorption to the soil, since they can be
transported easily by surface water without being attached to soil particles.
Soil erosion has an effect on the spreading of pesticides. If
there is heavy erosion many soil particles disappear, and along with them the
pesticides. Certain methods and the time of soil preparation can increase the
risk of erosion. Erosion is often caused by heavy precipitation (see booklet No.
1 "Agriculture" and No. 7, Water supply").
b) Spreading through the soil: The following conditions can
affect the pesticide transport through the soil: the chemical nature, drainage
and structure of the soil and the root systems of plants.
The chemical nature of the soil: The mobility of the pesticide
is affected by its solubility in water and the binding to soil particles - the
greater solubility and weaker binding, the greater mobility. A phenoxide such as
MPCA is highly soluble, whereas DDT is practically non-soluble. The solubility
depends among other things on whether the pesticide is acidic or alkaline. In
the tropical climates the soil is generally acidic and contains iron and
aluminium oxides (see booklet No.1 "Agriculture"). Kaolinite is the most common
clay soil. Kaolinite, oxide and hydroxide are contributing factors to low ion
exchange capacity. When this occurs in acidic soils, pesticides with negative or
neutral ion charge are easily leached out. The soil in tropical climates
generally contains little organic material and clay. This is a highly
contributing factor to increasing the mobility of the pesticide or causing it to
be leached out of the soil.
The drainage and structure of the soil: The transport of
pesticides through the soil also depends on the amount of water in circulation
and the saturation capability of the soil. If the soil contains a lot of water,
which can occur after heavy rainfall, pesticides will be leached out easily.
Pesticides can thus pollute the ground water (see chapter 3.3). The saturation
of the soil depends on the amount of cracks, worm holes or old root systems.
When there is a surplus of water in the soil, tile drains can also transport
water containing pesticides. Both water soluble pesticides as well as pesticides
with strong binding to soil particles will be transported through soil profiles
with many cracks. However, in compact soil such as clay soil, which is rich in
organic materials, it is mainly water soluble pesticides and pesticides with
weak binding to soil particles which will be washed out. Soil preparation and a
more permanent vegetation surface will prevent the water from flowing downwards
and thus reduce the vertical transport of highly soluble pesticides.
Spreading by the root systems: Pesticides can be absorbed by
some plants and transported through the plant to the root system where it is
subsequently released into the soil. This can happen relatively quickly, but is
usually of little importance because only small quantities are being
c) Mitigative measures: The following measures can be taken to
hinder unintended transport of pesticides by water:
· To hinder acute discharges by:
- Establishing safe conditions for diluting the pesticide and
filling it on sprayers.
- Establishing safe pesticide handling routines.
Establishing safe routines for cleaning the spraying equipment.
· Pesticide resits in sprayers
may be avoided by diluting a smaller quantity of pesticide than is normally
recommended for the last portion, and then using it more sparingly.
· Very poisonous pesticides
should not be used on fields sloping down to watercourses during seasons with
· The risk of soil erosion and
surface flow should be reduced.
· Pesticides with high risk of
leaching, such as triazines, should not be used on porous soil above important
ground water sources. The ground water then risks being polluted for several