Initial Environmental Assessment: Plant Protection - Series no 13 (NORAD, 1995): Part I: General account: 3 Possible environ mental impacts: 3.4 Impacts of slow degradation in the soil

Initial Environmental Assessment: Plant Protection - Series no 13 (NORAD, 1995)

Part I: General account

3 Possible environ mental impacts

		(introduction...)
		3.1 Unintended spreading by air
		3.2 Unintended spreading on or through the soil
		3.3 Pollution of water
		3.4 Impacts of slow degradation in the soil
		3.5 Impacts on flora, fauna and vulnerable ecosystems
		3.6 Health problems
		3.7 Impacts on local communities, traditional ways of life and utilisation of natural resources

3.4 Impacts of slow degradation in the soil

A significant portion of the pesticides used will sooner or later be degraded to harmless substances. The faster the pesticide is degraded, the smaller the risk of spreading. The degradation speed depends on a number of factors. Generally speaking, the degradation speed will be reduced by drought, poor supply of nutrients and low temperatures. For every ten-degree drop in temperature the degradation speed is increased 2-4 times. Furthermore, highly soluble pesticides degrade more quickly than non-soluble pesticides, and pesticides with strong binding to soil particles degrade more slowly. It is largely the micro-organisms in the soil which degrade pesticides. Such organisms will be positively affected by the same factors as the cultivated plants. This means that soil preparation, irrigation and fertilisation will accelerate the micro-organisms' degradation of the pesticides.

Toxicologists often stress the half life period as one of the factors affecting the official approval of a pesticide. The half life period is the time required for the degradation of half the pesticide substance. For some pesticides the half life period is several years, for others months, weeks, days or hours. The half life period for one single pesticide can vary significantly, depending on the conditions of the soil mentioned above. For example, the half life period for phenoxide may vary from 4-18 weeks and from 15 years for parathion.

When using pesticides, one should aim at a total degradation of the pesticide by the time the effect of the pesticide treatment has been achieved. This will not only reduce the extent of unintended spreading, but also ensure that farmers have a choice as to what kind of crops they wish to grow next season, unhindered by any pesticide residues from the previous season. If a pesticide has not been degraded by the time there is a drought, active pesticides agents can remain in the soil the following season.

Some mitigative measures:

· Avoid pesticides with slow degradation, particularly if local environmental conditions lengthen the degradation period.

· Avoid that sprayed areas lie fallow. Cultivation increases the degradation speed of pesticides.