Chemicals used to control pests and diseases in plants and animals can have undesirable toxic side-effects on the non-target soil biota. Pesticides have variable effects on soil biota. The same pesticide may affect different species of soil biota in different ways. Also, effects of pesticides depend on the nature of the chemical, its dose, the method of application, temperature and moisture conditions in soil, crop residue management, the rate of decay of the chemical and the extent of leaching from the site. For example, some herbicides are more persistent if sprayed onto a mulch than if sprayed onto bare soil as they may be quickly inactivated by adsorption onto soil particles.

However, some generalisations of effects of pesticides on soil biota can be made. A general rule-of-thumb is that if the toxic effect of a pesticide on a non-target organism lasts longer than 60 days, then the chemical can be regarded as persistent and its toxic effects only slowly reversible.

Another general rule is that toxicity of pesticides from the least to the most toxic to soil biota, follows the order: herbicides < insecticides < fungicides. The following points can be made about the eco-toxic effects of pesticides.

Spraying herbicides.


  • Newer herbicides affect enzyme pathways in plants which are not found in soil biota
  • Older herbicides can have some toxic effects on soil biota e.g. springtails, mites
  • Many fungi are tolerant to herbicides
  • Earthworms can increase in numbers following herbicide application as their food supply, in the form of dead plants, is increased.


  • Organochlorines are more active in soil than organophosphates or carbamates
  • Earthworms are not very susceptible to insecticides except carbamates.


  • Benomyl is toxic to earthworms
  • Some fungicides contain copper which is toxic to earthworms and microbes - see the inhibition of the decay of cellulose by soil microbes in cotton strips buried in soil containing copper. Staining of the strips are areas where microbes have colonised.

Toxic effect of a Copper-based pesticide (+Cu strip) on soil microbes decomposing a cotton strip.