The heavy metal copper, for example, is an essential tool in organic farming to control fungal infections. Not only do copper salts accumulate in soil, but they also harm soil organisms, as well as aquatic organisms if they get into the surface water. The EU regulation on organic farming permits up to 6 kilograms of pure copper per hectare and year. In Germany, up to 3 kilograms per hectare/year are permitted in organic wine-, potato- and fruit-growing.
Organic farmers employ beneficial organisms to control insects, but when the pests become too numerous they also use plant toxins such as pyrethrum extracts or pyrethrin, azadirachtin and rotenone (no longer permitted in Germany).
Pyrethrum extracts/pyrethrin are neurotoxins that also can attack the human central nervous system and accumulate in the brain. In cases of long-term exposure, they can accumulate in fatty tissue. They are also suspected of causing thyroid and liver cancer. They are toxic to all types of insects, including beneficial insects, and are extremely toxic to fish. Azadirachtin, a substance found in extracts of the neem tree, is harmful, even at low concentrations, to bumble bees. Source
Organic farmers in Germany are now requesting approval to use the synthetic fungicide potassium phosphonate, as they would otherwise lack an effective product to combat fungal infestation during wet summers.
In addition to their toxicity to humans and the environment, these substances are also inefficient and cause non-specific damage - disadvantages that have largely been elimintated in synthetic pesticides.