Critics often reproach organic farmers for the following: by having to refrain from using synthetic chemicals, they instead have to use large quantities of copper to fight disease, especially mildew. And, since copper is a heavy metal, it would accumulate in the soil and its toxicity would be responsible for environmental pollution that would be even worse than that caused by synthetic chemicals.
This remark must be taken seriously as copper is toxic in high doses. A few decades ago, it was not unusual to see growers spraying annual doses of Bordeaux mixture (bouillie bordelaise) equal to more than 10 kg of copper per hectare (ha). Today, organic specifications limit copper use to an average of 6 kg/ha per year over five years. Some biodynamic producers associations are more strict: Demeter aims for 3 kg/ha per year on average over five years. These growers are aware of copper’s drawbacks and are constantly seeking ways to minimise its use. But for me, when copper is used in moderation, there is no comparison between it and synthetic pesticides in terms of harmful impact.
First of all, copper is a contact product. In other words, it stays on the surface, on the outside of the leaves and the grape. It forms a sort of barrier that stops attacks from the fungus responsible for mildew. Chemical fungicides, however, are generally penetrating or systemic. Penetrating products, as their name suggests, penetrate plant cells and act at the biochemical level. Systemic products go even deeper into the plant’s system to the sap. From there, they spread to all the organs, to the grapes and even the roots. Clearly their effectiveness is far superior to that of copper … as is their potential for disrupting the vine’s balance. In addition, being a heavy metal, copper is eliminated after pressing during clarification. On the other hand, traces of synthetic products can be found at the biochemical level of grape cells, and therefore in the wine. For the consumer, the difference is indeed substantial.
Secondly, it is important to add that copper is generally used in the form of a mineral salt, for example copper sulphate. In very small doses it is a trace element and, contrary to chemical products, essential for life. In humans and mammals for example, who are regulated by the liver, copper plays a role in the functioning of the immune system. Copper is stored, excreted in the bile and distributed to the organs. We are talking about doses of a few mg/kg. Its anti-infective properties were already known and used in ancient Egypt and are still used for their purifying properties in certain cases: roofs or gutters to collect rainwater, piping, and preserving pans for jam. Thus, even in moderate doses, it is toxic for microorganisms such as mildew, and it disrupts soil’s microbial life. It is all a question of dosage.
Let us make a quick calculation: one hectare (ha) of vines, 10,000 m2, receives an average dose of 3 kg of copper annually. Let us suppose that this copper is deposited entirely in the soil which has an average depth of 50 cm and a volume of 5,000 m3/ha. Soil density is approximately 1,200 kg/m3 (considerably higher for land in Burgundy which is rich in clay and limestone), thus approximately 6,000 tonnes/hectare. According to this hypothesis, the use of copper would be equal to 0.5 mg/kg, which is a concentration relatively compatible to humans. Reputed soil microbiology specialist, Claude Bourguignon, estimates that, depending on the nature of the terroir, a well-functioning soil can eliminate approximately 1 to 3 kg copper per hectare per year via metabolisation.
For this dilemma as for all, it is a question of moderation. From my perspective, I believe that for many biodynamic producers, the amount of copper used is compatible with the maintenance of a living soil (which the land itself displays), and is a thousand times more desirable than the use of synthetic chemicals.