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Uses of Copper Compounds: Agricultural Uses

Copper compounds have their most extensive employment in agriculture where the first recorded use was in 1761, when it was discovered that seed grains soaked in a weak solution of copper sulphate inhibited seed-borne fungi. By 1807 the steeping of cereal seeds in a copper sulphate solution for a limited time and then drying them with hydrated lime became the standard farming practice for controlling stinking smut or bunt of wheat, which by then was endemic wherever wheat was grown. Flour milled from bunted wheat had to be fed to animals or sold cheaply to ginger bread makers who had found a way of masking its bad taste and color with ginger and treacle. Within a few decades, so general and effective had become the practice of treating seed grains with copper sulphate that the appearance of more than a few bunted ears in a field of wheat was looked upon as a sign of neglect on the part of the farmer. So well have copper compounds controlled bunt that today this seed-borne disease is no longer of any economic importance.

The greatest breakthrough for copper salts undoubtedly came in the 1880's when the French scientist Millardet, while looking for a cure for downy mildew disease of vines in the Bordeaux district of France, chanced to notice that those vines, bordering the highways and which had been daubed with a paste of copper sulphate and lime in water in order to make the grapes unattractive to passers-by, appeared freer of downy mildew. This chance observation led to experiments with mixtures of copper sulphate, lime and water and in 1885 Millardet announced to the world that he had found a cure for the dreaded mildew. This mixture became known as Bordeaux mixture and saw the commencement of protective crop spraying.

Within a year or two of the discovery of Bordeaux mixture, Burgundy mixture, which also takes its name from the district of France in which it was first used, appeared on the scene. Burgundy mixture is prepared from copper sulphate and sodium carbonate (soda crystals) and is analogous to Bordeaux mixture.

Trials with Bordeaux and Burgundy mixtures against various fungus diseases of plants soon established that many plant diseases could be prevented with small amounts of copper applied at the right time and in the correct manner. From then onwards copper fungicides have been indispensable and many thousands of tons are used annually all over the world to prevent plant diseases.

As a generalization, soils would be considered copper deficient if they contain less than two parts per million available copper in the context of plant health. However, where the soil contains less than five parts per million available copper, symptoms of copper deficiency may be expected in animals. The increasing use of chemical fertilizers which contain little or no copper are denuding soils of readily available copper and creating a deficiency of the element in plants and through them in animals. Copper compounds are now being added to the ever increasing copper deficient soils either direct or in combination with commercial fertilizers. This is particularly the case where the fertilizers are rich in nitrogen and phosphorus. Animals grazing on copper deficient pastures or obtaining an inadequate amount of copper through their normal diet will benefit from mineral supplements containing copper.

Copper sulphate, because of its fungicidal and bactericidal properties, has been employed as a disinfectant on farms against storage rots and for the control and prevention of certain animal diseases, such as foot rot of sheep and cattle.