Copper salts are frequently used as fungicides, directly added to plants, e.g. against mild dew, or indirectly as additive to cattle feeding to prevent intestinal diseases. In both cases, copper ultimately results to be added to soils. In some cases, in short times, soils experimented transitions from deficiency to nearly toxicity. This paper aims to establish geochemical controls on copper solubility by pH and redox potential, that are levers on which farmers can act through liming, nitrogen fertilization, input of organic matter, irrigation and drainage. A proper control of these parameters implies considering soil components/soil solution as an open system. Interactions with organic matter, iron oxides and sulphate/sulphide redox couple are the main processes. Though globally toxic, important biological functions of copper were selected by evolution and must be considered too. Toxicity levels cannot be defined on the sole basis of total element content, as soil solid compenents maintain copper concentration in soil solution at levels five orders of magnitude smaller than cupric hydroxide solubility. However, steadily additions of copper remain a concern in both conventional and organic agriculture.