Haselhoff (1892) extracted several lots of 25 kgm. soil, each with 25 litres of water in which quantities of mixed copper salts varying from 0–200 mg. had been dissolved, the mixture consisting of three parts copper sulphate and one part copper nitrate. This operation was repeated 15 times, the soils being allowed to drain thoroughly after each treatment, so that altogether each 25 kgm. soil was extracted with 375 litres water. The drainage waters were analysed, so that the amount of copper absorbed by the soils could be estimated. It was found that by extracting with water containing such soluble copper salts as sulphate and nitrate, the food salts of the soil, especially those of calcium and potassium, were dissolved and washed out, copper oxide being retained by the soil. In this way a double action was manifest, whereby the fertility of the soil was reduced by the loss of plant food, while its toxicity was increased by the accumulation of copper oxide. So long as the soil contained a good supply of undissolved calcium carbonate the harmful action of the copper-containing water was diminished, but as soon as the store was exhausted by solution and leaching, the toxic influence became far more evident. (_e_) _Mode of action of copper on plants._ Quite early in the investigations on the effect of copper on plants the question arose as to its mode of activity--whether the toxicity was merely due to some mechanical action on the root from outside, whereby the absorptive power of the root was impaired, or whether the poisonous substance was absorbed into the plant, so acting directly on the internal tissues. Gorup-Besanez made definite experiments towards ascertaining the truth of these theories as far back as 1863, endeavouring first of all to see whether the plants take up any appreciable quantity of poisons which exist in the soil as mixtures or combinations and which are capable of solution by the cell-sap. Salts of arsenic, copper, lead, zinc and mercury were intimately mixed with soil, 30 grams of the poison being added to 30·7 cubic decimetres of soil, two plants separated by a partition being grown on this quantity. The test plants were _Polygonum Fagopyrum_, _Pisum sativum_, _Secale cereale_ and _Panicum italicum_, and all the plants developed strongly and normally except the last named. The Panicum developed very badly coloured leaves in an arsenic-containing soil, and the plants were killed soon after they started in soils containing copper. After harvesting, the crops were analysed and no trace of copper was found in any one of the experimental plants by the methods adopted. Also the absorption capacity of different soils for different poisons was shown to vary, for basic salts are absorbed, while acids may pass completely through the soil into the drainage water.