Other investigators have shown that the presence of insoluble substances has a similar effect in reducing toxicity to an even greater degree. True and Oglevee ([1904], [1905]) again used Lupinus albus as a test plant in the presence of solutions of various poisons in pure distilled water, copper sulphate, silver nitrate, mercuric chloride, hydrochloric acid, sodium hydroxide, thymol and resorcinol all coming under consideration. Clean sea sand, powdered Bohemian glass, shredded filter paper, finely divided paraffin wax and pure unruptured starch grains were respectively added to the solutions, and seedlings were suspended over glass rods so that their roots were in the solutions for 24–48 hours. The solids varied in their action on the different poisons; while the toxic influence of mercuric chloride was reduced by sand and crushed glass, the action of silver nitrate was modified by nearly all the solids. Lupin roots proved unable to withstand an exposure of 24 hours to a concentration of copper sulphate of 1 molecular weight in 60,000 litres of water (i.e. about 1 part by weight CuSO4 . 5H2O in 240·4 parts water), but the addition of solids caused a great decrease in toxicity. When the amount of copper was diminished an advantage was regularly obtained in favour of the cultures containing the solid bodies. On the whole the ameliorating action of solids is more clearly marked with dilute solutions of strong poisons than with relatively concentrated solutions of weaker poisons. As a general rule, filter paper and potato starch grains exert a more marked modifying action than the denser bodies, such as sand, glass or paraffin.
[Breazeale (1906)] tested the same point with extracts of certain soils which proved toxic to wheat seedlings grown in them as water cultures. The toxicity was wholly or partly removed by the addition of such substances as carbon black, calcium carbonate or ferric hydrate. Other experiments showed that the toxic substances of ordinary distilled water are removed by ferric hydrate and carbon black, and further that the latter substance will take out copper from copper solutions, rendering them far less poisonous.
Further corroboration of True and Oglevee’s work was obtained by [Fitch (1906)] who worked in a similar way with fungi, arriving at the general conclusion that insoluble substances in a solution act as agents of dilution or absorption whereby poisonous ions or molecules are in some way removed. He found that n/256 of copper sulphate in beet concoction exercised a stimulating effect on Penicillium glaucum, but the addition of fine glass to the solution increased the stimulation, while large or medium sized pieces did not have the same effect.
This action of solid bodies in reducing the deleterious effects of poisonous solutions is attributed to the process of “adsorption” whereby a layer of greater molecular density is formed on the surfaces of solids immersed in solutions. The solids presumably withdraw a certain proportion of poisonous ions or molecules from the body of the solution (retaining them in a molecularly denser layer over their own surfaces), so that the toxic properties of the solution are reduced owing to the withdrawal of part of the poison from the field of action. In some cases this reduction may be so great as to relieve the solution of its toxic properties, or even to cause an abnormal acceleration to replace a marked retardation. Also, if the solution is of such a dilution as to cause acceleration of growth in plants, the addition of insoluble substances may increase this acceleration. The progressive addition of quantities of solids causes progressive dilution of the toxic medium, the underlying cause of these results being the gradual removal of molecules or ions from the solutions by the insoluble body present.
Fitch’s results are also in accordance with the well-known fact that the physical condition and properties of the added solid play a considerable part in determining its efficacy as an adsorbing agent.
(d) Effect of copper on plant growth when present in soils.
As has already been shown the toxic property of copper with regard to plants was recognised almost as soon as that element was found to occur in the vegetable kingdom, but little notice was taken of the discovery for many years. In 1882 [F. C. Phillips] asserted, as the result of experiments with various cultivated flowering plants, including geraniums, coleas, ageratum, pansies, &c., that under favourable conditions plants will absorb small quantities of copper by their roots, and that such compounds exercise a distinctly retarding influence even if in very small amount, while if large quantities are present they tend to check root formation, either killing the plants outright or so far reducing their vitality as seriously to interfere with nutrition and growth. Two years later [Knop] confirmed both the absorption and the toxicity of copper by his experiments on maize.
[Jensen (1907)] worked with “artificial” soils, under sterile conditions, using finely ground quartz flour for his medium and wheat for a test plant, parallel experiments being carried on with solutions. Every precaution was taken to ensure sterility—the corks were boiled first in water and then in paraffin, the seeds were sterilised in 2% copper sulphate solution for 3⁄4 hour, washed in sterilised water, planted in sterilised sphagnum, the transplanting being done in a sterile chamber into sterilised solutions. The criteria used to determine the toxic and stimulation effects were the total transpiration, average length of sprout, the green weight and dry weight of plants. The results obtained with the different substrata showed that it does not follow that a salt highly toxic in solution is equally so in soil, or that one which holds a relatively high toxic position in soil should occupy the same relative position in solution cultures. For instance, while in soil cultures nickel compounds were the most toxic of all the substances tried, in solution cultures silver compounds were more poisonous than nickel. The range of concentrations, both fatal and accelerating, was found to be much greater in solution than in soil cultures.
In the sand cultures the toxicity of the copper sulphate was found to decrease as the ratio of the quartz sand to the poisonous solution increased, provided that a water content suitable for growth was present. Jensen states that the fatal concentration of copper sulphate in solution cultures is approximately 1⁄10th that of the fatal concentration in his artificial soil.
When copper salts are added to soil a complication at once sets in due to the double decomposition which is always likely to occur when any soluble salt is added to soil. The reaction may be graphically expressed as follows, in a much simplified form—