Fig. 13. Curve showing the mean value of the dry weights of ten series of barley plants grown in the presence of sodium arsenite and nutrient salts. (Feb. 10th–April 18th, 1913.)
Fig. 14. Curve showing the mean value of the dry weights of ten series of pea plants grown in the presence of sodium arsenite and nutrient salts. (June 27th–Aug. 10th, 1911.)
(b) Toxic effect of arsenic compounds in sand cultures.
Comparatively few tests seem to have been made as to the action of arsenical solutions in sand cultures. [Stoklasa (1898)] repeated his water culture work, using sand as a medium, and found analogous results by the two methods, i.e. that arsenites are far more toxic than arsenates, and also that the degree of toxicity of a salt varies with the plant to which it is applied, as was shown by the fact that different plants lived for varying times when treated with similar strengths of solution.
(c) Toxic effect of arsenic when applied to soil cultures.
[Daubeny (1862)] watered barley plants with a solution of arsenious acid, 1 ounce in 10 gallons, five times in succession, and found that the crop arrived at maturity about a fortnight earlier than the untreated part of the crop, though the amount harvested was rather less. With turnips four waterings had no effect upon the time of maturity, but again the crop was slightly decreased. The analyses made indicated that no arsenic was taken into the tissues, but that it merely adhered to the external surfaces.
[Gorup-Besanez (1863)] mixed 30 grams arsenious acid with 30·7 litres[9] soil, growing two plants on this quantity of earth. Most of his experimental plants (Polygonum Fagopyrum, Pisum sativum, and Secale cereale) developed normally, but Panicum italicum died soon after the plants appeared above the surface, the leaves being very badly coloured. Analyses by Marsh’s test showed no trace of arsenic in 20 grams dry matter from Secale cereale, but in 148 grams Polygonum Fagopyrum the presence of arsenic was evident, though the mirror formed was weak. With such a large proportion of arsenious acid in the soil it seems hardly conceivable that the plants were not injured to some extent, and also it is probable that with more careful analyses arsenic would have been detected in those instances in which its presence was denied. Yet it must be remembered that [Davy (1859)] had treated pea plants in pots with a saturated solution of arsenious acid for a short time and had stated that the plants were uninjured. Thus both Gorup-Besanez and Davy concur in the opinion that Pisum sativum is indifferent to relatively large quantities of arsenious acid when presented in the soil, whereas the Rothamsted experiments show that in water cultures the plant is extremely sensitive even to minute traces of the substance. It is possible that the arsenic in the solution added to the soil enters into combination with other substances, forming insoluble compounds, thus being removed from the sphere of action and rendered unable to affect plant life. If this be so, the apparent immunity of certain plants to arsenious acid is explained. [F. C. Phillips (1882)], in his experiments on various flowering plants, such as geraniums, coleas and pansies, found that compounds of arsenic in the soil exercised a distinct poisoning influence, tending, when present in large amount, to check the formation of roots, so that the vitality of the plant was so far reduced as to interfere with nutrition and growth, or even to kill it outright. He also stated that traces of arsenic were found in all the plants grown upon the poisoned soil.
In this connection it is interesting to note that a certain proportion of arsenic is frequently present in the superphosphate used as manure. In view of the known toxicity of arsenical compounds to plant life the question arose as to whether superphosphate manuring would exercise a detrimental influence on account of its arsenic content. Experiments carried out by [Stoklasa (1898)], however, indicate that there is not sufficient arsenic in maximum doses of superphosphate to exercise a toxic action in the field.