Fig. 8. Curve showing the mean values of the dry weights of nine series of pea plants grown in the presence of anhydrous zinc sulphate and nutrient salts. (May 18th–June 28th, 1910.)
(c) Effect of zinc compounds on plant growth when they are present in soils.
As soon as the presence of zinc in members of the vegetable kingdom was established the question arose as to its effect upon both the plant and the soil.
[Gorup-Besanez (1863)] grew plants in soil with which 30 grams of metallic poisons such as CuSO4, ZnSO4, HgO, were intimately mixed with 30·7 litres (“cubik Decimeter”) of soil[6]. On analysing the ash of Secale cereale, Polygonum Fagopyrum, and Pisum sativum after six months growth he failed to detect the presence of zinc in any one of the three. As the results varied with different poisons on different plants he concluded that the absorption capacity of the various kinds of soils for different poisons varies, that basic salts are absorbed, while the acid salts may pass completely through the soil in the drainage water.
[Freytag (1868)] stated that zinc is retained by the soil in the form of oxide, which is derived from dilute zinc compounds as they filter through the soil, by decomposition by the salts of the soil. For field earth the limit of absorption of zinc oxide from zinc sulphate is between ·21%–·24% of the earth.
[F. C. Phillips (1882)] corroborated Freytag’s statement as to the absorption of small quantities of zinc by the roots of plants, but he states as a fact that both lead and zinc may enter plant tissues without causing any disturbance in the growth, nutrition or functions of the plants, a conclusion that is obviously incorrect or at least incomplete in view of later work on the subject. His choice of plants was certainly unusual, including geraniums, coleas, ageratums and pansies, the poison used being zinc carbonate.
[Holdefleiss (1883)] stated that in spite of a soil content of 2% zinc the vegetation was not in any way harmed, clover fields and meadow lands on zinc soil presenting a normal appearance. This observation was quite inconclusive, as the author proceeds to say that of the plants that were able to absorb zinc salts without disadvantage the most luxuriant were the so-called zinc plants—the exceptions that prove the rule. Two years later [Baumann] showed that such insoluble zinc salts as the carbonate and sulphide in the soil cannot hurt plants. These salts are certainly dissolved to some extent by water containing CO2 but solution is hindered by the constitution of the soil. He also found that the various kinds of soil act differently upon zinc solutions, the absorptive power of pure humus soils (“reinem Humusboden”) for zinc solutions being the strongest. Clay and chalk soils also decompose such solutions energetically, while poor sandy soils have only a weak power of absorption. This selectivity of absorption may account for the difference in the toxicity of zinc salts to plants in the various soils.
[Storp (1883)] experimented to determine the changes in the various characters of the soil by the action of zinc salts on it, and he makes the remarkable statement that in some soils the presence of zinc generates free sulphuric acid, which is particularly injurious to plant life. Grasses, young oaks and figs showed a decrease in dry weight, nitrogen and fat, as the quantity of zinc compounds increased in the water added to the soil. Both the quality and the quantity of the crop were adversely affected. This decrease in the dry weight due to the presence of zinc was confirmed by [Jensch] later on, and also by [Nobbe, Baessler and Will (1884)], who state that both lead and zinc compounds work disadvantageously to vegetation even when they are present in such small quantities that the plants are outwardly sound, the harmful action appearing in the decrease of dry weight. Contrary to Baumann’s opinion, zinc carbonate is said to be one of the salts that exercises this insidious poisonous action. [Storp (1883)] noticed that the direct poisonous action of zinc compounds is largely destroyed by their admixture with soil, but he suggests that a secondary cause of harm is introduced by the accumulation of insoluble zinc salts, so that the fertility of the soil is impaired to the detriment of the vegetation.
[Ehrenberg (1908)] throws out a suggestion that zinc is specially harmful to plant life when it occurs in conjunction with ammonia, but no further evidence has come to light.