(c) Toxic action of manganese compounds in soil cultures.
A large body of work has been done with manganese in soil cultures, but the toxic effect is hardly indicated, possibly because it is less manifest under soil conditions, possibly because the observation of the toxic action has been almost completely overshadowed by the interest in the stimulation observed under the same circumstances. [Namba] stated that ·5 gm. MnSO4 added to 8 kgm. Japanese soil exerted a depressing influence on the growth of various plants. The [Hills Experiments (1903)] indicated some toxic effect. Various soluble and insoluble salts of manganese were added to soil in pots at the rate of 2 cwt. per acre, wheat being sown. On the whole the plants from untreated pots were as good as any with manganese except those that received manganese nitrate or phosphate. Manganese iodide distinctly retarded growth. The plants that grew did well eventually, but development of the ear was greatly or entirely retarded. If the seeds were soaked in the iodide, a concentration of 10% was found to be harmful, 5% allowing normal growth. Similar experiments with barley showed that plants treated with manganese carbonate and sulphate were both inferior to the untreated plants; with iodide less plants were obtained and their development was abnormal. Soaking the seeds in the iodide, even in 10% solution, did not do damage as it did with wheat. The oxides were apparently innocuous, but gave no increase either in corn or straw.
[Kelley (1909)] found that on soils in Hawaii in which excessive quantities of manganese are present (5·61% Mn3O4) pineapples do not flourish, but turn yellow and produce poor fruits, and also that if rather less manganese is present (1·36% Mn3O4) the pineapples show the toxic effect by yellowing during the winter months, but they recover completely during the hot summer months. Kelley also observed that the deleterious effect is hardly noticeable during the first twelve months of growth, and that after a time a darkening occurs in the colour of the soil, which he attributes to some change in the constitution of the manganese compounds.
Some interesting observations were made by [Guthrie and Cohen (1910)] on certain Australian soils. A bowling green that was initially covered with a healthy mat of couch grass developed a number of small patches after about three years growth, on which the grass died off. No reason was apparent for this phenomenon, as the cultural conditions were uniform and to all appearances the soil over the whole area was similar in character. Analyses of soil samples from the dead patches and from the neighbouring healthy parts of the green showed that the chemical composition in both cases was practically the same, except that while no manganese occurred in the soil from the unharmed part, as much as ·254% Mn2O3 was found in that from the dead patches. As no other differences were found it was argued that the manganese, present in such large quantities, acted as a toxic agent and killed off the grass. Other instances of manganese poisoning in which wheat and barley were affected are quoted by these authors, the analytical results indicating that possibly barley is able to withstand without injury a greater quantity of manganese compounds in the soil than is wheat.
2. Effect of manganese compounds on germination.
[Nazari (1910)] rolled wheat grains in a paste of manganese dioxide, iron sesquioxide (both with and without organic matter), and in what he terms “artificial oxydases.” The seeds rolled in the last-named showed the greatest energy in germination, while those with manganese gave an appreciable acceleration. The presence of organic matter decreased the action of manganese. The plants from the manganese seedlings gave an increased yield in both straw and grain, while those treated with sesquioxide of iron showed no gain over the check plants.
The [Hills Experiments] yielded some information as to the differing effects of various compounds of manganese on germination. With wheat plants in pot experiments manganese oxide (MnO2) distinctly retarded germination when applied at the rate of 2 cwt. per acre. With barley MnO2, manganese carbonate and sulphate all retarded germination, while with the iodide 50% of the seeds were entirely prevented from germinating.
3. Does manganese stimulate higher plants?
With manganese the evidence in favour of stimulation is more weighty than with such poisons as copper, zinc and arsenic, and the literature on the subject is correspondingly plentiful.