Minor elements and plant growth

Summary During recent years more work has been done on the practical value of minor elements than on their physiological function in the economy of the plant. There are still very few elements of which there is definite proof that traces are essential to growth. The importance of boron in plant nutrition is firmly established. Large numbers of species are now known to suffer if the boron supply is insufficient and much commercial loss can be avoided by the judicious admixture of boron compounds with fertilizers. The value of Chilean nitrate is enhanced by its boron content. The part played by boron in the metabolism of the plant, however, still remains obscure. With copper attention has been focused on the value of copper salts in rendering fertile peat, or other highly organic soils, where they are brought under the plough. ?Reclamation disease? can be prevented by small dressings of copper sulphate before sowing. There is little direct evidence that copper is actually essential to the plant, though the assumption is generally made from the weight of indirect proof. Plants may play an important part as carriers of the iodine that is so necessary for animals and human beings, but owing to technical difficulties it has not been possible to establish whether or not iodine is equally necessary to the plants themselves. The value of manganese as an ameliorating agent for certain plant diseases has become increasingly evident, grey speck of cereals, marsh spot of peas and certain chlorotic disorders yielding to applications of manganese salts to the soil or as sprays on the foliage. The plant composition may be affected by treatment. Toxicity due to excess manganese may be controlled to some extent by variations in light intensity. Molybdenum in herbage is recognized as an important causal factor in some animal disorders as ?teart? disease. In plants excess produces characteristic morphological and cytological conditions, but there are indications that in some circumstances traces of the element may prove beneficial. Selenium toxicity in plants can be controlled by the judicious use of sulphur on seleniferous soils. As the absorption of selenium is reduced in this way, the risk of poisoning for animals feeding on the herbage is also minimized. Little definite evidence of benefit to growth due to traces of the element is yet available. Among other elements comparatively little is known as to whether any one or more may be essential for plant life. Isolated instances, particularly with fungi, indicate that such possibilities exist, but much work remains to be done before any assumption in this connexion can be made. The weight of evidence, indirect if not direct, with boron, copper, manganese, and zinc goes to indicate that each of these may play a vital role in plant development, though the exact conditions required and the extent to which each element is essential still remain subjects for investigation.