The essential nature of certain minor elements for plant nutrition. II

In spite of war conditions, work on the relations between minor elements and plants has made considerable progress, and the literature on the subject has greatly increased, especially in regard to boron. Relatively little advance has been made in discovering fresh elements that can be considered as essential, using the word in its strict sense. Attention has rather been focussed on the practical aspects of the problem, such as the improvement in crops brought about by the use of certain elements as subsidiary manures, or the damage done by others in similar conditions. The effect of minor elements, absorbed by the roots, on the nutritive or toxic properties of crops and herbage in relation to animal nutrition has received more consideration, as also has the value of spraying for remedying minor element deficiencies, and for controlling various forms of plant disease. Apart from boron most of the work has related to arsenic, copper, manganese, mercury, molybdenum, selenium and zinc, all of which were recognised before the war as having important relations with plant growth in one way or another. Although so much work has been done, the fundamental problem of the function of minor elements in plant metabolism still remains unsolved, and a vast field remains open for investigation.