New Science

November 2021

What is a plant nutrient? Changing definitions to advance science and innovation in plant nutrition. Plant Soil S97

Brown, P.H., Zhao, F.J., Dobermann, A. 2021.

Summary by Dobermann, A.

The benefits of adding ash or other forms of minerals to soils to improve plant growth have been known for more than 2000 years. However, current definitions of ‘essential‘ or ’beneficial‘ elements for plant growth rely on old and narrowly defined criteria that do not fully represent a new vision for plant nutrition. They also compromise fertilizer regulation and practice. In this paper, the authors first review the historical origins and evolution of the currently used definitions. They then discuss some of the specific shortcomings, illustrating how modern scientific understanding has not been captured properly in scientific definitions as well as fertilizer regulations. For example, mineral elements such as Na, Si, Se, Al, Co or I, are known to also beneficially impact plant growth, but they are relegated to a legal and practical ‘no man’s land’ that by and large does not support their use as fertilizers.

The authors propose a new definition of what is a ‘plant nutrient’: “A mineral plant nutrient is an element which is needed for plant growth and development or for the quality attributes of the harvested product, of a given plant species, grown in its natural or cultivated environment.” Reframing the definition around the term ‘plant nutrient’ and emphasizing that this explicitly includes both the essential and the demonstrated beneficial mineral elements provides greater clarity. It will enable regulators to consider the beneficial elements as legitimate fertilizer components, while encouraging more scientific and commercial inquiry for optimizing yield and quality oriented plant production strategies in different crops and environments. An open scientific debate to refine and implement this updated definition of plant nutrients is proposed, including an independent scientific body to regularly review the list of essential and beneficial elements. The debate could also attempt to refine the definition of plant nutrients to better align with nutrients deemed essential for animal and human nutrition, thus following a more holistic ’one nutrition‘ concept.

Classic Plant Nutrition Paper

The essentiality of certain elements in minute quantity for plants with special reference to copper. Plant Physiology 14:371–375.

Arnon, D.I., Stout, P.R. (1939)

Summary by Dobermann, A.

It was only in the 19th and 20th century that a broader understanding of the role of different chemical elements arose, providing the scientific foundation for using mineral fertilizers in crop production. Nicolas de Saussure, 1804 in France, was perhaps the first to show that developing plants require mineral nutrients, insisting that some elements absorbed by plants were indispensable (‘essential’), while others were not. In the 1820s and 1830s, Carl Sprengel in Germany, listed up to 20 elements that he considered to be plant nutrients. It became clear that identifying the essentiality of specific elements required new techniques. Using nutrient solution culture, Sachs in 1865 identified 10 elements as essential because they were either an integral component of the chemical formula of plant substances (C, O, H, N, S), or because it was demonstrated that the plant cannot complete its vegetation cycle without uptake of any of these elements (P, K, Ca, Mg, Fe). For a long time, it seemed that the list of essential elements would remain at these, but it grew quickly in the 1920s and 1930s, when Mn, B, Zn, Cu and Mo were added to it.

Terms such as micronutrients or trace elements emerged at that time too, to depict nutrients that were required only in very small amounts in the physiology of the plant. The question arose, which of those were indispensable to growth or not. Working at the University of California, Daniel Arnon and Perry Stout tackled that issue in this elegant paper by defining three rigid criteria that an element would have to meet in order to be called an ‘essential’ plant nutrient. One practical implication of this was that a favorable response from adding a given element to the growth medium does not constitute conclusive evidence of its indispensability in plant nutrition. The authors were aware of some of the theoretical and experimental limitations of their definition. They also pointed out that, as scientific methods and knowledge advance, every element in the periodic table might at some point be shown as being essential to plants. Perhaps going far beyond the original intentions Arnon & Stout had, their strict definition of ‘essential’ plant nutrients has prevailed until today. It is still found in many textbooks, and it is also used in standards and regulations for fertilizers worldwide. Their 1939 paper was an important milestone in plant nutrition research, but it is now necessary to replace their definition with one that takes a broader approach to plant nutrition.

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