Plant nutrition is a study that deals with plants’ need for certain chemical elements including their specific and interactive effects on all aspects of plant growth and development, their availabiliy, absorption, transport, and utilization. These chemical elements are referred to as plant nutrients.
Those nutrients which have been proved absolutely needed by plants are commonly called essential elements. They include the carbon (C), oxygen (O), and hydrogen (H) which are derived from atmospheric carbon dioxide (CO2) and from water (H2O), as well as the mineral nutrients which are mainly absorbed from the soil in ionic forms through the roots.
Plant nutrition can be differentiated from animal nutrition based on distinctive traits that plants are autotrophic while animals are heterotrophic. This means that plants produce their own food but animals do not. In both organisms it is this food which serve as source of energy (chemical energy) which is required in life processes.
Plants by themselves obtain their own supply of energy from the sun and absorb nutrients from the air and soil. Ultimately they convert these nutrients into organic compounds which serve as storage of chemical energy for later use and as structural components of the plant body. They are therefore self-sustaining and aptly called "autotrophic".
In contrast, animals are heterotrophic, that is, they are not capable of utilizing directly the radiant energy from the sun. They must obtain their supply from plants in the form of chemical energy either directly or indirectly. They also need to harvest most nutrient elements which their body need from those absorbed by plants from the soil and incorporated into the organic compounds which form the latter's body.
Animals do this harvesting of chemical energy and most nutrient elements by eating plants or by eating those animals which eat plants or the animals which eat plant-eating animals.
Further here are some definitions / meaning of nutrition:
Maynard et al. (1980), in their book Animal Nutrition, explained: “Nutrition involves various chemical reactions and physiological processes which transform foods into body tissues and activities. It involves the ingestion, digestion, and absorption of the various nutrients, their transport to all body cells, and the removal of unusable elements and waste products of metabolism.”
Pertaining to human nutrition, Wardlaw et al. (2004) provided the following definition from the Council on Food and Nutrition of the American Medical Association: Nutrition is “the science of food, the nutrients and the substances therein, their action, interaction, and balance in relation to health and disease, and the process by which the organism ingests, digests, absorbs, transports, utilizes, and excretes food substances.”Therefore, the needed chemical elements enter the animal body in the form of whole food and water. But plants absorb these elements in compound form (e.g. CO2 and H2O) and also in ionic forms (e.g. NH4+, NO3-, H2PO4-, K+).
It took plenty of years for the modern concept of plant nutrition to evolve. First, it was necessary to disprove the Humus Theory of plant nutrition, that is, that plants are soil eaters. Specifically, it was believed then that plants feed upon the organic matter of the soil, or humus, their major source of nutrition. Rather, it was demonstated by numerous researchers that it is some chemical elements in the air and water and the minerals in the soil that sustain plant life.
Plants likewise absorb mineral elements mainly from the soil which become components of the plant body. These chemical compounds and minerals are in turn harvested by animals either directly as plants or plant parts are utilized as food, or indirectly from animals or animal-based food.
In his classic pot experiment starting with a stem of the willow tree, Johann Baptista Van Helmont (1577-1644) concluded that it was water, not soil, that was responsible for the increase in plant size. He was correct as to soil, but only partially as to water. John Woodward (1665-1728) himself demonstrated that the growth of plants was more enhanced when they were grown in muddy water than in rain water. Although it was not apparent then, it is because soil contains some nourishment which is essential to plant growth.
In 1804, Nicolas Thiodore de Saussure (1767-1845) confirmed that plants need the carbon dioxide in the air as well as water that is mainly absorbed through their roots. He concluded that the chemical composition of the plant body consists mainly of carbon dioxide that is fixed from the atmosphere with a portion coming from the soil solution.
He argued that the minerals in the ash of plants, though in small amounts, are essential to plant growth and development. He classified essential elements into two types: one is effective in large amounts (now called major elements or macronutrients) while the other is needed in trace amounts (now called trace elements or micronutrients).
According to Devlin (1975), de Saussure should be credited for discovering the essentiality of minerals in the soil to plant growth. Likewise, Hart (2005) declared that he (de Saussure) founded the modern theory of plant nutrition, along with Jan Ingenhousz and Jean Senebier. However, it was not until confirmed by Justus von Liebeg that the mineral nutrient theory of plant nutrition became popular.
Justus von Liebeg (1803-1873) likewise found that plants are composed of carbon, hydrogen, and oxygen, as well as nitrogen, sulfur, phosphorus and other mineral matter. The minerals are obtained by plants from the soil and these can be detected in their ashes. As a result of a study that involved the application of manures, he concluded that the soil must be replenished by adding the minerals that agricultural crops have withdrawn (Shenstone 1901).
In destroying the humus theory, Liebeg advanced the following argument (in the wording of Shenstone (1901): “since it is universally admitted that humus is only produced by the decay of plants, no primitive humus can have existed for the first plants, for plants must have preceded the humus.”
The hydroponics culture introduced by Woodward made it possible later to shed more enlightenment on plant nutrition by growing plants in nutrient solutions with varying composition. The deliberate exclusion of certain chemical elements in the growth medium particularly showed if indeed they are essentially required in plant nutrition based on the criteria of essentiality.
At least 16 essential plant nutrients or essential elements have now been identified according to approved criteria. Of these, carbon, hydrogen, and oxygen combined comprise the main bulk of the plant body. The carbohydrates alone comprise about 75 percent of the dry matter content of all plants (Maynard et al. 1980).
A carbohydrate molecule is generally composed of C, H, and O in the ratio of 1:2:1 (ex: C6H12O6); therefore, the respective proportions of the elements in the molecule taking into consideration their atomic weights are: C = 40.0%, H = 6.7%, and O = 53.3%. Both C and O are sourced from atmospheric CO2 while H is derived from water or H2O via photosynthesis.
(Ben G. Bareja, Dec. 2012)