Plant nutrition is the study of the chemical elements that are necessary for plant growth. There are several principles that apply to plant nutrition.
Some elements are essential, meaning that the absence of a given mineral element will cause the plant to fail to complete its life cycle; that the element cannot be replaced by the presence of another element; and that the element is directly involved in plant metabolism (Arnon and Stout, 1939). However, this principle does not leave any room for the so-called beneficial elements, whose presence, while not required, has clear positive effects on plant growth.
Plants require specific elements for growth and, in some cases, for reproduction.
Major nutrients include:
- C = Carbon 450,000 ppm
- H = Hydrogen 60,000 ppm
- O = Oxygen 450,000 ppm
- P = Phosphorus 2,000 ppm
- K = Potassium 10,000 ppm
- N = Nitrogen 15,000 ppm
- S = Sulfur 1,000 ppm
- Ca = Calcium 5,000 ppm
- Mg = Magnesium 2000 ppm
- Fe = Iron 100 ppm
- Mo = Molybdenum 0.1 ppm
- B = Boron 20 ppm
- Cu = Copper 6 ppm
- Mn = Manganese 50 ppm
- Zn = Zinc 20 ppm
- Cl = Chlorine 100 ppm
These nutrients are further divided into the mobile and immobile nutrients. A plant will always supply more nutrients to its younger leaves than its older ones, so when nutrients are mobile, the lack of nutrients is first visible on older leaves. When a nutrient is less mobile, the younger leaves suffer because the nutrient does not move up to them but stays lower in the older leaves. Nitrogen, phosphorus, and potassium are mobile nutrients, while the others have varying degrees of mobility. Concentration of ppm (parts per million) represents the dry weight of a representative plant.
Plant uses for essential nutrients
Each of these nutrients are used in a different place for a different essential function.
- Carbon is what most of the plant is made of. It forms the backbone of many plant biomolecules, including starches and cellulose. Carbon is fixed through photosynthesis from the carbon dioxide in the air and is a part of the carbohydrates that store energy in the plant.
- Hydrogen also is necessary for building sugars and building the plant. It is obtained from air and liquid water.
- Oxygen is necessary for cellular respiration. Cellular respiration is the process of generating energy-rich adenosine triphosphate (ATP) via the consumption of sugars made in photosynthesis. It is obtained from the air.
- Phosphorus is important in plant bioenergetics. As a component of ATP, phosphorus is needed for the conversion of light energy to chemical energy (ATP) during photosynthesis. Phosphorus can also be used to modify the activity of various enzymes by phosphorylation, and can be used for cell signalling. Since ATP can be used for the biosynthesis of many plant biomolecules, phosphorus is important for plant growth and flower/seed formation.
- Potassium regulates the opening and closing of the stoma by a potassium ion pump. Since stomata are important in water regulation, potassium reduces water loss from the leaves and increases drought tolerance. Potassium deficiency may cause necrosis or interveinal chlorosis.
- Nitrogen is an essential component of all proteins, and as a part of DNA, it is essential for growth and reproduction as well. Nitrogen deficiency most often results in stunting.
- Sulfur is another important component of amino acids and proteins, and is therefore important in plant growth.
- Calcium a part of cell walls. It also regulates transport of other nutrients into the plant. Calcium deficiency results in stunting.
- Magnesium is an important part of chlorophyll, a critical plant pigment important in photosynthesis. It is important in the production of ATP through its role as an enzyme cofactor. There are many other biological roles for magnesium-- see Magnesium in biological systems for more information. Magnesium deficiency can result in interveinal chlorosis.
- Iron is necessary for photosynthesis and is present as an enzyme cofactor in plants. Iron deficiency can result in interveinal [[chlorosis] and necrosis.
- Molybdenum is a cofactor to enzymes important in building amino acids.
- Boron is important in sugar transport, cell division, and synthesizing certain enzymes. Boron deficiency causes necrosis in young leaves and stunting.
- Copper is important for photosynthesis. Symptoms for copper deficiency include chlorosis.
- Manganese is necessary for building the chloroplasts. Manganese deficiency may result in coloration abnormalities, such as discolored spots on the foliage.
- Zinc is required in a large number of enzymes and plays an essential role in DNA transcription. A typical symptom of zinc deficiency is the stunted growth of leaves, commonly known as "little leaf" and is caused by the oxidative degredation of the growth hormone auxin
- Nickel is required in a nitrogen metabolism, however the requirement is vague in all but a very few select plants.
Additional elements include silicon, also used only in a few select plants. Cobalt has proven to be beneficial to at least some plants, but is essential in others, such as legumes where it is required for nitrogen fixation. Vanadium may be required by some plants, but at very low concentrations. It may also be substituting for molybdenum. Selenium and sodium may also be beneficial. Sodium can replace potassium's regulation of stomatal opening and closing.
Plant nutrition is a difficult subject to understand completely, partially because of the variation between different plants and even between different species or individuals of a given clone. Elements present at low levels may demonstrate deficiency, and toxicity is possible at levels that are too high. Further, deficiency of one element may present as symptoms of toxicity from another element, and vice-versa. Carbon and oxygen are absorbed from the air, while other nutrients are absorbed from the soil. Green plants obtain their carbohydrate supply from the carbon dioxide in the air by the process of photosynthesis.