Plant nutrition


Plant nutrition is the study of the chemical elements and compounds necessary for plant growth and reproduction, plant metabolism and their external supply. In its absence the plant is unable to complete a normal life cycle, or that the element is part of some essential plant constituent or metabolite. This is in accordance with Justus von Liebig's law of the minimum. The total essential plant nutrients include seventeen different elements: carbon, oxygen and hydrogen which are absorbed from the air and water, whereas other nutrients including nitrogen are typically obtained from the soil.
Plants must obtain the following mineral nutrients from their growing medium:
These elements stay beneath soil as salts, so plants absorb these elements as ions. The macronutrients are taken up in larger quantities; hydrogen, oxygen, nitrogen and carbon contribute to over 95% of a plant's entire biomass on a dry matter weight basis. Micronutrients are present in plant tissue in quantities measured in parts per million, ranging from 0.1 to 200 ppm, or less than 0.02% dry weight.
Most soil conditions across the world can provide plants adapted to that climate and soil with sufficient nutrition for a complete life cycle, without the addition of nutrients as fertilizer. However, if the soil is cropped it is necessary to artificially modify soil fertility through the addition of fertilizer to promote vigorous growth and increase or sustain yield. This is done because, even with adequate water and light, nutrient deficiency can limit growth and crop yield.

History

, hydrogen and oxygen are the basic nutrients plants receive from air and water. Justus von Liebig proved in 1840 that plants needed nitrogen, potassium and phosphorus. Liebig's law of the minimum states that a plant's growth is limited by nutrient deficiency. Plant cultivation in media other than soil was used by Arnon and Stout in 1939 to show that molybdenum was essential to tomato growth.

Processes

Plants take up essential elements from the soil through their roots and from the air through their leaves. Nutrient uptake in the soil is achieved by cation exchange, wherein root hairs pump hydrogen ions into the soil through proton pumps. These hydrogen ions displace cations attached to negatively charged soil particles so that the cations are available for uptake by the root. In the leaves, stomata open to take in carbon dioxide and expel oxygen. The carbon dioxide molecules are used as the carbon source in photosynthesis.
The root, especially the root hair, a unique cell, is the essential organ for the uptake of nutrients. The structure and architecture of the root can alter the rate of nutrient uptake. Nutrient ions are transported to the center of the root, the stele, in order for the nutrients to reach the conducting tissues, xylem and phloem. The Casparian strip, a cell wall outside the stele but in the root, prevents passive flow of water and nutrients, helping to regulate the uptake of nutrients and water. Xylem moves water and mineral ions in the plant and phloem accounts for organic molecule transportation. Water potential plays a key role in a plant's nutrient uptake. If the water potential is more negative in the plant than the surrounding soils, the nutrients will move from the region of higher solute concentration—in the soil—to the area of lower solute concentration - in the plant.
There are three fundamental ways plants uptake nutrients through the root:
  1. Simple diffusion occurs when a nonpolar molecule, such as O2, CO2, and NH3 follows a concentration gradient, moving passively through the cell lipid bilayer membrane without the use of transport proteins.
  2. Facilitated diffusion is the rapid movement of solutes or ions following a concentration gradient, facilitated by transport proteins.
  3. Active transport is the uptake by cells of ions or molecules against a concentration gradient; this requires an energy source, usually ATP, to power molecular pumps that move the ions or molecules through the membrane.
Nutrients can be moved in plants to where they are most needed. For example, a plant will try to supply more nutrients to its younger leaves than to its older ones. When nutrients are mobile in the plant, symptoms of any deficiency become apparent first on the older leaves. However, not all nutrients are equally mobile. Nitrogen, phosphorus, and potassium are mobile nutrients while the others have varying degrees of mobility. When a less-mobile nutrient is deficient, the younger leaves suffer because the nutrient does not move up to them but stays in the older leaves. This phenomenon is helpful in determining which nutrients a plant may be lacking.
Many plants engage in symbiosis with microorganisms. Two important types of these relationship are
  1. with bacteria such as rhizobia, that carry out biological nitrogen fixation, in which atmospheric nitrogen is converted into ammonium ; and
  2. with mycorrhizal fungi, which through their association with the plant roots help to create a larger effective root surface area. Both of these mutualistic relationships enhance nutrient uptake.
The Earth's atmosphere contains over 78 percent nitrogen. Plants called legumes, including the agricultural crops alfalfa and soybeans, widely grown by farmers, harbour nitrogen-fixing bacteria that can convert atmospheric nitrogen into nitrogen the plant can use. Plants not classified as legumes such as wheat, corn and rice rely on nitrogen compounds present in the soil to support their growth. These can be supplied by mineralization of soil organic matter or added plant residues, nitrogen fixing bacteria, animal waste, through the breaking of triple bonded N2 molecules by lightning strikes or through the application of fertilizers.

Functions of nutrients

At least 17 elements are known to be essential nutrients for plants. In relatively large amounts, the soil supplies nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur; these are often called the macronutrients. In relatively small amounts, the soil supplies iron, manganese, boron, molybdenum, copper, zinc, chlorine, and cobalt, the so-called micronutrients. Nutrients must be available not only in sufficient amounts but also in appropriate ratios.
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 cause deficiency symptoms, and toxicity is possible at levels that are too high. Furthermore, deficiency of one element may present as symptoms of toxicity from another element, and vice versa. An abundance of one nutrient may cause a deficiency of another nutrient. For example, K+ uptake can be influenced by the amount of NH available.
Nitrogen is plentiful in the Earth's atmosphere, and a number of commercially-important agricultural plants engage in nitrogen fixation. However, plants mostly receive their nitrogen through the soil, where it is already converted in biological useful form. This is important because the nitrogen in the atmosphere is too large for the plant to consume, and takes a lot of energy to convert into smaller forms. These include soybeans, edible beans and peas as well as clovers and alfalfa used primarily for feeding livestock. Plants such as the commercially-important corn, wheat, oats, barley and rice require nitrogen compounds to be present in the soil in which they grow.
Carbon and oxygen are absorbed from the air while other nutrients are absorbed from the soil. Green plants ordinarily obtain their carbohydrate supply from the carbon dioxide in the air by the process of photosynthesis. Each of these nutrients is used for a different essential function.

Basic nutrients

The basic nutrients are derived from air and water.

Carbon

forms the backbone of most plant biomolecules, including proteins, starches and cellulose. Carbon is fixed through photosynthesis; this converts carbon dioxide from the air into carbohydrates which are used to store and transport energy within the plant.

Hydrogen

is necessary for building sugars and building the plant. It is obtained almost entirely from water. Hydrogen ions are imperative for a proton gradient to help drive the electron transport chain in photosynthesis and for respiration.

Oxygen

is a component of many organic and inorganic molecules within the plant, and is acquired in many forms. These include: O2 and CO2 and H2O, NO, H2PO and SO. Plants produce oxygen gas along with glucose during photosynthesis but then require O2 to undergo aerobic cellular respiration and break down this glucose to produce ATP.

Macronutrients (primary)

Nitrogen

is a major constituent of several of the most important plant substances. For example, nitrogen compounds comprise 40% to 50% of the dry matter of protoplasm, and it is a constituent of amino acids, the building blocks of proteins. It is also an essential constituent of chlorophyll. In many agricultural settings, nitrogen is the limiting nutrient for rapid growth.

Phosphorus

Like nitrogen, phosphorus is involved with many vital plant processes. Within a plant, it is present mainly as a structural component of the nucleic acids: deoxyribonucleic acid and ribonucleic acid, as well as a constituent of fatty phospholipids, that are important in membrane development and function. It is present in both organic and inorganic forms, both of which are readily translocated within the plant. All energy transfers in the cell are critically dependent on phosphorus. As with all living things, phosphorus is part of the Adenosine triphosphate, which is of immediate use in all processes that require energy with the cells. Phosphorus can also be used to modify the activity of various enzymes by phosphorylation, and is used for cell signaling. Phosphorus is concentrated at the most actively growing points of a plant and stored within seeds in anticipation of their germination.