Nutrients

Most plants require the same nutrients, although absolute requirements may vary. Nutrients are categorized into two classes depending upon their proportion in the plant body. Macronutrients compose more than 1% of the weight of plants and include carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, calcium, magnesium, and potassium. Most of these have a structural or important physiological role in plants. Carbon, hydrogen, and oxygen are incorporated into plants through photosynthesis. The remaining nutrients are absorbed from the soil. Micronutrients make up less than 1% of plant weight. These include iron, molybdenum, copper, zinc, manganese, nickel, and boron. These elements are associated with enzymatic functions. Some plants may contain sodium, chlorine, selenium, and silicon, but the evidence for the requirement for these elements is less clear.

Soil nutrient availability may be divided into two groups: nutrients whose availability is largely controlled biologically and those largely controlled abiotically. Common examples of these groups are nitrogen and phosphorus, which are nutrients most commonly limiting to plants. The nitrogen cycle consists of biological processes involving fixation of atmospheric nitrogen into organic nitrogen (nitrogen fixation), release of organic nitrogen into the soil solution (mineralization), and conversion of ammonia into nitrate (nitrification), which is the form of nitrogen most commonly absorbed by plants. Each of these steps is mediated by soil microbes that control the rate of availability of nitrogen to plants. Soil factors such as oxygen content and pH have important effects on the availability of nitrogen to plants because of effects on soil microbes.

Phosphorus is most commonly available to plants as phosphate and is ultimately derived from the substrate. The availability of phosphorus is controlled by pH and the abundance of other ions that immobilize it in the soil. Calcium, silicates, and oxides of aluminum and iron bind phosphorus at different soil pH resulting in low soil availability of phosphorus. Phosphorus availability to many plants is enhanced by mutualistic relationships between plants and mycorrhizal fungi. These fungi derive carbon energy from the host plant and have hyphae that extend through the mineral soil and absorb great quantities of phosphorus, which is shared with the host plant. So, despite the abiotic control of phosphorus availability in the soil, the absorption by plants at low concentrations of phosphorus is often dependent upon an association with fungi.

Toxicity results from the presence of certain elements in the soil. In general, excessive concentrations of any element, including nitrogen, can result in toxicity, but the most common forms of nutrient toxicity are salinity, chromium, and nickel. Salinization of soils frequently results from irrigation in arid regions, but it can also result from the concentration of salt by some plants. The invasive saltcedar (Tamarixspp.) transpires copious quantities of water that it absorbs deep in the soil. The result is the accumulation of salt in its tissues, which it exudes from salt glands. The salt accumulates in the upper regions of the soil to the detriment of surrounding plants both by toxicity and decreased soil water potentials.

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