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'Soil fertility' is the characteristic of soil that supports abundant plant life. In particular the term is used to describe agricultural and garden soil.
Fertile soil has the following properties:

★ It is rich in nutrients necessary for basic plant nutrition, including nitrogen, phosphorus and potassium.

★ It contains sufficient minerals (trace elements) for plant nutrition, including boron, chlorine, cobalt, copper, iron, manganese, magnesium, molybdenum, sulfur, and zinc.

★ It contains soil organic matter that improves soil structure and soil moisture retention.

★ Soil pH is in the range 6.0 to 6.8.

★ Good soil structure, creating well drained soil.

★ A range of microorganisms that support plant growth.

★ It often contains large amounts of topsoil.
In lands used for agriculture and other human activities, fertile soil typically arises from the use of soil conservation practices.

Contents

Soil Fertilization

Air Fertilization

Soil depletion

Soil Fertilization

Nitrogen is the element in the soil that is most often lacking. Phosphorous and potasium are also needed in substantial amounts. For this reason these three elements are always included in commercial fertilizers and the content of each of these items is included on the bags of fertilizer. For example a 10-10-15 fertilizer has 10 percent nitrogen, 10 percent available phosphorus and 15 percent water soluble potassium Inorganic fertilizers are generally less expensive and have higher concentrations of nutrients than organic fertilizers. Some have criticized the use of inorganic fertilizers claiming that the water-soluble nitrogen doesn't provide for the long-term needs of the plant and creates water pollution. Slow-release fertilizer, however, is not soluble and eliminates the biggest negative of fertilization fertilizer burn. Additionally, most soluble fertilizers are coated, such as sulfur-coated urea.

Air Fertilization

Photosynthesis is the process by which a plant grows. Water is taken up by the roots of the plant and combined with carbon dioxide in the atmosphere. This endothermic reaction requires outside energy, which is usually supplied by the sun in the form of light, but can also be provided by artificial lighting in the blue spectrum. If light supplementation is required the decision is normally made to purchase a metal halide bulb, which provide higher amounts of light with lower amounts of heat per watt. The light intensity is concentrated in the blue spectrum, which is good for overall plant growth and is normally used to supplement the amount of time per day that a plant receives lighting (up to 16 hours per day). If light supplementation will be provided during daylight hours a red spectrum high pressure sodium bulb is normally chosen to promote flowering and budding. A plant cannot grow more quickly than the available amount of light and carbon dioxide. Generally the limiting factor for a plant is the amount of free carbon dioxide available. Free carbon dioxide in the atmosphere is generally expressed as parts per million (ppm) and is about 0.38 percent of the atmosphere. The carbon dioxide level has been rising at around 2ppm per year, but levels of carbon dioxide fluctuate between summer and winter months and during the day and night. Plants generally thrive in concentrations between 1 to 2 percent (1000ppm to 2000ppm). Beyond that amount additional light must be supplied to offset the effects of global dimming.
Supplying carbon dioxide to the plants is a tricky business as carbon dioxide can be easily dispersed by the wind, unlike normal fertilizers and light energy. Since carbon dioxide is heavier than air it can be chanelled into one specific area by growing in a depression, but generally the decision is made to build a greenhouse to trap the carbon dioxide and not permit it to escape. This works well in colder, northern climes but is more difficult to carry out in warmer climates because the greenhouse also causes excess heat to accumulate. Generally plants grow better the colder it is within their range of life as additional heat require plants to transpire in order to cool themselves, which results in additional loss of water and is detrimental to the process of photosynthesis.
Additional carbon dioxide is normally provided by burning a relatively clean hydrocarbon gas such as methane or propane, which also produces heat as a byproduct. This method is selected because it is the least expensive alternative, but care must be taken to limit the amount of NO_x released as this is detrimental to plant growth.

Soil depletion

Soil depletion occurs when the components which contribute to fertility are removed and not replaced, and the conditions which support soil fertility are not maintained. This leads to poor crops, which may in turn affect the health of the animals that consume those crops. In agriculture, depletion is often due to inadequate soil management.
One of the most widespread occurrences of soil depletion as of 2006 is in tropical zones where nutrient content of soils is low. The combined effects of growing population densities, large-scale industrial logging, slash-and-burn agriculture and ranching, and other factors, have in some places depleted soils through rapid and almost total nutrient removal.
Depletion may occur through a variety of other effects, including overtillage which damages soil structure, and overuse of inputs such as synthetic fertilizers and herbicides, which leave residues and buildups that inhibit microorganisms.