Fertility (soil)

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.

Soil Fertilization
Nitrogen is the element in the soil that is most often lacking. Phosphorus and potassium 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 (P2O5) available phosphorus and 15 percent (K2O) 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.

Light and CO2 limitations
Photosynthesis is the process whereby plants use light energy to drive chemical reactions which convert CO2 into sugars. As such, all plants require access to both light and carbon dioxide to produce energy, grow and reproduce.

While typically limited by nitrogen, phosphorus and potassium, low levels of carbon dioxide can also act as a limiting factor on plant growth. Peer reviewed and published scientific studies have shown that increasing CO2 is highly effective at promoting plant growth up to levels over 300ppm. Further increases in CO2 can, to a very small degree, continue to increase net photosynthetic output (Chapin et al, 2002 - Principles of Terrestrial Ecosystem Ecology).

Since higher levels of CO2 have only a minimal impact on photosynthetic output at present levels (presently around 380 ppm and increasing), we should not consider plant growth to be limited by carbon dioxide. Other biochemical limitations, such as soil organic content, nitrogen in the soil, phosphorus and potassium, are far more often in short supply. As such, neither commercial nor scientific communities look to air fertilization as an effective or economic method of increasing production in agriculture or natural ecosystems. Furthermore, since microbial decomposition occurs faster under warmer temperatures, higher levels of CO2 (which is one of the causes of unusually fast climate change) should be expected to increase the rate at which nutrients are leached out of soils and may have a One of the most widespread occurrences of soil depletion as of 2008 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.

Topsoil depletion is when the nutrient rich organic topsoil that takes hundreds to thousands of years to build up under natural conditions is eroded or depleted of its original organic material. Historically, many past civilizations collapses can be attributed to the depletion of the topsoil. Since the beginning of agricultural production in the Great Plains of North America in the 1880's about one half of its topsoil has disappeared.

Depletion may occur through a variety of other effects, including overtillage which damages soil structure, overuse of inputs such as synthetic fertilizers and herbicides, which leave residues and buildups that inhibit microorganisms, and salinization of soil.