Fertile soil has the following properties:
In lands used for agriculture and other human activities, fertile soil typically arises from the use of soil conservation practices.
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 negative impact on soil fertility.
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.