Soil fertility


Soil fertility refers to the ability of soil to sustain agricultural plant growth, i.e. to provide plant habitat and result in sustained and consistent yields of high quality. A fertile soil has the following properties:
The following properties contribute to soil fertility in most situations:
In lands used for agriculture and other human activities, maintenance of soil fertility typically requires the use of soil conservation practices. This is because soil erosion and other forms of soil degradation generally result in a decline in soil quality with respect to one or more of the aspects indicated above.
Soil fertility and quality of land have been impacted by the effects of colonialism and slavery both in the U.S. and globally. The introduction of harmful land practices such as intensive and non-prescribed burnings and deforestation by colonists created long-lasting negative results to the environment. Also, the rise of intensive farming and intensive sylviculture contributed to the collapse of soil quality in developed countries.
Soil fertility and depletion have different origins and consequences in various parts of the world. The intentional creation of dark earth in the Amazon promoted the tight relationship between indigenous communities and their land during Pre-Columbian times and are still searched as areas of high fertility. In African and Middle Eastern regions, humans and the environment are also altered due to soil depletion.

Soil fertilization

is the element in soil that is most often lacking, in particular in humid tropical soils. Nitrogen and potassium are also needed in substantial amounts. For this reason these three elements are always identified on a commercial fertilizer analysis. For example, a 10-10-15 fertilizer has 10 percent nitrogen, 10 percent available phosphorus and 15 percent water-soluble potassium. Sulfur is the fourth element that may be identified in a commercial analysis—e.g. 21-0-0-24 which would contain 21% nitrogen and 24% sulfate.
Inorganic fertilizers are generally less labour intensive and have higher concentrations of nutrients than organic fertilizers. Also, since nitrogen, phosphorus and potassium generally must be in the inorganic forms to be taken up by plants, inorganic fertilizers are generally immediately bioavailable to plants without modification. However, studies suggest that chemical fertilizers have adverse health impacts on humans including the development of chronic disease from the toxins. As for the environment, over-reliance on inorganic fertilizers disrupts the natural nutrient balance in the soil, resulting in lower soil quality, loss of organic matter, and higher chances for erosion in the soil.
Additionally, the water-soluble nitrate nitrogen in inorganic fertilizers does not provide for the long-term needs of the plant and creates water pollution. Slow-release fertilizers may reduce leaching loss of nutrients and may make the nutrients that they provide available over a longer period of time.
Soil fertility is a complex process that involves the constant cycling of nutrients between organic and inorganic forms. As plant material and animal wastes are decomposed by micro-organisms, they release inorganic nutrients to the soil solution, a process referred to as mineralization. Those nutrients may then undergo further transformations which may be aided or enabled by soil micro-organisms. Like plants, many micro-organisms require or preferentially use inorganic forms of nitrogen, phosphorus or potassium and will compete with plants for these nutrients, tying up the nutrients in microbial biomass, a process often called immobilization. The balance between immobilization and mineralization processes depends on the balance and availability of major nutrients and organic carbon to soil microorganisms. Natural processes such as lightning strikes may fix atmospheric nitrogen by converting it to nitric oxide and nitrogen dioxide. In soil, nitrogen fixation is performed by free-living and symbiotic bacteria. Denitrification occurs generally under anaerobic conditions in the presence of denitrifying bacteria, but it may also occur in aerobic environments where oxygen concentration is fluctuating and reduced carbon is available. Nutrient cations, including potassium and many micronutrients, are held in relatively strong electrostatic interaction bonds with the negatively charged portions of the soil in a process known as cation exchange which has a prominent influence on soil fertility.
Phosphorus is a primary factor of soil fertility as it is essential for cell division and plant development, especially in seedlings and young plants. However, phosphorus is becoming increasingly harder to find and its reserves are starting to be depleted due to its excessive use as a fertilizer. The widespread use of phosphorus in fertilizers has led to pollution and eutrophication. The term peak phosphorus has been coined, due to the limited occurrence of rock phosphate in the world, estimating that U.S. peak phosphorus occurred in 1988 and for the world in 1989.
A wide variety of materials have been described as soil conditioners due to their ability to improve soil quality, including biochar, offering multiple soil health benefits.
Food waste compost was found to have better soil improvement than manure based compost.

Light and CO2 limitations

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 300 ppm. Further increases in CO2 can, to a very small degree, continue to increase net photosynthetic output.

Soil depletion

occurs when the components which contribute to fertility are removed and not replaced, and the conditions which support soil's fertility are not maintained. This leads to poor crop yields, now becoming a global problem. In agriculture, soil depletion can be due to excessively intensive cultivation and inadequate soil management. Depletion may occur through a variety of other effects, including over-tillage, overuse of nutrient inputs which leads to mining of the soil nutrient bank, and salinization of soil.

Colonial impacts on soil depletion

Soil fertility can be severely challenged when land-use changes rapidly. For example, in Colonial New England, colonists made a number of decisions that depleted the soils, including: allowing herd animals to wander freely, not replenishing soils with manure, and a sequence of events that led to erosion. William Cronon wrote that "...the long-term effect was to put those soils in jeopardy. The removal of the forest, the increase in destructive floods, the soil compaction and close-cropping wrought by grazing animals, ploughing—all served to increase erosion." Cronon continues, explaining, "Where mowing was unnecessary and grazing among living trees was possible, settlers saved labor by simply burning the forest undergrowth...and turning loose their cattle...In at least one ill-favored area, the inhabitants of neighboring towns burned so frequently and graze so intensively that...the timber was greatly injured, and the land became hard to subdue...In the long run, cattle tended to encourage the growth of woody, thorn-bearing plants which they could not eat and which, once established, were very difficult to remove". These practices were methods of simplifying labor for colonial settlers in new lands when they were not familiar with traditional Indigenous agricultural methods. Those Indigenous communities were not consulted but rather forced out of their homelands so European settlers could commodify their resources. The practice of intensive land burning and turning loose cattle ruined soil fertility and prohibited sustainable crop growth.
While colonists utilized fire to clear land, certain prescribed burning practices are common and valuable to increase biodiversity and in turn, benefit soil fertility. However, without consideration of the intensity, seasonality, and frequency of the burns, the conservation of biodiversity and the overall health of the soil can be negatively impacted by fire.
In addition to soil erosion through using too much fire, colonial agriculture also resulted in topsoil depletion. Topsoil depletion occurs when the nutrient-rich organic topsoil, which takes hundreds to thousands of years to build up under natural conditions, is eroded or depleted of its original organic material. The Dust Bowl in the Great Plains of North America is a great example of this with about one-half of the original topsoil of the Great Plains having disappeared since the beginning of agricultural production there in the 1880s. Outside of the context of colonialism topsoil depletion can historically be attributed to many past civilizations' collapses.