Soil fertility

Soil scientists use the capital letters O, A, B, C, and E to identify the master horizons, and lowercase letters for distinctions of these horizons. Most soils have three major horizons—the surface horizon (A), the subsoil (B), and the substratum (C). Some soils have an organic horizon (O) on the surface, but this horizon can also be buried. The master horizon, E, is used for subsurface horizons that have a significant loss of minerals (eluviation). Hard bedrock, which is not soil, uses the letter R.

Desert east of Birdsville, Australia. Much of Australia is sparsely populated as its desert soils are mostly infertile; thus unable to support larger scale human habitation.

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. It also refers to the soil's ability to supply plant/crop nutrients in the right quantities and qualities over a sustained period of time. A fertile soil has the following properties:

• The ability to supply essential plant nutrients and water in adequate amounts and proportions for plant growth and reproduction; and
• The absence of toxic substances which may inhibit plant growth e.g. Fe²⁺ which leads to nutrient toxicity.

The following properties contribute to soil fertility in most situations:

• Sufficient soil depth for adequate root growth and water retention;
• Good internal drainage, allowing sufficient aeration for optimal root growth (although some plants, such as rice, tolerate waterlogging);
• Topsoil or horizon O is with sufficient soil organic matter for healthy soil structure and soil moisture retention;
• Soil pH in the range 5.5 to 7.0 (suitable for most plants but some prefer or tolerate more acid or alkaline conditions);
• Adequate concentrations of essential plant nutrients in plant-available forms;
• Presence of a range of microorganisms that support plant growth.

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 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 create long-lasting negative results to the environment. The institution of slavery reproduced distress to the natural world and crop production.

Soil fertility and depletion have different origins and consequences in various parts of the world. The intentional creation of dark earth in the Amazon promotes the important relationship between indigenous communities and their land. In African and Middle Eastern regions, humans and the environment are also altered due to soil depletion.

Soil fertilization

Main article: Fertilizer

Bioavailable phosphorus (available to soil life) is the element in soil that is most often lacking. 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 (P₂O₅) and 15 percent water-soluble potassium (K₂O). 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 expensive 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 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 (NO₂). Denitrification may occur under anaerobic conditions (flooding) in the presence of denitrifying bacteria. Nutrient cations, including potassium and many micronutrients, are held in relatively strong bonds with the negatively charged portions of the soil in a process known as cation exchange.

Phosphorus is a primary factor of soil fertility as it is an element of plant nutrients in the soil. 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 the excessive use as a fertilizer. The widespread use of phosphorus in fertilizers has led to pollution and eutrophication. Recently the term peak phosphorus has been coined, due to the limited occurrence of rock phosphate in the world.

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

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

Soil depletion

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. In agriculture, depletion can be due to excessively intense cultivation and inadequate soil management. Depletion may occur through a variety of other effects, including overtillage (which damages soil structure), underuse 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. This introduction of thorn-bearing plants, turning loose cattle, and intensive burning of land 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. 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. Prescribed burning is used in many cultural rituals as a vital spiritual and ecological practice. The Karuk Tribe of contemporary California represent one community where this is utilized. The Karuk Tribe live in the Klamath River basin, and their cultural burning methods have been “actively suppressed by the USFS (United States Forest Service)”. A study by sociologist Kirsten Vineyta analyzes “how the USFS deployed anti-Indigenous rhetoric to justify its own unsubstantiated forest management agenda. USFS leadership racialized light burning by deridingly referring to it as ‘Piute Forestry.’ The agency has also discredited, downplayed, and erased Indigenous peoples and knowledges in ways that invoke tropes of the ‘Indian savage,’ the ‘Vanishing Indian,’ and the concept of ‘Terra Nullius’”. In the 1960s, the Civil Rights and American Indian Movements began to take flight, changing public and government sentiments on these century-long practices. After years of colonial oppression at the hands of the United States against the Karuk Tribe, the USFS started to reconsider the impact of these prescribed burnings. The USFS has since reversed its stance on the ecological role of fire to further support prescribed fire as a management tool.

In addition to soil erosion through using too much or too little 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 Southern United States, as reinterpreted from its mainstream analysis by sociologist Hannah Holleman, is an example of “one dramatic regional manifestation of a global socio-ecological crisis generated by the realities of settler colonialism and imperialism”. Beginning with a period of new imperialism starting in the 1870s, conditions of economic expansion were emphasized globally and were demonstrated through policies such as increased seizure of Indigenous lands. Economic expansion is supported by the commodification of natural resources and the globalization of the ecological rift. Settler colonialists found they were able to profit from the divide in the relationship between humanity and nature since it is irreparable within capitalist society. These theories and policies of ecological degradation and social domination shaped global farming practices. As a result, agricultural regions were impacted, including, specifically argued by Holleman, the U.S. Southern Plains’ Dust Bowl due to the soil erosion crisis that developed in the 1930s.

The need to produce agricultural commodities represented in colonial and dustbowl soil depletion has its roots, according to Karl Marx, in capitalism. In 1867, Marx wrote of the role of capitalism in soil depletion, describing how “all progress in capitalistic agriculture is a progress in the art, not only of robbing the worker, but of robbing the soil; all progress in increasing the fertility of the soil for a given time, is a progress towards ruining the lasting sources of that fertility”.

Soil Depletion and Enslavement

The sacred relationship between Black people and the soil can be traced back to Cleopatra’s reign in Egypt in 51 BCE. According to farmer Leah Penniman, in Egypt, earthworms of the Nile River Valley contributed to the significant fertility of the soils. As a result, Cleopatra declared the earthworm and sacred animal to recognize the animal’s positive impact. No one, including farmers, was “allowed to harm or remove an earthworm for fear of offending the deity of fertility”. Today, that soil-human relationship is maintained and in West African communities “the depth of highly fertile anthropogenic soils serves as a “meter stick” for the age of communities”. In Ghana and Liberia, it is a long-withstanding practice to combine different types of waste to create fertile soil. This “black gold” as it’s known, contains high concentrations of calcium, phosphorus, and carbon. However, when African communities were removed from their homelands during periods of enslavement, it’s known that “The further the population gets from its connection to earth, the more likely we are to ignore and exploit those who work the soil”.

Enslavement of Black people, particularly in the United States, had a negative impact on the relationship between those people and the land as well as the soil health. As Wendell Berry described in The Hidden Wound, “The white man, preoccupied with the abstractions of the economic exploitation and ownership of the land, necessarily has lived in the country as a destructive force, an ecological catastrophe, because he assigned the hand labor, and in that the possibility of intimate knowledge of the land, to a people he considered racially inferior; in this debasing labor, he destroyed the possibility of meaningful contact with the earth. He was literally blinded by his presuppositions and prejudices. Because he did not know the land, it was inevitable that he would squander its natural bounty, deplete its richness, corrupt and pollute it, or destroy it altogether. The history of the white man’s use of the earth in America is a scandal”.

As historian David Silkenat explains, the goals of Southern plantation and slave owners, instead of measuring productivity based on outputs per acre, were to maximize the amount of labor that could be extracted from the enslaved workforce. The landscape was seen as disposable, and the African slaves were seen as expendable. Once these Southern farmers forced slaves to leach soils and engage in mass deforestation, they would discard the land and move towards more fertile prospects. The forced slave practices created extensive destruction on the land. The environmental impact included draining swamps, clearing forests for monocropping and fuel steamships, and introducing invasive species, all leading to fragile ecosystems. In the aftermath, these ecosystems left hillsides eroded, rivers clogged with sterile soil, and extinction of native species. Silkenat summarizes this phenomenon of the relationship between enslavement and soil, “Although typically treated separately, slavery and the environment naturally intersect in complex and powerful ways, leaving lasting effects from the period of emancipation through modern-day reckonings with racial justice…the land too fell victim to the slave owner’s lash”.

South American Soil Fertility

The details of Indigenous societies prior to European colonization in 1492 within the Amazonian regions of South America, particularly the size of the communities and the depth of interactions with the environment, are continually debated. Central to the debate is the influence of Dark Earth. Dark Earth is a type of soil found in the Amazon that has a darker color, higher organic carbon content, and higher fertility than soil in other regions of South America which makes it highly coveted even today. Dark Earth deposits have been found, through ethnographic and archaeological studies, to have been created through ancient Indigenous practices by intentional soil management.

Ethnoarchaeologist Morgan Schmidt outlines how this carbon-rich soil was intentionally created by communities in the Amazon. While Dark Earth, and other anthropic soils, can be found all throughout the world, Amazonian Dark Earth is particularly significant because “it contrasts too sharply with the especially poor fertility of typical highly weathered tropical upland soils in the Amazon”. There is much evidence to suggest that the development of ancient agricultural societies in the Amazon was strongly influenced by the formation of Dark Earth. As a result, Amazonian societies benefitted from the dark earth in terms of agricultural success and enhanced food production. Soil analyses have been completed on the modern and ancient Kuikuro Indigenous Territory in the Upper Xingu River basin in southeastern Amazonia through archaeological and ethnographic research to determine the human relation to the soil. The “results demonstrate the intentional creation of dark earth, highlighting how Indigenous knowledge can provide strategies for sustainable rainforest management and carbon sequestration”.

Dark Earth and other tropical soils are the second largest potential source of atmospheric carbon dioxide after fossil fuels. These dark earths could be a substantial carbon reservoir that has not been considered. When incorporated into land management practices, dark earths can be carbon sinks, meaning that they absorb more carbon from the atmosphere than they release and are an essential part of combatting climate change. Research has shown hundreds to thousands of tons of carbon and nutrients stored in the dark earth in those regions of the Amazon, which confirms the implications of utilizing the soil for carbon sequestration (when carbon dioxide is captured and stored in the atmosphere and is a method associated with reducing the amount of CO₂ in the environment which in turn aids in the reduction of climate change) through its formation.

Global Soil Depletion

One of the most widespread occurrences of soil depletion as of 2008 is in tropical zones where nutrient content of soils is low. The depletion of soil has affected the state of plant life and crops in agriculture in many countries. In the Middle East for example, many countries find it difficult to grow produce because of droughts, lack of soil, and lack of irrigation. The Middle East has three countries that indicate a decline in crop production, the highest rates of productivity decline are found in hilly and dryland areas.

Many countries in Africa also undergo a depletion of fertile soil. In regions of dry climate like Sudan and the countries that make up the Sahara Desert, droughts and soil degradation is common. Cash crops such as teas, maize, and beans require a variety of nutrients in order to grow healthy. Soil fertility has declined in the farming regions of Africa and the use of artificial and natural fertilizers has been used to regain the nutrients of ground soil.

Humans and Soil

Human Health

Albert Howard is credited as the first Westerner to publish Native techniques of sustainable agriculture. As noted by Howard in 1944, “In all future studies of disease we must, therefore, always begin with the soil. This must be gotten into good condition first of all and then the reaction of the soil, the plant, animal, and man observed. Many diseases will then automatically disappear...Soil fertility is the basis of the public health system of the future...”. Howard connects the health crises of crops to the impacts of livestock and human health, ultimately spreading the message that humans must respect and restore the soil for the benefit of the human and non-human world. He continues that industrial agriculture disrupts the delicate balance of nature and irrevocably robs the soil of its fertility.

Soil has been known to have positive mental health effects as well. The exposure to microbiomes in quality soil aids human depression. Specifically, a study by scientist Christopher Lowry treated mice with the bacteria, Mycobacterium vaccae, which is found in the soil on the shores of Lake Kyoga in Uganda. Lowry has studied this bacteria’s impact on the brain since 2001. The brains of these mice produced more serotonin, known as the mood-regulating hormone, and increased levels are known to help with depression.

Reconnecting Communities with the Soil

Many community organizations work to reconnect humans with the soil, bring people together based on a shared passion, and strengthen autonomy and power for individuals. For example, one community-based organization in Portland, Oregon, Black Futures Farm, strives to rebuild the relationship between humans and the soil. Community programs and outreach coordinator for Black Futures Farm, Nia Harris, describes the goal to be “to heal the connection between Black people and the land…We achieve this by cultivating a healthy place for the Black community to gather in joy…In a state where we’re so beaten down by so many factors that have to do with identity and politics and just pure hatred and greed, to have a space that’s literally a sanctuary for so many people — not just those of us who work here, but our community — that’s what makes it so special”. The farm has programming for people to engage in healing, meditative, and artistic wellness practices. All of the produce grown on the farm is distributed to Black individuals who do not have healthy food readily accessible to them. The Black Food Sovereignty Coalition which the Black Futures Farm is part of, has a similar mission to create events and spaces in which Black and brown communities are liberated through community-building practices and food sovereignty. According to the co-founders Malcolm Hoover and Mirabai Collins, farming is an act of resistance against the disconnection between Black people and the land.

Irrigation effects

Irrigation is a process by which crops are watered by man-made means, such as bringing in water from pipes, canals, or sprinklers. Irrigation is used when the natural rainfall patterns of a region are not sustainable enough to maintain crops. Ancient civilizations heavily relied on irrigation and today about 18% of the world's cropland is irrigated, mainly in Asia, Africa, and South America. The quality of irrigation water is very important to maintain soil fertility and tilth, and for using more soil depth by the plants. When soil is irrigated with high alkaline water, unwanted sodium salts build up in the soil which would make soil draining capacity very poor. So plant roots can not penetrate deep into the soil for optimum growth in Alkali soils. When soil is irrigated with low pH / acidic water, the useful salts (Ca, Mg, K, P, S, etc.) are removed by draining water from the acidic soil and in addition unwanted aluminium and manganese salts to the plants are dissolved from the soil impeding plant growth. When soil is irrigated with high salinity water or sufficient water is not draining out from the irrigated soil, the soil would convert into saline soil or lose its fertility. Saline water enhance the turgor pressure or osmotic pressure requirement which impedes the off take of water and nutrients by the plant roots.

Top soil loss takes place in alkali soils due to erosion by rain water surface flows or drainage as they form colloids (fine mud) in contact with water. Plants absorb water-soluble inorganic salts only from the soil for their growth. Soil as such does not lose fertility just by growing crops but it lose its fertility due to accumulation of unwanted and depletion of wanted inorganic salts from the soil by improper irrigation and acid rain water (quantity and quality of water). The fertility of many soils which are not suitable for plant growth can be enhanced many times gradually by providing adequate irrigation water of suitable quality and good drainage from the soil.

Global distribution

Global distribution of soil types of the USDA soil taxonomy system. Mollisols, shown here in dark green, are a good (though not the only) indicator of high soil fertility. They coincide to a large extent with the world's major grain producing areas like the North American Prairie States, the Pampa and Gran Chaco of South America and the Ukraine-to-Central Asia Black Earth belt.

See also

• Arable land
• Plaggen soil
• Shifting cultivation
• Soil contamination
• Soil life
• Terra preta
• Cation-exchange capacity