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Topsoil is the upper layer of
soil
Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
. It has the highest concentration of
organic matter
Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have c ...
and
microorganism
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s and is where most of the
Earth's biological soil activity occurs.
Description
Topsoil is composed of mineral particles and organic matter and usually extends to a depth of 5-10 inches (13–25 cm). Together these make a substrate capable of holding water and air which encourages biological activity.
There are generally a high concentration of roots in topsoil since this is where plants obtain most of their vital
nutrient
A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...
s. It also plays host to significant
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
l,
fungal
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from th ...
and
entomological
Entomology () is the scientific study of insects, a branch of zoology. In the past the term "insect" was less specific, and historically the definition of entomology would also include the study of animals in other arthropod groups, such as arach ...
activity without which soil quality would degrade and become less suitable for plants. Bacteria and fungi can be essential in facilitating nutrient exchange with plants and in breaking down organic matter into a form that roots can absorb. Insects also play important roles in breaking down material and aerating and rotating the soil. Many species directly contribute to the health of the soil resulting in stronger plants. A healthy topsoil layer is a very rich
microbiome
A microbiome () is the community of microorganisms that can usually be found living together in any given habitat. It was defined more precisely in 1988 by Whipps ''et al.'' as "a characteristic microbial community occupying a reasonably wel ...
that hosts a wide array of species.
Organic matter
Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have c ...
provides nutrition for living organisms and varies in quantity between different soils with the strength of the soil structure decreasing when more is present. It condenses and settles over time in different ways depending upon conditions such as beneath roadbeds and foundations vs uncovered and exposed to the elements. The structure becomes affected once the soil is dehydrated. Dehydrated topsoil volume substantially decreases and may suffer
wind erosion
Aeolian processes, also spelled eolian, pertain to wind activity in the study of geology and weather and specifically to the wind's ability to shape the surface of the Earth (or other planets). Winds may erode, transport, and deposit materials ...
.
Production
Topsoil is naturally produced in the process of soil formation or
pedogenesis
Soil formation, also known as pedogenesis, is the process of soil genesis as regulated by the effects of place, environment, and history. Biogeochemical processes act to both create and destroy order ( anisotropy) within soils. These alterations ...
. Natural topsoil is mined and conditioned for human use and makes up the bulk of commercial topsoil available. The current rate of use and erosion outpaces soil generation.
It is possible to create artificial topsoil which supports some of the engineering or biological uses of topsoil.
[ More traditional examples of artificial plant-growth media include ]terra preta
''Terra preta'' (, locally , literally "black soil" in Portuguese) is a type of very dark, fertile anthropogenic soil ( anthrosol) found in the Amazon Basin. It is also known as "Amazonian dark earth" or "Indian black earth". In Portuguese its f ...
and potting mix
Potting soil or growing media, also known as potting mix or potting compost (UK), is a substrate used to grow plants in containers. The first recorded use of the term is from an 1861 issue of the ''American Agriculturist''. Despite its name, lit ...
. Manufactured topsoil based on minerals, biosolids
Biosolids are solid organic matter recovered from a sewage treatment process and used as fertilizer. In the past, it was common for farmers to use animal manure to improve their soil fertility. In the 1920s, the farming community began also to use ...
, compost
Compost is a mixture of ingredients used as plant fertilizer and to improve soil's physical, chemical and biological properties. It is commonly prepared by decomposing plant, food waste, recycling organic materials and manure. The resulting m ...
and/or paper mill sludge is available commercially. A Victorian open-cut
In civil engineering, a cut or cutting is where soil or rock from a relative rise along a route is removed. The term is also used in river management to speed a waterway's flow by short-cutting a meander.
Cuts are typically used in road, rail, ...
coal mine was rehabilitated with low-quality artificial topsoil made from local materials.
Classification
In soil classification systems, topsoil is known as the O Horizon or A Horizon. Soil horizon
A soil horizon is a layer parallel to the soil surface whose physical, chemical and biological characteristics differ from the layers above and beneath. Horizons are defined in many cases by obvious physical features, mainly colour and texture. ...
s are layers parallel to the soil surface whose physical, chemical and biological characteristics differ from the layers above and beneath. The depth of the topsoil layer is measured as the depth of the surface to the first densely packed soil layer, known as subsoil
Subsoil is the layer of soil under the topsoil on the surface of the ground. Like topsoil, it is composed of a variable mixture of small particles such as sand, silt and clay, but with a much lower percentage of organic matter and humus, and it ...
.
Evaluation
Organisations such as the British Standards Institution (BSI) and the North Carolina Department of Agriculture
The North Carolina Department of Agriculture & Consumer Services (NCDA&CS) is a state agency of North Carolina headed by the North Carolina Commissioner of Agriculture, Commissioner of Agriculture. It is headquartered in the Agriculture Building in ...
publish guidelines for soil quality and the desired levels of topsoil nutrients broadly suitable for many plants.
Two common types of commercial topsoil are Bulk and Bagged Topsoil. The following table illustrates major differences between the two.
Alternatively the BSI relates the following values:
The preceding tables are for a multipurpose grade and certain levels can alter with regard to soil pH.
Standards also exist for specialist soils suitable for plants with specific needs including acidic or ericaceous soil and calcareous
Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.
In zoology
''Calcareous'' is used as an adje ...
soil. These have different pH levels to typical soil and are meant for growing different plant species. Low fertility, low fertility acidic and low fertility calcareous are other soil classifications designed for plants which thrive in nutrient sparse soil.
Examples of specialist plants include the Venus flytrap
The Venus flytrap (''Dionaea muscipula'') is a carnivorous plant native to subtropical wetlands on the East Coast of the United States in North Carolina and South Carolina. It catches its prey—chiefly insects and arachnids—with a trapping ...
which is found in low nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
and phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
environments so is less tolerant of highly nutrient rich environments than other plants and less able to compete in them. Whereas blueberries require ericaceous soil to grow well and clover grows well in calcareous soil. Soils must therefore be selected to suit the plants which are intended to be grown and hence standards are required.
Carbon to nitrogen ratio
Topsoil is the primary resource for plants to grow and crops to thrive. The main two parameters for this are carbon and nitrogen. The carbon provides energy and nitrogen is required for plants to build proteins and hence tissues. Plants require them in a range of ratios to enable suitable growth. An optimum figure for topsoil in the UK is a C:N ratio of less than 20:1. A sawdust base typically has a high C:N ratio in the order of 400:1 while an alfalfa hay has a low carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
aceous content and can typically have a C:N ratio around 12:1.
Commercial application
A variety of soil mixtures are sold commercially
Commerce is the large-scale organized system of activities, functions, procedures and institutions directly and indirectly related to the exchange (buying and selling) of goods and services among two or more parties within local, regional, nation ...
as topsoil. Typical uses for this product are improving garden
A garden is a planned space, usually outdoors, set aside for the cultivation, display, and enjoyment of plants and other forms of nature. The single feature identifying even the wildest wild garden is ''control''. The garden can incorporate bot ...
s and lawn
A lawn is an area of soil-covered land planted with grasses and other durable plants such as clover which are maintained at a short height with a lawnmower (or sometimes grazing animals) and used for aesthetic and recreational purposes. L ...
s or for use in container gardens. Potting soil
Potting soil or growing media, also known as potting mix or potting compost (UK), is a substrate used to grow plants in containers. The first recorded use of the term is from an 1861 issue of the ''American Agriculturist''. Despite its name, lit ...
, compost
Compost is a mixture of ingredients used as plant fertilizer and to improve soil's physical, chemical and biological properties. It is commonly prepared by decomposing plant, food waste, recycling organic materials and manure. The resulting m ...
, manure
Manure is organic matter that is used as organic fertilizer in agriculture. Most manure consists of animal feces; other sources include compost and green manure. Manures contribute to the Soil fertility, fertility of soil by adding organic ma ...
and peat
Peat (), also known as turf (), is an accumulation of partially decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, moors, or muskegs. The peatland ecosystem covers and is the most efficien ...
are also sold for domestic uses with each having specific intended purposes. Topsoil products typically are not as suitable for potting plants or growing fruit and veg as potting soil or compost. Using it for this purpose can also work out prohibitively expensive compared to other alternatives.
Topsoil is also used for proper surface grading near residential buildings. In order to protect against flooding the International Residential Code requires a 2% slope () for the first ten feet away from the home.Energy Star
Energy Star (trademarked ''ENERGY STAR'') is a program run by the U.S. Environmental Protection Agency (EPA) and U.S. Department of Energy (DOE) that promotes energy efficiency. The program provides information on the energy consumption of pr ...
requires a rate of .
Commercially available topsoil (manufactured or naturally occurring) in the United Kingdom must be classified to British Standard BS 3882, with the current version dated 2015. The standard has several classifications of topsoil with the final classification requiring material to meet certain threshold criteria such as nutrient content, extractable phytotoxic elements, particle size distribution, organic matter content, carbon:nitrogen ratio, electrical conductivity, loss on ignition, pH, chemical and physical contamination. The topsoil must be sampled in accordance with the British Standard and European Norm BS EN 12579:2013 Soil improvers and growing media – Sampling.
Erosion
Topsoil erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...
occurs when the topsoil layer is blown or washed away. The estimated annual costs of public and environmental health losses related to soil erosion in the United States exceed $45 billion. Conventional industrial agriculture
Industrial agriculture is a form of modern farming that refers to the industrialized production of crops and animals and animal products like eggs or milk. The methods of industrial agriculture include innovation in agricultural machinery and f ...
practices such as plough
A plough or plow ( US; both ) is a farm tool for loosening or turning the soil before sowing seed or planting. Ploughs were traditionally drawn by oxen and horses, but in modern farms are drawn by tractors. A plough may have a wooden, iron or ...
ing and spraying high quantities of synthetic liquid fertilisers
A fertilizer (American English) or fertiliser (British English; see spelling differences) is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from ...
can degrade the quality of the soil. Intensive farming methods to satisfy high food demands with high crop yield
In agriculture, the yield is a measurement of the amount of a crop grown, or product such as wool, meat or milk produced, per unit area of land. The seed ratio is another way of calculating yields.
Innovations, such as the use of fertilizer, the c ...
s and growing crops in monoculture
In agriculture, monoculture is the practice of growing one crop species in a field at a time. Monoculture is widely used in intensive farming and in organic farming: both a 1,000-hectare/acre cornfield and a 10-ha/acre field of organic kale are ...
s can deplete the soil nutrients and damage the soil microbiome
A microbiome () is the community of microorganisms that can usually be found living together in any given habitat. It was defined more precisely in 1988 by Whipps ''et al.'' as "a characteristic microbial community occupying a reasonably wel ...
. These factors can affect the consistency and quality of the soil resulting in increased erosion.
Surface runoff
Surface runoff (also known as overland flow) is the flow of water occurring on the ground surface when excess rainwater, stormwater, meltwater, or other sources, can no longer sufficiently rapidly infiltrate in the soil. This can occur when th ...
from farm fields is a type of nonpoint source pollution
Nonpoint source (NPS) pollution refers to diffuse contamination (or pollution) of water or air that does not originate from a single discrete source. This type of pollution is often the cumulative effect of small amounts of contaminants gathered ...
. Topsoil as well as farm fertilizer
A fertilizer (American English) or fertiliser (British English; see spelling differences) is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from ...
s and other potential pollutants
A pollutant or novel entity is a substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource. These can be both naturally forming (i.e. minerals or extracted compounds like o ...
run off unprotected farm fields when heavy rains occur. This can result in polluting waterways and groundwater
Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidate ...
and may potentially contaminate drinking water sources. Algae blooms can occur when high quantities of nutrients flood rivers, lakes or oceans often as a result of farm runoff or from sewage. These harmful algal bloom
A harmful algal bloom (HAB) (or excessive algae growth) is an algal bloom that causes negative impacts to other organisms by production of natural phycotoxin, algae-produced toxins, mechanical damage to other organisms, or by other means. HABs are ...
s can be toxic and have devastating impacts on ecosystems and wildlife. They are often referred to as red tides due to the presence of toxic red algae which can impact human food sources by contaminating seafood.
Sustainable techniques attempt to slow erosion through the use of cover crops
In agriculture, cover crops are plants that are planted to cover the soil rather than for the purpose of being harvested. Cover crops manage soil erosion, soil fertility, soil quality, water, weeds, pests, diseases, biodiversity and wildlife in ...
in order to build organic matter in the soil. The United States loses almost 3 tons of topsoil per acre per year. of topsoil can take between 500 and 1,000 years to form naturally, making the rate of topsoil erosion a serious ecological concern. Based on 2014 trends, the world has about 60 years of topsoil left.
Conservation
See also
* Agricultural wastewater
Agricultural wastewater treatment is a farm management agenda for controlling pollution from confined animal operations and from surface runoff that may be contaminated by chemicals in fertilizer, pesticides, animal slurry, crop residues or i ...
* Desertification
Desertification is a type of land degradation in drylands in which biological productivity is lost due to natural processes or induced by human activities whereby fertile areas become increasingly arid. It is the spread of arid areas caused by ...
* Dust bowl
The Dust Bowl was a period of severe dust storms that greatly damaged the ecology and agriculture of the American and Canadian prairies during the 1930s. The phenomenon was caused by a combination of both natural factors (severe drought) an ...
* Erosion control
Erosion control is the practice of preventing or controlling wind or water erosion in agriculture, land development, coastal areas, river banks and construction. Effective erosion controls handle surface runoff and are important techniques in ...
* No-till farming
No-till farming (also known as zero tillage or direct drilling) is an agricultural technique for growing crops or pasture without disturbing the soil through tillage. No-till farming decreases the amount of soil erosion tillage causes in certai ...
* Sustainable agriculture
Sustainable agriculture is farming in sustainable ways meeting society's present food and textile needs, without compromising the ability for current or future generations to meet their needs. It can be based on an understanding of ecosystem ser ...
* Waterway degradation
Water pollution (or aquatic pollution) is the contamination of water bodies, usually as a result of human activities, so that it negatively affects its uses. Water bodies include lakes, rivers, oceans, aquifers, reservoirs and groundwater. Water ...
References
Further reading
The lowdown on topsoil: It's disappearing
*
External links
USDA Electronic Field Office Technical Guide
- Detailed soil conservation guides tailored to individual states/counties
{{Authority control
Soil
Horticulture
Soil improvers
Lawn care
Types of soil