Topsoil is the upper layer ofsoil. It has the highest concentration oforganic matter andmicroorganisms and is where most of theEarth'sbiological soil activity occurs.

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 is generally a high concentration of roots in topsoil since this is where plants obtain most of their vitalnutrients. Topsoil also hosts significantbacterial,fungal andentomological activity without whichsoil quality would degrade. 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.^[1] A healthy topsoil layer is a very richmicrobiome that hosts a wide array of species.^[2]

Organic matter provides nutrition for living organisms and varies in quantity between different soils; more organic matter increases the strength of the soil structure. It condenses and settles over time in different ways depending upon conditions such as exposure. Dehydration also affects the structure. Dehydrated topsoil volume substantially decreases and is more prone towind erosion.^[3]

Topsoil isnaturally produced. Natural topsoil which has been 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.^[4]

It is also possible to create artificial topsoil which supports some of the engineering or biological uses of topsoil.^[4] More traditional examples of artificial plant-growth media includeterra preta andpotting mix. Manufactured topsoil based on minerals,biosolids,compost and/or paper mill sludge^[5] is available commercially.^[6] Anopen-cut coal mine inVictoria, Australia, was rehabilitated with low-quality artificial topsoil made from local materials.^[7]

In soil classification systems, topsoil is known as the O Horizon or A Horizon.^[8][9]Soil horizons 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 assubsoil.

In theUnited States, there is no federal, legal definition of the word topsoil when used in commerce.

Organisations such as theBritish Standards Institution (BSI) and theNorth Carolina Department of Agriculture publish guidelines for soil quality and the desired levels of topsoil nutrients broadly suitable for many plants.^[10]

Two common types of commercial topsoil are Bulk and Bagged Topsoil. The following table illustrates major differences between the two.^[10]

Alternatively theBSI relates the following values:

The preceding tables are for a multipurpose grade and certain levels can alter with regard tosoil pH.

Standards also exist for specialist soils suitable for plants with specific needs including acidic orericaceous soil andcalcareous 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 theVenus flytrap which is found in lownitrogen andphosphorus 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.

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 lowcarbonaceous content and can typically have a C:N ratio around 12:1.^[11]

A variety of soil mixtures are soldcommercially as topsoil. Typical uses for this product are improvinggardens andlawns or for use incontainer gardens.Potting soil,compost,manure andpeat 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 surfacegrading near residential buildings. In order to protect against flooding theInternational Residential Code requires a 2% slope (2.4 in (61 mm)) for the first ten feet away from the home.^[12]Energy Star requires a rate of 0.5 in/ft (42 mm/m).

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.^[13]

Topsoilerosion 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.^[14] Conventionalindustrial agriculture practices such asploughing and spraying high quantities of synthetic liquidfertilisers can degrade the quality of the soil.Intensive farming methods to satisfy high food demands with highcrop yields and growing crops inmonocultures can deplete the soil nutrients and damage the soilmicrobiome. These factors can affect the consistency and quality of the soil resulting in increased erosion.

Surface runoff from farm fields is a type ofnonpoint source pollution. Topsoil as well as farmfertilizers and other potentialpollutants run off unprotected farm fields when heavy rains occur. This can result in polluting waterways andgroundwater 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. Theseharmful algal blooms 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.^[15]

Sustainable techniques attempt to slow erosion through the use ofcover crops in order to build organic matter in the soil. The United States loses almost 3 tons of topsoil per acre per year.^[16] 1 inch (2.5 cm) of topsoil can take between 500^[17] and 1,000 years^[18] 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.^[18][19]

This section is an excerpt fromSoil conservation.[edit]

Soil conservation is the prevention of loss of the topmost layer of thesoil fromerosion or prevention of reduced fertility caused by over usage,acidification,salinization or other chemicalsoil contamination.

Slash-and-burn and otherunsustainable methods ofsubsistence farming are practiced in some lesser developed areas. A consequence ofdeforestation is typically large-scaleerosion, loss of soil nutrients and sometimes totaldesertification. Techniques for improved soil conservation includecrop rotation,cover crops,conservation tillage and plantedwindbreaks, affect botherosion andfertility. When plants die, they decay and become part of the soil. Code 330 defines standard methods recommended by the U.S.Natural Resources Conservation Service. Farmers have practiced soil conservation for millennia. In Europe, policies such as theCommon Agricultural Policy are targeting the application of best management practices such as reducedtillage, winter cover crops,^[20] plant residues and grass margins in order to better address soil conservation.Political and economic action is further required to solve the erosion problem. A simplegovernancehurdle concerns how we value the land and this can be changed by cultural adaptation.^[21]Soil carbon is acarbon sink, playing a role inclimate change mitigation.^[22]

• USDA Electronic Field Office Technical Guide - Detailed soil conservation guides tailored to individual states/counties

• Soil
• Horticulture
• Soil improvers
• Lawn care
• Soil fertility
• Types of soil

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• Articles with excerpts