Topsoil is the upper, outermost layer of soil, usually the top 5–10 inches (13–25 cm). It has the highest concentration of organic matter and microorganisms, and is where most of the Earth's biological soil activity occurs. There is generally a high concentration of roots in topsoil, as this is where plants obtain most of their vital nutrients. Topsoil is composed of mineral particles, organic matter, water, and air. Organic matter varies in quantity between different soils. The strength of soil structure decreases with the presence of organic matter, creating weak bearing capacities. Organic matter condenses and settles in different ways under certain conditions, such as roadbeds and foundations. The structure becomes affected once the soil is dehydrated. Dehydrated topsoil volume substantially decreases and may suffer wind erosion.
In nature, topsoil is produced in the process of pedogenesis. Natural topsoil, mined and conditioned, 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 and potting mix. Manufactured topsoil based on minerals, biosolids, compost, and/or paper mill sludge is available commercially. A Victorian open-cut coal mine was rehabilitated with low-quality artificial topsoil made from local materials.
In soil classification systems, topsoil is known as the "O Horizon or A Horizon,". The depth of the topsoil layer is measured as the depth of the surface to the first densely packed soil layer, known as subsoil.
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.
The North Carolina Department of Agriculture and British Standard publish guidelines for "desired levels of topsoil nutrients" broadly suitable for many plants.
|pH Level||5.0 to 6.2|
|Phosphorus (P-I)||Index of 50|
|Potassium (K-I)||Index of 50|
|Calcium (Ca%)||40-60% of Cation Exchange Capacity (CEC)|
|Magnesium (Mg%)||8-10% of CEC|
|Base saturation (BS%)||35-80% of CEC|
|Manganese (Mn-I)||Index > 25|
|Zinc (Zn-I)||Index > 25|
|Copper (Cu-I)||Index > 25|
The two common types of commercial topsoil are Bulk and Bagged Topsoil. The following table illustrates major differences between the two.
Alternatively the British Standard relates the following values:
|pH Level||5.5 to 8.5|
|Phosphate (PO4)||16 to 140 mg/L|
|Potassium (K)||121 to 1500 mg/L|
|Magnesium (Mg)||51 to 600 mg/L|
|Nickel (Ni)||from <60 mg/kg|
|Zinc (Zn)||from <200 mg/kg|
|Copper (Cu)||from <100 mg/kg|
The preceding tables are for a multipurpose grade and certain levels can alter with regard to soil pH. Other uses specified in the standard that allows for a variety of uses in different and specific scenarios include "acidic", "calcareous", "low fertility", "low fertility acidic" and "low fertility calcareous".
Topsoil is the primary resource for plants to grow and crops to thrive and the main two parameters for this are carbon and nitrogen. The carbon provides energy and nitrogen is required for 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 on the order of c. 400:1 while an alfalfa hay has a low carbonaceous content and can typically have a C:N ratio around 12:1.
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 [2.4 in] for the first ten feet away from the home while Energy Star requires a rate of 0.5 in/ft [5 in].
Topsoil erosion occurs when the topsoil layer is blown or washed away. The estimated annual costs of public and environmental health losses related to soil erosion exceed $45 billion. Conventional agriculture encourages the depletion of topsoil because the soil must be plowed and replanted each year. Sustainable techniques attempt to slow erosion through the use of cover crops in order to build organic matter in the soil. The United States loses almost 3 tons of topsoil per acre per year. One inch (2.5 centimeters) of topsoil can take between 500 and 1,000 years to form naturally, making the rate of topsoil erosion an ecological concern. Based on 2014 trends, the world has about 60 years of topsoil left.
Soil conservation is the prevention of loss of the top most layer of the soil from erosion or prevention of reduced fertility caused by over usage, acidification, salinization or other chemical soil contamination.Slash-and-burn and other unsustainable methods of subsistence farming are practiced in some lesser developed areas. A sequel to the deforestation is typically large scale erosion, loss of soil nutrients and sometimes total desertification. Techniques for improved soil conservation include crop rotation, cover crops, conservation tillage and planted windbreaks, affect both erosion and fertility. 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 the Common Agricultural Policy are targeting the application of best management practices such as reduced tillage, winter cover crops, plant residues and grass margins in order to better address the soil conservation. Political and economic action is further required to solve the erosion problem. A simple governance hurdle concerns how we value the land and this can be changed by cultural adaptation. Soil carbon is a carbon sink, playing a role in climate change mitigation.