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, little or no

soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

is usually used in potting soil.

Materials

Materials used for growing mediums include:

peat Peat is an accumulation of partially Decomposition, decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, Moorland, moors, or muskegs. ''Sphagnum'' moss, also called peat moss, is one of the most ...

, coconut coir, wood products like bark and wood fiber,

perlite Perlite is an amorphous volcanic glass that has a relatively high water content, typically formed by the Hydrate, hydration of obsidian. It occurs naturally and has the unusual property of greatly expanding when heated sufficiently. It is an indu ...

, stone wool, soils/tufts, and recycled paper and cardboard. Other materials used include rice hulls, sand,

vermiculite Vermiculite is a hydrous phyllosilicate mineral which undergoes significant expansion when heated. Exfoliation occurs when the mineral is heated sufficiently; commercial furnaces can routinely produce this effect. Vermiculite forms by the weathe ...

, and calcined clays.

Properties

Typical potting mixes include one or more materials which retain moisture, one or more materials which aid in aeration and drainage, and fertilizer. Moisture-retaining materials and aerating materials can be combined in any ratio, depending on the particular needs of the plant. Soils are minimally used as growing media because they compact and lose pore space after repeated watering and can be too heavy for growing potted plants. Mediums used for growing plants in pots typically are a mix of organic and inorganic ingredients. Good growing mediums have a number of properties including moisture and nutrient retention capacity, quick water infiltration, pore space for aeration (plants

root In vascular plants, the roots are the plant organ, organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often bel ...

s need oxygen), drainage for excess water, decompose slowly, and provide support for the plants growing in them. They also have an optimal range of pH, cation exchange properties, and lack substance that are toxic to plants These are also dependent on the type of plant grown since there is wide variation in moisture and nutrient needs among different plants.

Moisture retention

This part is usually made up of

(usually with

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

to reduce acidity) or coconut

coir Coir (), also called coconut fibre, is a natural fibre extracted from the outer husk of coconut, and used in products such as floor mats, doormats, brushes, and mattresses. Coir is the fibrous material found between the hard, internal shell ...

. It serves to absorb water and nutrients.

Tree bark Bark is the outermost layer of stems and roots of woody plants. Plants with bark include trees, woody vines, and shrubs. Bark refers to all the tissues outside the vascular cambium and is a nontechnical term. It overlays the wood and consist ...

, mainly of pine, may also be used. The use of peat is controversial since the harvesting of peat moss from

peatland A peatland is a type of wetland whose soils consist of Soil organic matter, organic matter from decaying plants, forming layers of peat. Peatlands arise because of incomplete decomposition of organic matter, usually litter from vegetation, du ...

s (which includes unique habitats such as bogs and fens) can degrade these peatlands. Peatlands are home to a diverse range of plant and animal species. Peat also has a very slow accumulation rate, as little as 1mm per year, so they take a long time to regenerate. Peatlands are also carbon sinks, constituting 3% of the world's surface but storing up to 30% of the carbon sequestered in the soil. The removal of the layer of absorbing plants releases into the atmosphere, contributing to climate change. As such, alternatives such as coconut

are promoted by some organisations.

Drainage

Sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...

and grit may be used for drainage and aeration.

Perlite Perlite is an amorphous volcanic glass that has a relatively high water content, typically formed by the Hydrate, hydration of obsidian. It occurs naturally and has the unusual property of greatly expanding when heated sufficiently. It is an indu ...

and

improve both aeration and water retention.

Nutrients and chemistry

All plants need essential plant nutrients to grow, so it is important to make sure there is a sufficient amount in the potting soil. Some nutrients may be already present in the bulking ingredients. Peat contains 1% nitrogen that is almost never released. Limestone (for raising pH) contains mostly calcium (

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

), but can also contain magnesium ( dolomitic). The latter is preferred as it supplies both elements. A typical proportion of limestone to peat is . Coir contains a high amount of electrolytes (salts). In fact, untreated coir contains too much sodium and potassium for plant growth, so it is washed and then buffered (partially replacing salts with other minerals, usually calcium and magnesium) to produce the growth medium. Vermiculite contains some calcium and magnesium, but more importantly it helps retain water and nutrients in the porous structure. Nutrients not supplied by the bulk will need to be supplied by the fertilizer. In conventional mixes they may be slow-release formulae of synthetic fertilizers, while organic mixes will use organic source such as

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 Decomposition, decomposing plant and food waste, recycling organic materials, and man ...

(e.g. leaf mold,

bark Bark may refer to: Common meanings * Bark (botany), an outer layer of a woody plant such as a tree or stick * Bark (sound), a vocalization of some animals (which is commonly the dog) Arts and entertainment * ''Bark'' (Jefferson Airplane album), ...

compost or recycled mushroom compost). Overuse of fertilizers will, as with in normal soils, risk damaging the plant. For compost, the maximum recommended amount is 1 part compost to 1 part bulking material. A

soil test A soil test is a laboratory or in-situ analysis to determine the chemical, physical or biological characteristics of a soil. Possibly the most widely conducted soil tests are those performed to estimate the plant-available concentrations of nutri ...

may be done to analyze the chemistry of a potting mix, despite the mix not necessarily being made of soil. As an approximation for indoor home planting, the mix is generally treated as

greenhouse A greenhouse is a structure that is designed to regulate the temperature and humidity of the environment inside. There are different types of greenhouses, but they all have large areas covered with transparent materials that let sunlight pass an ...

growth medium. The main method is a saturated media extract (SME), which tests the chemical contents of a water extract of the mix.

Different mixes for different uses

The growth medium should be adapted to each plant's (and growth stage's) preference for aeration, drainage, nutrition, and pH.What is Potting Soil?
/ref> For seed starting, a "germination mix" is typically light-weight and suitable for starting small-seeded plants. A "seed starting" mix is suitable for larger seeded crops. Following early growth, most plants prefer a potting mix that is more well-draining. Cacti and succulents require sharp drainage, thus requiring a much larger percentage of perlite or sand. Carnivorous plants, such as the Venus flytrap and the pitcher plant, prefer the nutrient-poor, acidic soils common to bogs and fens, while water-based plants thrive in a heavier topsoil mix.

Sterilization

Commercially available potting soil is sterilized, in order to avoid the spread of weeds and plant-borne diseases. As with garden soil, potting soil can attract insects. For example, the

fungus gnat Fungus gnats are small, dark, short-lived gnats, of the families Sciaridae, Diadocidiidae, Ditomyiidae, Keroplatidae, Bolitophilidae, and Mycetophilidae (order Diptera); they comprise six of the seven families placed in the superfamily Sc ...

is often found around houseplants because it lays eggs in moist potting soil. Infections of

Legionnaires' disease Legionnaires' disease is a form of atypical pneumonia caused by any species of ''Legionella'' bacteria, quite often ''Legionella pneumophila''. Signs and symptoms include cough, shortness of breath, high fever, myalgia, muscle pains, and headach ...

due to potting mix have been reported in

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...

, the

Netherlands , Terminology of the Low Countries, informally Holland, is a country in Northwestern Europe, with Caribbean Netherlands, overseas territories in the Caribbean. It is the largest of the four constituent countries of the Kingdom of the Nether ...

, the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

, and Japan.

References

{{Authority control Soil improvers Gardening aids Soil Garden vases