Potting soil

Potting soil, also known as potting mix or miracle soil, is a substrate in which to grow plants, herbs and vegetables in a pot or other durable container. The first recorded use of the term is from an 1861 issue of the American Agriculturist.[1]

A flowerpot filled with potting soil

Ingredients

Bulk

Most of the potting soil plays a structural role.

  • Plant material. This part is usually made up of peat (usually with limestone to reduce acidity)[2] or coconut coir. It serves to absorb water and nutrients (as a polyelectrolyte). Tree bark, mainly of pine, may also be used.
  • Minerals. Sand and grit may be used for drainage and aeration. Perlite and vermiculite improve both aeration and water retention.

Despite its name, little or no soil is used in potting soil because it is considered too heavy for growing houseplants.[3] (Some authors and suppliers do, however, make a distinction between "potting soil" and "potting mix" in that the former contains soil and is heavier.[4] The two can also be combined in any proportion to result in a spectrum of planting media. Soil-containing mixes are more acceptable in raised beds, as the larger surface area improves aeration.[5])

Peat
Main article: Peat § Environmental and ecological issues

The use of peat is controversial since the harvesting of peat moss from peatlands (which includes unique habitats such as bogs and fens) degrades 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. Also, the removal of the layer of CO2 absorbing plants releases CO2 into the atmosphere, contributing to climate change.[6][7]

As such, alternatives such as coconut coir are promoted by some organisations.[8][9]

Nutrients and chemistry

All plants need essential plant nutrients to grow, so it is quite important to make sure there is a sufficient amount in the potting soil.[10] Some nutrients may be already present in the bulking ingredient:[11]

  • Peat contains 1% nitrogen that is almost never released.
    • Limestone (for raising pH) contains mostly calcium (calcite), but can also contain magnesium (dolomitic). The latter is preferred as it supplies both elements. A typical proportion of limestone to peat is 8.5 pounds per cubic yard (5.0 kg/m3).[11]
  • 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 (e.g. leaf mold, bark compost or recycled mushroom compost). Overuse of fertilizers will, as with in normal soils, risk damaging the plant.[11] For compost, the maximum recommended amount is 1 part compost to 1 part bulking material.[5]

A soil test 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 growth medium. The main method is a saturated media extract (SME), which tests the chemical contents of a water extract of the mix.[11][12]

Reference levels for potting mix by the SME method[11][lower-alpha 1]
AnalysisAcceptable, minOptimum, minOptimum, maxAcceptable, max
Soluble salt, mS/cm[lower-alpha 2] 0.752.03.55.0
Nitrate-N, ppm[lower-alpha 3] 40100200299
Phosphorus, ppm 36918
Calcium, ppm 80200N/AN/A
Magnesium, ppm 3070N/AN/A

Different mixes for different uses and plants

The growth medium should be adapted to each plant's (and growth stage's) preference for aeration, drainage, nutrition, and pH.[5]

By life-cycle

For seed starting and cuttings, a mix can be made using 40% coconut coir or peat moss with limestone, 40% vermiculite, and 20% sand.[14] Besides sand, it is also possible to use perlite, using near-similar sized percentages (33%-33%-33%).[15] 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.[16] Following early growth, most plants prefer a potting mix that is more well-draining, often with less than 20% of peat moss or other fine-grained materials.

By plant

An African violet would grow better in potting soil containing extra peat moss.

Cacti and succulents require sharp drainage, thus requiring a much larger percentage of perlite or sand.[17]

Insectivorous plants, such as the Venus flytrap and the pitcher plant, prefer nutrient-poor soils common to bogs and fens,[18] while water-based plants thrive in a heavier topsoil mix.[19]

Sterilization

As with garden soil, potting soil can attract insects. For example, the fungus gnat is often found around houseplants because it lays eggs in moist potting soil.[20]

Commercially available potting soil is sterilized, in order to avoid the spread of weeds and plant-borne diseases. It is possible to reuse commercial potting soil, provided that the remnants of plant roots, fungus, weeds and insects are removed from the mixture through heating before new planting can take place.[21] Packaged potting soil is sold in bags ranging from 5 to 50 pounds (2.3–22.7 kg).[22]

Infections of Legionnaires' disease due to potting mix have been reported in Australia,[23] New Zealand,[24] the Netherlands,[25] the United States,[26] and Japan.[27]


See also

References
  1. UVM data from the 1980s. An extended and updated (~1995) version of this table may be found from UConn.[13] Significant variation in reference ranges is possible due to procedural differences, so it is recommended to use the test provider's values.
  2. May be also written mmhos/cm using the "mho" synonym of S (siemens).
  3. Generally written mg/L in newer sources.
  1. Oxford English Dictionary
  2. Nissen, Dante. "The Indoor Plant Bible." Page 21. Barron’s. ISBN 0-7641-5769-8
  3. Pleasant, Barbara. "The Complete Houseplant Survival Guide." Pages 314. Storey Publishing. ISBN 1-58017-569-4
  4. "The Difference Between Potting Soil and Potting Mix". The Spruce.
  5. What is Potting Soil?
  6. Gardeners urged to stop using peat-based compost
  7. Peat moss: Good for plants but bad for the planet?
  8. Peat alternatives in the garden
  9. Alternatives to peat
  10. Leichty, Corey. "Does Potting Soil Need Fertilizer?". Do Not Disturb Gardening.
  11. "pottingmix". www.uvm.edu.
  12. "Methods of Greenhouse Media Testing and How They Differ". Center for Agriculture, Food, and the Environment. 6 March 2015.
  13. Dawn Pettinelli, Extension Educator and Dr. Richard McAvoy. "Interpretation of SME Results for Greenhouse Media" (PDF). Soil Nutrient Analysis Laboratory, University of Connecticut.
  14. DIY Potting Soil: 6 Homemade Potting Mixes
  15. 3-ingredient seed starting mix
  16. "Guidelines for Starting Seeds Indoors, from Johnny's Research Team". www.johnnyseeds.com. Retrieved 2021-01-19.
  17. Burne, Geoffrey. "Encyclopedia of Container Gardening." Page 22. Fog City Press. ISBN 978-1-877019-43-2
  18. "Dirty Buttons - Arts and Culture - Philly Weekly". philadelphiaweekly.com. Archived from the original on 2012-09-07.
  19. "Creating a water garden in a tub". The Christian Science Monitor.
  20. "How to minimize dents in carpet from furniture". azcentral.com.
  21. Carol Cloud Bailey. "Carol Cloud Bailey: Heat potting soil to prepare it for replanting". TCP. Archived from the original on 2016-05-30. Retrieved 2016-05-30.
  22. Reader's Digest Association. "Care-free Plants". Page 298. Readers Digest. ISBN 0-7621-0358-2.
  23. Speers DJ, Tribe AE (October 1994). "Legionella longbeachae pneumonia associated with potting mix". Med. J. Aust. 161 (8): 509. doi:10.5694/j.1326-5377.1994.tb127576.x. PMID 7935133. S2CID 28552691.
  24. Kingston M, Padwell A (March 1994). "Fatal legionellosis from gardening". N. Z. Med. J. 107 (974): 111. PMID 8127508.
  25. den Boer, JW; Yzerman, EP; Jansen, R; Bruin, JP; Verhoef, LP; Neve, G; van der Zwaluw, K (January 2007). "Legionnaires' disease and gardening". Clin. Microbiol. Infect. 13 (1): 88–91. doi:10.1111/j.1469-0691.2006.01562.x. PMID 17184293.
  26. "Legionnaires' disease associated with potting soil--California, Oregon, and Washington, May-June 2000". Can. Commun. Dis. Rep. 26 (22): 189–92. November 2000. PMID 11131692.
  27. "Legionnaires' Disease Associated with Potting Soil --- California, Oregon, and Washington, May--June 2000". Centers for Disease Control and Prevention.
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.