Editing Soil (section)

==Horizons==
{{Main|Soil horizon}}

A horizontal layer of the soil, whose physical features, composition and age are distinct from those above and beneath, is referred to as a [[soil horizon]]. The naming of a horizon is based on the type of material of which it is composed. Those materials reflect the duration of specific processes of soil formation. They are labelled using a shorthand notation of letters and numbers which describe the horizon in terms of its colour, size, texture, structure, consistency, root quantity, pH, voids, boundary characteristics and presence of [[Nodule (geology)|nodules]] or [[Concretion|concretions]].<ref>{{cite web |url=https://soilsofcanada.ca/soil-formation/horizons.php |title=Horizons |website=Soils of Canada |access-date=11 May 2025 |archive-url=https://web.archive.org/web/20190922153041/https://soilsofcanada.ca/soil-formation/horizons.php |archive-date=22 September 2019 |url-status=live }}</ref> No soil profile has all the major horizons. Some, called [[entisols]], may have only one horizon or are currently considered as having no horizon, in particular incipient soils from unreclaimed [[mining waste]] deposits,<ref>{{cite journal |last1=Frouz |first1=Jan |last2=Prach |first2=Karel |last3=Pizl |first3=Václav |last4=Háněl |first4=Ladislav |last5=Starý |first5=Josef |last6=Tajovský |first6=Karel |last7=Materna |first7=Jan |last8=Balík |first8=Vladimír |last9=Kalčík |first9=Jiří |last10=Řehounková |first10=Klára |year=2008 |title=Interactions between soil development, vegetation and soil fauna during spontaneous succession in post mining sites |journal=European Journal of Soil Biology |volume=44 |issue=1 |pages=109–21 |url=https://www.academia.edu/14019971 |doi=10.1016/j.ejsobi.2007.09.002 |bibcode=2008EJSB...44..109F |access-date=11 May 2025 }}</ref> [[moraines]],<ref>{{cite journal |last1=Kabala |first1=Cezary |last2=Zapart |first2=Justyna |year=2012 |title=Initial soil development and carbon accumulation on moraines of the rapidly retreating Werenskiold Glacier, SW Spitsbergen, Svalbard archipelago |journal=Geoderma |volume=175–176 |pages=9–20 |url=https://www.academia.edu/31221217 |doi=10.1016/j.geoderma.2012.01.025 |bibcode=2012Geode.175....9K |access-date=11 May 2025 }}</ref> [[volcanic cones]]<ref>{{cite journal |last1=Ahmad |first1=Asmita |last2=Solle |first2=Muchtar Salam |last3=Lopulisa |first3=Christianto  |year=2019 |title=Soil development from volcanic ash based on different pyroclastic composition |journal=Journal of Tropical Soils |volume=24 |issue=3 |pages=135–40 |doi=10.5400/jts.2019.v24i3.135-140 |doi-access=free }}</ref> [[sand dunes]] or [[alluvial terrace]]s.<ref>{{cite journal |last=Huggett |first=Richard J. |year=1998 |title=Soil chronosequences, soil development, and soil evolution: a critical review |journal=Catena |volume=32 |issue=3 |pages=155–72 |url=https://www.academia.edu/2116704 |doi=10.1016/S0341-8162(98)00053-8 |bibcode=1998Caten..32..155H |access-date=11 May 2025 }}</ref> Upper soil horizons may be lacking in truncated soils following wind or water ablation, with concomitant downslope burying of soil horizons, a natural process aggravated by agricultural practices such as tillage.<ref>{{cite journal |last1=De Alba |first1=Saturnio |last2=Lindstrom |first2=Michael |last3=Schumacher |first3=Thomas E. |last4=Malo |first4=Douglas D. |year=2004 |title=Soil landscape evolution due to soil redistribution by tillage: a new conceptual model of soil catena evolution in agricultural landscapes |journal=Catena |volume=58 |issue=1 |pages=77–100 |url=https://www.academia.edu/22300477 |doi=10.1016/j.catena.2003.12.004 |bibcode=2004Caten..58...77D |access-date=11 May 2025 }}</ref> The growth of trees is another source of disturbance, creating a micro-scale heterogeneity which is still visible in soil horizons once trees have died.<ref>{{cite journal |last1=Phillips |first1=Jonathan D. |last2=Marion |first2=Daniel A. |year=2004 |title=Pedological memory in forest soil development |journal=[[Forest Ecology and Management]] |volume=188 |issue=1 |pages=363–80 |url=https://www.srs.fs.usda.gov/pubs/ja/ja_phillips004.pdf |doi=10.1016/j.foreco.2003.08.007 |bibcode=2004ForEM.188..363P |access-date=11 May 2025 }}</ref> By passing from a horizon to another, from the top to the bottom of the soil profile, one goes back in time, with past events registered in soil horizons like in [[sediment]] layers. Sampling [[pollen]], [[testate amoebae]] and plant remains in soil horizons may help to reveal environmental changes (e.g. climate change, [[land use]] change) which occurred in the course of soil formation.<ref>{{cite journal |last1=Mitchell |first1=Edward A.D. |last2=Van der Knaap |first2=Willem O. |last3=Van Leeuwen |first3=Jacqueline F.N. |last4=Buttler |first4=Alexandre |last5=Warner |first5=Barry G. |last6=Gobat |first6=Jean-Michel |year=2001 |title=The palaeoecological history of the Praz-Rodet bog (Swiss Jura) based on pollen, plant macrofossils and testate amoebae(Protozoa) |journal=[[The Holocene]] |volume=11 |issue=1 |pages=65–80 |url=https://www.academia.edu/31915005 |doi=10.1191/095968301671777798 |bibcode=2001Holoc..11...65M |s2cid=131032169 |access-date=11 May 2025 }}</ref> Soil horizons can be dated by several methods such as [[radiocarbon]], using pieces of charcoal provided they are of enough size to escape [[pedoturbation]] by [[earthworm]] activity and other mechanical disturbances.<ref>{{cite journal |last=Carcaillet |first=Christopher |year=2001 |title=Soil particles reworking evidences by AMS <sup>14</sup>C dating of charcoal |journal=[[Comptes Rendus de l'Académie des Sciences, Série IIA]] |volume=332 |issue=1 |pages=21–28 |url=https://fr.1lib.sk/book/50412117/352654 |doi=10.1016/S1251-8050(00)01485-3 |bibcode=2001CRASE.332...21C |access-date=11 May 2025 }}</ref> Fossil soil horizons from [[paleosols]] can be found within [[sedimentary rock]] sequences, allowing the study of past environments.<ref>{{cite journal |last=Retallack |first=Gregory J. |year=1991 |title=Untangling the effects of burial alteration and ancient soil formation |journal=[[Annual Review of Earth and Planetary Sciences]] |volume=19 |issue=1 |pages=183–206 |doi=10.1146/annurev.ea.19.050191.001151 |bibcode=1991AREPS..19..183R |url=https://www.researchgate.net/publication/234148901 |access-date=11 May 2025 }}</ref>

The exposure of parent material to favourable conditions produces mineral soils that are marginally suitable for plant growth, as is the case in eroded soils.<ref>{{cite journal |last1=Bakker |first1=Martha M. |last2=Govers |first2=Gerard |last3=Jones |first3=Robert A. |last4=Rounsevell |first4=Mark D.A. |year=2007 |title=The effect of soil erosion on Europe's crop yields |journal=Ecosystems |volume=10 |issue=7 |pages=1209–19 |doi=10.1007/s10021-007-9090-3 |bibcode=2007Ecosy..10.1209B |doi-access=free }}</ref> The growth of vegetation results in the production of organic residues which fall on the ground as litter for plant aerial parts ([[leaf litter]]) or are directly produced belowground for subterranean plant organs (root litter), and then release [[dissolved organic matter]].<ref>{{cite journal |last1=Uselman |first1=Shauna M. |last2=Qualls |first2=Robert G. |last3=Lilienfein |first3=Juliane |year=2007 |title=Contribution of root vs. leaf litter to dissolved organic carbon leaching through soil |journal=[[Soil Science Society of America Journal]] |volume=71 |issue=5 |pages=1555–63 |url=https://www.academia.edu/34475958 |doi=10.2136/sssaj2006.0386 |bibcode=2007SSASJ..71.1555U |access-date=11 May 2025 }}</ref> The remaining surficial organic layer, called the [[forest floor|O horizon]], produces a more active soil due to the effect of the organisms that live within it. Organisms colonise and break down organic materials, making available nutrients upon which other plants and animals can live.<ref>{{cite journal |last1=Schulz |first1=Stefanie |last2=Brankatschk |first2=Robert |last3=Dümig |first3=Alexander |last4=Kögel-Knabner |first4=Ingrid|author4-link=Ingrid Kögel-Knabner |last5=Schloter |first5=Michae |last6=Zeyer |first6=Josef |year=2013 |title=The role of microorganisms at different stages of ecosystem development for soil formation |journal=[[Biogeosciences]] |volume=10 |issue=6 |pages=3983–96 |doi=10.5194/bg-10-3983-2013 |bibcode=2013BGeo...10.3983S |doi-access=free }}</ref> After sufficient time, [[humus]] moves downward and is deposited in a distinctive organic-mineral surface layer called the [[A horizon]], in which organic matter is mixed with mineral matter through the activity of burrowing animals, a process called [[Perturbation (geology)|pedoturbation]]. This natural process does not go to completion in the presence of conditions detrimental to soil life such as strong acidity, cold climate or pollution, stemming in the accumulation of undecomposed organic matter within a single organic horizon overlying the mineral soil<ref>{{cite journal |last1=Gillet |first1=Servane |last2=Ponge |first2=Jean-François |year=2002 |title=Humus forms and metal pollution in soil |journal=European Journal of Soil Science |volume=53 |issue=4 |pages=529–39 |url=https://www.academia.edu/45705588 |doi=10.1046/j.1365-2389.2002.00479.x |bibcode=2002EuJSS..53..529G |s2cid=94900982 |access-date=11 May 2025 }}</ref> and in the juxtaposition of humified organic matter and mineral particles, without intimate mixing, in the underlying mineral horizons.<ref>{{cite journal |last1=Bardy |first1=Marion |last2=Fritsch |first2=Emmanuel |last3=Derenne |first3=Sylvie |last4=Allard |first4=Thierry |last5=do Nascimento |first5=Nadia Régina |last6=Bueno |first6=Guilherme |year=2008 |title=Micromorphology and spectroscopic characteristics of organic matter in waterlogged podzols of the upper Amazon basin |journal=Geoderma |volume=145 |issue=3 |pages=222–30 |doi=10.1016/j.geoderma.2008.03.008 |bibcode=2008Geode.145..222B |url=https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=9370526affebf1bc012ec66afdca723fcdd4a940#page=2.43 |access-date=11 May 2025 }}</ref>