Editing Soil (section)

====Cation exchange capacity (CEC)====
[[Cation exchange capacity]] is the soil's ability to remove cations from the soil water solution and sequester those to be exchanged later as the plant roots release hydrogen ions to the solution.<ref>{{cite journal |last=Brown |first=John C. |year=1978 |title=Mechanism of iron uptake by plants |journal=[[Plant, Cell & Environment|Plant, Cell and Environment]] |volume=1 |issue=4 |pages=249–57 |doi=10.1111/j.1365-3040.1978.tb02037.x |bibcode=1978PCEnv...1..249B |url=https://fr.1lib.sk/book/41304841/1381d1 |access-date=16 February 2025 }}</ref> CEC is the amount of exchangeable hydrogen cations (H<sup>+</sup>) that will combine with 100&nbsp;grams dry weight of soil and whose measure is one [[milliequivalent]] per 100&nbsp;grams of soil (1&nbsp;meq/100&nbsp;g). Hydrogen ions have a single charge and one-thousandth of a gram of hydrogen ions per 100&nbsp;grams dry soil gives a measure of one milliequivalent of hydrogen ion. Calcium, with an atomic weight 40 times that of hydrogen and with a [[Valence (chemistry)|valence]] of two, converts to {{nowrap|(40 ÷ 2) × 1 milliequivalent}} = 20 milliequivalents of hydrogen ion per 100&nbsp;grams of dry soil or 20&nbsp;meq/100&nbsp;g.{{sfn|Donahue|Miller|Shickluna|1977|p=114}} The modern measure of CEC is expressed as centimoles of positive charge per kilogram (cmol/kg) of oven-dry soil.

Most of the soil's CEC occurs on clay and humus colloids, and the lack of those in hot, humid, wet climates (such as [[tropical rainforest]]s), due to leaching and decomposition, respectively, explains the apparent sterility of tropical soils.<ref>{{cite journal |last1=Singh |first1=Jamuna Sharan |last2=Raghubanshi |first2=Akhilesh Singh |last3=Singh |first3=Raj S. |last4=Srivastava |first4=S. C. |year=1989 |title=Microbial biomass acts as a source of plant nutrient in dry tropical forest and savanna |journal=[[Nature (journal)|Nature]] |volume=338 |issue=6215 |pages=499–500 |url=https://www.researchgate.net/publication/236941524 |doi=10.1038/338499a0 |access-date=16 February 2025 |bibcode=1989Natur.338..499S |s2cid=4301023 }}</ref> Live plant roots also have some CEC, linked to their [[specific surface area]].<ref>{{cite journal |last1=Szatanik-Kloc |first1=Alicja |last2=Szerement |first2=Justyna |last3=Józefaciuk |first3=Grzegorz |year=2017 |title=The role of cell walls and pectins in cation exchange and surface area of plant roots |journal=[[Journal of Plant Physiology]] |volume=215 |pages=85–90 |url=https://daneshyari.com/article/preview/5517999.pdf |doi=10.1016/j.jplph.2017.05.017 |pmid=28600926 |bibcode=2017JPPhy.215...85S |access-date=16 February 2025 }}</ref>

{| class="wikitable" style="border-spacing: 5px; margin:auto;"
|+ Cation exchange capacity for soils; soil textures; soil colloids{{sfn|Donahue|Miller|Shickluna|1977|pp=115–116}}
|-
! scope="col" style="width:200px;"| Soil
! scope="col" style="width:100px;"| State
! scope="col" style="width:100px;"| CEC meq/100&nbsp;g
|-
| Charlotte fine sand ||Florida|| 1.0
|-
| Ruston fine sandy loam ||Texas|| 1.9
|-
| Glouchester loam ||New Jersey || 11.9
|-
| Grundy silt loam || Illinois || 26.3
|-
| Gleason clay loam || California || 31.6
|-
| Susquehanna clay loam || Alabama || 34.3
|-
| Davie mucky fine sand || Florida || 100.8
|-
| Sands || {{n/a}} || 1–5
|-
| Fine sandy loams || {{n/a}} || 5–10
|-
| Loams and silt loams || {{n/a}} || 5–15
|-
| Clay loams || {{n/a}} || 15–30
|-
| Clays || {{n/a}} || over 30
|-
| Sesquioxides || {{n/a}} || 0–3
|-
| Kaolinite || {{n/a}} || 3–15
|-
| Illite || {{n/a}} || 25–40
|-
| Montmorillonite || {{n/a}} || 60–100
|-
| Vermiculite (similar to illite) || {{n/a}} || 80–150
|-
| Humus || {{n/a}} || 100–300
|}