Editing Soil (section)

==== Base saturation percentage ====
There are acid-forming cations (e.g. hydronium, aluminium, iron) and there are base-forming cations (e.g. calcium, magnesium, sodium). The fraction of the negatively-charged soil colloid exchange sites (CEC) that are occupied by base-forming cations is called [[base saturation]]. If a soil has a CEC of 20&nbsp;meq and 5&nbsp;meq are aluminium and hydronium cations (acid-forming), the remainder of positions on the colloids ({{nowrap|1=20 − 5 = 15 meq}}) are assumed occupied by base-forming cations, so that the base saturation is {{nowrap|1=15 ÷ 20 × 100% = 75%}} (the compliment 25% is assumed acid-forming cations). Base saturation is almost in direct proportion to pH (it increases with increasing pH).<ref>{{cite journal |last1=McFee |first1=William W. |last2=Kelly |first2=J. Michael |last3=Beck |first3=Robert H. |year=1977 |title=Acid precipitation effects on soil pH and base saturation of exchange sites |journal=[[Water, Air, & Soil Pollution|Water, Air, and Soil Pollution]] |volume=7 |issue=3 |pages=401–08 |doi=10.1007/BF00284134 |bibcode=1977WASP....7..401M |url=https://www.researchgate.net/publication/226736129 |access-date=2 March 2025 }}</ref> It is of use in calculating the amount of lime needed to neutralise an acid soil (lime requirement). The amount of lime needed to neutralize a soil must take account of the amount of acid forming ions on the colloids (exchangeable acidity), not just those in the soil water solution (free acidity).<ref>{{cite journal |last1=Farina |first1=Martin Patrick W. |last2=Sumner |first2=Malcolm E. |last3=Plank |first3=C. Owen |last4=Letzsch |first4=W. Stephen |year=1980 |title=Exchangeable aluminum and pH as indicators of lime requirement for corn |journal=[[Soil Science Society of America Journal]] |volume=44 |issue=5 |pages=1036–41 |url=https://www.researchgate.net/publication/250123873 |access-date=2 March 2025 |doi=10.2136/sssaj1980.03615995004400050033x |bibcode=1980SSASJ..44.1036F }}</ref> The addition of enough lime to neutralize the soil water solution will be insufficient to change the pH, as the acid forming cations stored on the soil colloids will tend to restore the original pH condition as they are pushed off those colloids by the calcium of the added lime.{{sfn|Donahue|Miller|Shickluna|1977|pp=119–120}}