Agriculture Reference
In-Depth Information
nutrients they need in the proper ratio, but to keep the soil loose and friable while
retaining soil moisture.
Here is our ideal soil cation saturation ratio:
Calcium 68%
Magnesium 12%
Potassium 4%
Sodium 1.5%
meq/100g
vs
cmol
c
/kg
Theacceptedmodern
notationforscientific
audiencesiscmol
c
/kg
(centimolesofchargeperkg
soil).The“c”subscript
beforetheslashincmol
c
/kg
denotes“charge”.The
magnitudeofthenumbers
remainsthesame.
1meq/100g=1cmol
c
/kg.
Other bases 4% to 5%
Free Hydrogen H+ 10%
Asoil with this ratio of cation minerals will
self-adjust to a pH of about 6.4 given adequate
soil moisture. Most of these elements are being
held on the clay fraction in a clay soil, and on the
organic (humus) fraction in a sandy or
high-organic matter soil such as peat or muck
soils. Sand and gravel have almost no exchange
capacity.Aged clays, especially in the tropics
and subtropics, also have very low exchange
capacity. Methods of increasing the exchange
capacity of low-CEC soils include adding organic
matter, charcoal (biochar), high CEC clay (e.g.
montmorillonite/bentonite), or humic acid
sources. Biochar and montmorillonite clay will
both confer permanent exchange capacity.
~~
Let's take another look at the list from the end of
the
Cation Exchange Capacity Simplified
chapter:
Manysoiltesting
laboratoriesstilluse
meq/100g,andwewillbe
usingmeq/100ginthis topic
becausethatisthenotation
usedbyAlbrechtandwhat
willbefoundintheolder
researchthatmuchofour
knowledgeofexchange
capacityisbasedon.
Per 100 grams of soil,1 milligram equivalent (meq or ME)=
1 milligram Hydrogen H+ or
20 mg of Calcium Ca++ or
12 mg of Magnesium Mg++ or
39 mg of Potassium K+ or
23 mg of Sodium Na+
Again, if you take 100 grams of oven-dry soil, with a CEC of 1, one milligram of
free Hydrogen H+ will fill all of the negative exchange sites.As we will be working
