Agriculture Reference
In-Depth Information
A Review of Methods and Materials for Increasing Exchange Capacity
Increasing the soil's humus content
and thereby its exchange capacity by
growing and decomposing organic matter right in the soil is generally the least
costly method pf increasing exchange capacity. Given the proper amount and
balance of mineral nutrients, humus will be formed and conserved, but the level of
humus that can be maintained easily will be dictated by the climate. In a cool,
rainy climate, maintaining `above 5% SOM is easy; in humid tropical or warm
desert climate it is an endless struggle, even with large amounts of organic matter
brought in from off site.All growers should do their best to maintain optimum SOM
and humus levels in the soil, but that may not be enough to raise the CEC to the
desired level..
Humate ores
can add some exchange capacity, but are limited in the amount that
can be safely applied, generally 200 to 300 ppm annually. Presently it is unknown
if there will be a significant cumulative effect on exchange capacity from long-term
application of humates.
Expanded Vermiculite
is an excellent source of CEC, but has
a high amount
of
Magnesium
and often
Iron
already adsorbed to the exchange sites. The amount
of Mg and Fe being added must be taken into account when vermiculite is used.
Expanded vermiculite is relatively expensive and normally used only for potting
mixes and container growing media.
Perlite
is a naturally occurring volcanic glass that contains water. When heated
above 850
°
C the glass softens and the water expands the perlite to 7 to 16 times
its former volume, making a lightweight, porous, inert glass bead.
Perlite has
almost no exchange capacity and is not useful for raising CEC.
High CEC clays
may naturally occur not far from sandy or gravelly soils with low
CEC, and it may be economical to dig, haul, spread and mix them into a
low-CEC soil. One should first make certain that the clay being considered as an
amendment does have a high enough CEC to be worth the effort. Many sticky,
dense clays such as kaolin have a very low CEC. The only way to know is to have
them tested. For a soil with pH less than 7, a Mehlich 3 test would be appropriate;
for clays with a pH above 7, theAmmoniumAcetate pH8.2 test is better.
See
Chapter 9 on Calcareous and High-pH Soils for more info on the AA8.2 soil
test.
Calcium Bentonite
is an excellent CEC booster for soils that don't already have a
high clay content, but large amounts must be used to make much difference. An
application rate of 300 to 600 kg/are (600 to 1200 lbs/1000 ft
2
) works out to 30 to
60 metric tons per hectare or 13 to 26 avoirdupois tons per acre.Adding those
amounts will only be economical if there is access to an inexpensive source of Ca
bentonite and transportation costs are not too high.
