Agriculture Reference
In-Depth Information
High CEC Clays: Calcium Bentonite
Calcium bentonite is an amendment that only needs to be brought to an optimal
level in the soil one time; its exchange capacity and physical properties will then
remain in the soil permanently unless eroded away.
Bentonites are hydrated 2:1alumino-silicate layer clays. Structurally they consist
of alumina sheets sandwiched between SiO
2
tetrahedral layers. Unlike the
vermiculite micas, bentonite clays readily hydrate and expand in their natural state.
Of the two main varieties, Calcium and Sodium bentonite, Calcium bentonite is
the one used for agriculture. Sodium bentonite contains excessive amounts of Na,
attracts and retains too much water, and swells and shrinks excessively. Calcium
bentonites typically have a CEC of 70 to 100meq.
Calcium Bentonite is is an excellent amendment for loose, low-CEC “beach sand”
type soils. The Ca bentonite will disperse and coat the sand grains, helping them
stick together to form aggregates. Calcium bentonite also interacts chemically and
biologically with humus and decomposing organic matter, preserving it the soil
and providing habitat for soil microbes whose excretions further bind the sand
grains and give the soil a cohesive structure while adding much-needed CEC.
Calcium bentonite is also a significant source of exchangeable Ca, Mg, K, and Na.
It
may be used in any sand or silt soil
or in growing media where readily
available Ca, increased CEC, and better moisture and fertilizer retention are
desired.
For heavy clay soils with low CEC,
where adding more dispersed clay
is generally not desired; other CEC builders like
biochar, humates, granular
zeolite, and increased SOM are more appropriate.
See below.
Calcium bentonite works best when combined with mature compost, or mixed in
at the start of the composting process, or when used in soils with optimum humus
and organic matter levels.
Application rates for Calcium Bentonite:
Loose sandy soils: 300 to 600 kg/are (600 to 1200 lbs/1000 ft
2
) mixed into the top
15 to 30 cm (6 to 12 inches)
Established gardens and lawns: 100 to 200 kg/are (200 to 400 lbs/1000 ft
2
). If
bentonite cannot be tilled in, the application should be split into three or four doses
applied separately and raked and watered in. The clay will leach into the soil and
find its proper place. It may also be mixed with water at a 1:10 ratio by volume and
applied with a watering can.
Zeolites
Clinoptilolite (a naturally occurring hard clay compound with high CEC) is the
zeolite generally used in agriculture. It is mined as a hard clay compound that is
then crushed to a granular form. Unlike Ca bentonite, zeolites remain in granular
form and do not disperse in the soil. They may be used in any soil type, but are
