Agriculture Reference
In-Depth Information
Freshly made biochar is hydrophobic and difficult to wet.
Biochar may take a number of years to reach full exchange capacity.
Freshly applied biochar can lower the soil concentration of essential
nutrients via simple dilution.
Fresh biochar can strongly adsorb large amounts of nutrient anions and
cations from the soil/water solution, making them less available to plants
and soil organisms, at least temporarily.
Significant research work is ongoing with biochar. One promising avenue is using
it as a carrier for soluble fertilizers ranging from fish emulsion to ammonium
sulfate.
Biochar should not be considered a source of plant nutrients.
Adding
biochar alone will in general only add a modest amount of Potassium. Its main
value is to retain nutrient ions in soils that lack exchange capacity; its important
secondary values are its ability to hold water, loosen soil texture, and provide
habitat for soil organisms.
Low-Fired Pottery Sherds
Terra-cotta type low-fired pottery sherds are also reported as abundant in terra
preta soils. These highly-porous lightly-fused clays have been utilized for their
ability to capture and hold onto elements long before the concept of exchange
capacity was known:Ancient sailors reportedly used terra-cotta jugs to desalinate
sea water. If a sealed, empty jug was held under water, the pure H
2
O would filter
to the inside while the salt and other elements were adsorbed and held by the
negative charges in the terra cotta. The actual exchange capacity of terra cotta
pottery sherds is unknown but perhaps worthy of further investigation.
High-fired clays such as stoneware and porcelain are non-porous and would not
be expected to add any CEC to a soil.
Expanded Vermiculite
Vermiculite is a hydrous mica, a type of collapsed 2:1 alumino-silicate layer clay
where the space between the clay layers is filled with hydrated Magnesium ions.
When vermiculite is heated, the water molecules surrounding the Mg++ ions turn
to steam and the spaces between the layers expand greatly, making a large
surface area available for cation exchange. Expanded vermiculite has a CEC of
~100meq per 100 grams.
Expanded vermiculite is very light-weight and has excellent water-holding
properties as well as high CEC. It is primarily used in potting soils and
greenhouse beds. Its main disadvantage is that when fresh its exchange sites
are saturated with Magnesium and Iron, which can add undesired high levels of
Mg and Fe to the soil. When expanded vermiculite is used, the growing media
should be analyzed for Mg and Fe content, and the amount of vermiculite adjusted
so as not to overload the Mg saturation %. The Calcium level must also be
carefully adjusted to maintain the desired Ca:Mg ratios.
