Using Biochar in Soil

Erin Rasmussen

David Yarrow, May 2010

Using Biochar in Soil

Preparation & Application

Biochar Preparation

Applying raw biochar to soil can inhibit plant growth one or two
years while microbes inhabit the char, form diversified, stable,
functional communities, and gather balanced mineral supplies.
Microbes also consume tar residues that inhibit water absorbtion.
Several weeks to a few months are needed to age char for
soil. Proper preparation can reduce this time to two weeks, and
reduce char volume needed for vigorous plant response. Four
simple steps assure rapid response, high yield and healthy plants.


Biochar’s first service to soil is water digestion, retention and
slow release from its sponge-like micropore matrix. Char must
soak up water to be an effective substrate for microbial cultures
and mobilize minerals for ion exchange with plant roots. To
moisten char, hydrophobic residues must be broken down and
removed—a task done mostly by microbes.

Size Reduction (Micronize)

Reducing char particle size dramatically improves response by
increasing external surface area accessible to water, ions and
microbes. This increases the rate char becomes saturated.
Char powder changes soil structure and nutrient capacity by
interacting with humic acids and mycorrhizal glomalin to form
supra-molecules and aggregating into mega-structures.
Inch-size char must break down further for efficient action in
soil. Weedy biomass—crop food and wastes—crumble easily to
dust. Woody biomass—dense hardwood, fruit pits, nut shells—
need effort and energy to crush.
Screening separates larger particles from powder. Pea-size is
useful in potting soils to substitute for peat moss, vermiculite and
perlite. Sand-size grains are best for seed mixes.


An ideal media for minerals, biochar adsorps both cations and
anions to prevent loss by leaching. Clays only adsorp cations,
with no internal pore capacity. Minerals enhance char’s ability to
support microbial communities, and thus improve plant response.
Ions adsorped in char’s micropore matrix are easily available to
microbe or plant by simple ion exchange (CEC & AEC).
Assess your soil’s primary mineral needs, and add needed
elements to the char. Nitrogen is the primary anion (-). Most
Northeast soils need calcium (+) and phosphorus (-).
Fertilizers are soluble, but powdered rock minerals are most
effective for microbial digestion and liberation. Today, micronized
minerals are fine powders that quickly suspend in water.
Trace elements—crucial to biology at parts per million or less
—have huge effects as co-factors in complex biomolecules like
enzymes, vitamins, hormones, and regulators. The best trace
element source is the sea and sea products, since sea water has
all soluble elements (84+) in ideal ratios for biology, from primary
elements (parts per thousand) to pico-elements (parts per trillion).


With water and minerals, char is ready to inoculate. Microbial
biology gives biochar its ability to sustain fertility and structure.
Research shows char is an ideal, preferred microbial media.
Depending on particle size and microbial vigor, inoculation can
take two weeks to six months. Microbes first task is to devour
and scour pyrolysis residues—tars, resins, poly-aromatic
hydrocarbons (PAH). Fresh char is black and shiny from residual
hydrocarbons. Inoculated and aged, char becomes dull gray,
lusterless and ready for soil. So continue making compost.

Compost: Simplest way to inoculate char is to blend with
compost. In fact, char and compost need each other, since char
is sterile, and compost is unstable, digested by decay. Compost’s
decaying biomass releases nutrients to feed microbes, who feed
plants. A 50/50 blend is suitable, but other ratios are effective.

Compost Tea: Spray char and/or soil with compost tea. Add compost to water, then bubble air through to oxygenate the
solution. Aeration and stirring boost microbe propagation rates.

EM & Bokashi (Effective Micro-organisms): three primary
microbes (fermenting bacteria, photosynthetic bacteria, algae).
Bokashi is made by culturing EM on a mix of rice hulls, manure,
biochar, and other ingredients.

Biodynamic Preps: Some of Rudolf Steiner’s preparations
are microbial inoculants or stimulants to use on biochar.

Mycorrhizae prefer char, live in its micropores, send white
fungal tubes (hyphae) into soil to suck up water and nutrients, and
pump them to the char. Char becomes a microbial supermarket
stocked with water and nutrients, waiting to feed plant root hairs.

Nitrogen-fixing bacteria
are special concern. This fourth
most abundant chemical must be in ratio to carbon (C/N ratio),
and the entire N soil cycle is managed by microbes. Biochar
fosters N-fixing bacteria different from Rhizobia on legume roots.
Tropical N-fixing bacteria differ from temperate climate ones, so
inoculants must be local and regional cultures.


Putting biochar in soil is a new practice, with minimal science
and experience to guide decisions on how and how much. This
information deficit for clear, reliable recommendations will change
rapidly in the next few years as experiences accumulate, and
standard processes and materials are proven and perfected.


In Amazon terra preta, char is up to 9% (rarely 20%), two to
six feet deep. In trials, 10% is an upper saturation limit; beyond
this, further benefits are minimal. Per acre, this amounts to 10-20
tons. Such huge volume is unreasonable—physically, financially.
However, in farm or nursery work, 9-15 inch depth is enough
to affect the active root zone of most cultivated plants. This
shallow depth can reduce the volume to 1-5 tons per acre.
Inoculation can further reduce application rates to as little as a
few hundred pounds per acre with equally strong plant response.
Successive applications over a few years are advised, not a
large amount all in one year. Introduce other forms of carbon.


If applying uninoculated, unmineralized char, fertilizers must
be added with char the first year or two. Soil food web microbes
need this time to get established and fully functional. The wiser
approach is to apply char inoculated with minerals and microbes.
Soils with abundant organic carbon and microbial life are selfinoculating,
and may not need inoculation. Minerals may still be
needed. Four methods can add char to soils:
Broadcast: scatter over soil surface; till in immediately.
Banding: narrow placement directly in seed furrow or root zone
Side dressing: scatter over root zone; shallow surface tillage
Planting hole: blend one pint in one gallon of planting soil mix
Char is light and easily wind-blown. Minimize losses by wind
and water erosion by tilling char into soil as soon as possible


Equipment suitable to spread biochar isn’t currently available.
Growers must improvise with ingenuity to add char to soil.