Soil type refers to the
texture of your soil. Soil's basically
mineral particles, organic matter and air spaces.
For earthbags & plaster, you'll want soil that's between about 18%
and 30% clay.
After you sift out the rocks & gravel in your dirt, you're left
with soil. Soil
minerals come in three basic categories: sand, silt and clay. Sand is
particle. Silt particles are smaller than fine sand but can still be
seen by the human eye. Clay particles are microscopic. The relative
proportion of these particles determines the soil texture.
soil—tends to be very light and dries out swiftly. Water drains very
quickly and makes the soil easy to dig. It's the first to warm up in
- Silt soil—retains moisture and feels slippery when
dry out as quickly as the sandy soils.
soil—very heavy, holds moisture for long periods of time when
wet and dries hard as a brick. Besides being heavy, it tends to be
sticky and very hard to dig. It's the last to
warm up in the sun.
Loam soil—the ideal soil
texture - is
composed of all three categories (sand, silt and clay). The ideal loam
soil contains 40% silt,
20% clay and 40% sand (and organic matter, if you're gardening). Loam
is a separate category
because none of its components account for more than 50% of volume.
Test Soil Type by Hand
You'll want to bring
along a water bottle, and maybe some jars or
plastic sandwich bags for samples. For the latter, also bring a magic
marker & way
to label your samples so you don't get confused. These are very rough
tests, but allow you to narrow soils down to those that are worth
subjecting to the jar test (below).
1: Take a handful of soil, and pour some water on it. Feel it and rub
it between your
fingers. If it's rough and gritty and breaks up easily, it's
sandy. If it feels sticky and like plastic, it's probably mostly
clayey. If it
feels slippery with particles that are smaller than sand, it's
mostly silty. You want something that has elements of all three.
TEST 2: Squeeze the wet soil in your fist, trying to form a lump
with it. If it makes a solid
lump that doesn't crumble, it's got a high clay content. If it crumbles
at once, it's mostly silt
or sand. Rub it back & forth between your hands (like you would
with Play-Doh or, duh, clay). Does it hold up & make an elastic
snake that holds together? Partially?
Test Soil Type by Jar
This test can take several hours to a day or two.
- If you have more than one sample, label or identify
a piece of tape on a mason jar so you can keep things straight. What
part of your land did they come
from? Surface samples, or from depth?
- Place 5
inches(or 5 cm) of soil into the jar and add a tablespoon of powdered
dishwashing detergent. The detergent is a surfactant, which keeps the
soil particles separate, resulting in a more accurate test.
- Fill the jar to the top with water, screw the lid on,
the jar for three minutes to thoroughly combine the soap, soil, and
water, and to make sure no soil is stuck to the bottom or sides of the
- Set the jar somewhere stable &
out of the way to let the sediment settle.
- As the sedimentation progresses, check the sample
watch the layers form and to note the size of the particles settling
Sand particles are the heaviest of the three and
settle out of the solution after about a minute. The sand layer is
coarser in texture than the silt and clay.
Silt is the next heaviest particle and will settle out after about an
hour. The silt layer is darker than the sand.
the lightest particle in the mix, can take from one to two days to
settle out of the solution. The clay layer that settles on top is fine
textured and light in color.
If you have any organics, they'll be floating on the water's surface.
If you like, you can calculate
percentages and transfer the numbers to the soil texture triangle
(below, click to enlarge) to determine the composition of your soil.
1.) Measure each layer of sediment.
2.) Figure out the rough
silt, and clay in the sample.
For example, if you have
total inches of
soil in your jar, then every 1/2 inch will be 10%.
etc). If you're using centimeters, it's the same idea.
your "ideal" soil for earthbags should fall into the "loam" category
approximately 20% clay, 40% silt and 40% sand.
Amending by Soil Type
a soil tends toward an excess in clay, sand or silt,
you can amend it by simply adding & mixing the required components
in a wheelbarrow.
If you're doing much of this, do yourself a favor & invest in a
mortar hoe, which is heavy-duty and has 2 large holes in the
that makes mixing easier).
Both sand and silt are pretty commonly available in most areas. If
necessary, you can buy them from an aggregate yard for a few dollars
per cubic yard.
is a little more difficult to find, if it's not common in your region.
You might be able to find sources of clay-rich dirt in river bottoms,
lakes and ponds. Construction sites may have cleared topsoil, leaving
access to clay layers below that you might be able to get permission to
harvest. You can even buy powdered clay from pottery suppliers, but
this can get expensive. You don't want too much clay in your mix - it
tends to expand when it's wet & contract when it's dry, causing
cracking. Don't go over 30 percent.
You can also use
cement and/or lime to stabilize clay-poor soil.
dirt will be subject to cracking & doesn't offer as good insulative
properties as clay or lime. Lime is great but takes forever to dry and
tends to be caustic (wear gloves). It's also best when fresh, so if you
buy a bag of lime from your big-box hardware store know that it might
be old. Look for a date on the bag (not always there, but worth trying).
You can try try mixing a little lime with cement. Start with about 8%
make a few test batches with different percentages of clay, then set
them out in the sun to cure.
If you have access to straw, adding a small amount to your mud also
helps strengthen it (more useful for plaster than for bag fill). You'll
want to chop it up small - say, 2" pieces. This can easily be done by
tossing straw in a metal drum & using a weed eater to chop it up.
If you're really interested, check out the Internet for articles on
Roman concrete (which has lasted for 2,000 years, even underwater).
It's vastly superior to anything we've developed in modern times, and
still has chemists and engineers scratching their heads. The key seems
to be using volcanic sand, if you have access to it. We haven't tried
mixing it up yet, so if you're curious have a go at it & let us
know what you come up with. Here's a good article: http://www.romanconcrete.com/docs/spillway/spillway.htm.
Testing your soil
If you're going to try amending your soil, get out a measuring cup and
measuring spoons. There are 16 tablespoons to a cup, so
3.2 tablespoons = 20% of a cup
1.6 tablespoons = 10% of a cup
3/4 tablespoon, slightly rounded = about 5% of a cup
You don't have to be that precise; just be consistent. If you're using
metric measurements, 250 ml = 1.06 cups, so you can go from there.
If you're trying a few different formulas - say, one with 10% cement,
another with 15% cement, another with 5% cement + 10% lime, etc. then
be sure to label your samples.
Moisten your soil, roll it between your hands & make balls or
snakes, and leave them out in the sun (or put them in a 150-200 degree
oven) to dry & harden. Once they're fully hardened, try these to
see if you have earthbag-worthy
1.) For snakes or cylinders, try breaking them in half. Study what
happens. Do they crumble? Do you get a clean break? How hard is it to
snap them in half?
2,) The drop test. See what happens when your dried ball sample
it hits the ground from chest
height. Do it on soft ground, grass or carpeting. If it shatters, you
have too much sand. Cracking in half is better. No cracking is best.
3.) The nail test. Try to gently (gently!) tap a small nail into your
fully-cured sample. If you can
splintering or breaking apart, you've got gold.
Remember that a good soil mix can serve
both as both bag fill and as a base plaster.
The following is geared more
gardening, but might help with gaining a comprehensive sense of the
components & their qualities:
|Type of Soil
soils come from erosive sources and so are very common near the
mountain foothills, along rivers and
streams and certain coastal areas. Sandy soils are typically comprised
of approximately 80 - 100% sand, 0 - 10% silt and 0 - 10% clay by
Sandy soils are light and typically very free
draining, usually holding water very poorly due to very low organic
|Loam soils are common
in the valleys and flat areas (flood plains) surrounding rivers and
Loam soils are typically comprised of approximately 25 - 50% sand, 30 -
50% silt and 10 - 30% clay by volume.
soils are somewhat heavier than sandy soils, but also tend to be fairly
free draining, again, due to typically low organic content.
|Clay soils are very
common in certain areas, particularly around urban
areas where fill soils have been used to establish grade in
subdivisions and developments. Clay soils are typically
approximately 0 - 45% sand, 0 - 45% silt and 50 - 100% clay by
Clay soils are not typically free draining, and water tends to take a
long time to infiltrate. When wet, such soils tend to allow virtually
all water to run-off. Clay soils tend to be heavy and difficult to work