Agriculture Reference
In-Depth Information
Soil Texture
There are three sizes of soil particles. From biggest to smallest, they are sand, silt and
clay. Sand is gritty and it doesn't stick together. You've seen it on it's own on the beach or
in a sand box. Silt is smaller and more powdery, but is still separate grains. Clay particles
are much, much smaller and they join together to form a sticky substance kind of like play
dough.
So
soil texture
, a bit of a misleading term, is actually a specific way of describing the rel-
ative particle sizes in your soil. It refers to the percentages of sand, silt and clay that are
present. Note that even if you have a clay soil or sandy soil, you probably have all three
soil particles to some degree.
The quickest way to learn what kind of soil texture you might have is to take ⅓ cup of
loose soil, remove any debris, and slightly moisten it uniformly throughout the sample.
Then roll it into a ball. If you have a hard time doing this or if it breaks easily under pres-
sure, the predominant soil is sand. It will also feel a bit gritty. The more it stays together,
the more silt or clay you have.
Try to roll it out into a cylinder. The longer the cylinder you get, the less sand you have.
It can be tricky to differentiate between silt and clay, but silt is smoother with more indi-
vidual grains and clay is stickier with no discernible grains. Clay can also stain the hands.
There is another test to determine texture that I prefer called a sedimentation test, but it
is a little more involved. Take a clear one-quart jar with a lid and fairly straight sides — a
major curve at the bottom of the jar will distort the results. Go to your garden and dig down
6-8 inches, taking enough soil to fill the jar about ⅓ full. Be careful to remove any large
stones, gravel and organic matter, since this test is only meant to determine the mineral
particle size of the soil. Next, add a few tablespoons of liquid dish detergent, which helps
separate the soil aggregates, and fill the jar up with water to one inch below the top rim.
Cover and shake it for a minimum of 10 minutes. Set it down where it won't be dis-
turbed, being careful not to slosh it sideways as you set it down. When you come back,
after at least 24 hours, you should end up with a sand layer on the bottom, then a silt layer,
then clay, and perhaps a bit of dark organic matter. The sand will have settled out right
away and the silt within an hour, but the clay takes longer.
It's usually fairly easy to see the three distinct layers. Measure the height of each layer
and divide by the total height to determine your percentage of sand, silt and clay. To be ac-
curate, there is actually one more step. Your results are in volume, but the soil texture tri-
angle uses weight for units. To fix this, multiply the percentage of sand by 1.19, the per-
centage of silt by 0.87 and the percentage of clay by 0.94.
If you have 70% sand, 20% silt and 10% clay soil, you have a sandy loam. If you have
50% sand, 20% silt and 30% clay soil, you have a clay loam. There is no need to remember
