Friday, June 17, 2011

The how and why to determine soil texture

All soil is made up of three different sized particles clay being the smallest, sand being the largest, and silt somewhere in-between.  Different proportions of these particles results in different types of soils with different water holding properties.

Nearly all life on land is dependent on the soil beneath our feet.  For this reason, as a gardener or amateur scientist your ability to become familiar with soil types and textures can become critical.  Things that grow in one soil type will not grow in another soil type.  Or watering one way in one soil type will have completely different results in another soil type.  Even if you look at the lawn in your own back yard you may notice different species of grass growing in different patches, these patches often correspond to different soil types.  Or even patches of lawn where it is difficult to get grass to grow may correspond to a certain type of grass.  In the more natural world different habitats strongly correspond to the different soil types they are found on.  Think about it, if you were able to identify soil types and texture you would be able to explain gardens, habitats, plant growth, and water drainage among other things and amaze all your friends! 

So how can we learn how to explain a soil?  Well first of all, soil is not simply dirt but is a complex mixture of materials most of which are derived from a parent material, typically the rock common to an area.  As I mentioned in a previous blog entry, particles aggregated together to form a rock eventually breakdown (called weathering) into individual particles that will go onto form soil.  These individual rock particles (crystals) come in three different size classes, which determine the texture and properties of a soil.  Clay is the smallest particle size, sand is the largest, and silt is somewhere in-between.  Differing proportions of these three soil particle sizes within soil determines the overall texture of the soil.  While the chart at the top of this entry looks fairly complicated, its actually fairly strait forward; more clay particles in a soil means a clay soil, while more sand in a soil means a more sandy soil. 

Size comparison of sand, silt, and clay.
Sand, silt, clay, this is all known as soil texture.  More sandy soils are referred to as having course textures due to the course nature of sand, while clay soils are referred to as having fine texture due to the fine size of clay particles.  Why does this matter?  Well, soil texture determines its capacity for holding water.  And a soils water capacity will determine what grows where and how we should water or amend it to create an ideal plant environment.  Hmm... Apparently you need a dirty brown thumb before you can have a green thumb!

There are two quick and dirty ways for you to determine soil texture and they can be done just about anywhere.  The first is a touch or "ribbon" test.

Soil "ribbon and ball" test
1. Collect about a ping pong ball amount of soil and moisten it with water.
2. Once the soil is moist, try to form it into a ribbon.  If you can form the moist soil into a ribbon you know the soil has a lot of clay in it.  If it will not form a ribbon it has a lot of sand in it.  The stronger the ribbon the more clay the soil has in it.
3. Form the moistened soil into a ball and squeeze it.  The easier the ball breaks apart the more sand the soil has.  The harder it is for it to crack or break apart the more clay the soil has.
4. As you are handling the ribbon and ball note how the soil feels.  Gritty feeling means sand and a smoother feeling means clay.
From these steps you should be able to identify if you have more clay or sand in your soil.

This is a soil ribbon.  If you can form a ribbon like this it means your soil has a lot of clay in it.  If you can't and your ribbon keeps falling apart it means you have a more sandy soil.

Soil settling
1. You will need a clear glass or plastic jar with a lid.  A typical peanut butter jar works well.
2. Collect soil soil, enough to fill the jar up about half way.
3. Dry the soil out and break up all the dirt clods.  You can skip this step if you want but be sure to shake extremely well in step 4 in order to break the clods up.
4. Fill the jar about 90% full with water and shake like crazy, ensuring all clods are broken up and all soil particles are suspended in the water.
5. Shake the jar like crazy,   And when I say crazy I mean crazy, no excuses.   Do this for at least a minute, longer won't hurt.  Then, set the jar down and watch.  The heaviest sand particles will settle out within a minute, silt particles within two hours, and clay particles after that.  You can mark the jar at the top of the sediment mark after 1 minute and two hours to give you these divisions and give you an idea of what proportions of particle sizes the soil is made of.

The above experiments can be found in more depth at:
Now that you have patiently endured some of the theory and practice you are probably wondering, "why the heck is this important?"  As mentioned before soil texture determines what grows where and how you water.  Clay soils hold more water and absorb water slowly.  This is why wetland soils are usually very fine.  Clay soils also hold their water for long periods of time because water sticks to the surface of clay well.  Clay absorbs water slowly though and requires a longer but slower irrigation in the garden because of this.  So in nature clay soils are typically wetter and often associated with wetlands or plants that like moisture.  In the garden or landscape clay soils will also be wetter and require slower irrigation.  Sandy soils however are typically drier and absorb water quickly.  The large particle size of sand makes for a lot of space between particles for water to peculate through very quickly.  This also allows for water to evaporate from sand very quickly also, so sandy soils tend to be drier.  Sandy soils are often associated with dry habitats or plants that prefer drier soils.

So after you test a particular type of soil for sand-silt-clay content you can associate certain plants or habitats with that soil.  You can also assess your garden soil and adjust watering and planting accordingly.  Most garden plants can adapt to most soil conditions but do best in a "loam" soil.  Some plants to prefer sandy or clay soils though.  A loam soil has even amounts of sand, silt, and clay, and as a result it absorbs and holds just the right amount of water and air for plant roots to breath.  I have had several garden of my own, none of which have every been loam.  As a result, I have had to adjust soil texture by adding sand to clay soils, or adding clay to sandy soils, or add organic matter to adjust the water retention or drainage of a soil.

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