Sunlight is often the first thing that comes to mind when we think
about what plants need to grow, but just as important is water and
the nutrients that plants find in soil.
Soil is formed by weathered rock, referred to as parent material.
Weathered parent material is mixed with dead or decaying leaves, twigs,
and organisms, known as ‘organic matter’. Organic matter
and soil provide plants with important nutrients as well the medium
for which to grow a supporting root system. The amount of organic
matter and type of soil a plant grows on is extremely important due
to the fact that plants are immobile. Unlike humans, plants cannot
choose where they would like to live and grow. Whether a plant grows
on a sandy, silty, or clayey soil affects its ability to grow roots
and take up water and nutrients. Remember, we can alter a plant’s
ability to take up water and nutrients through such practices as irrigation
and fertilization, but these practices are not always feasible.
Notice the following pictures in which a layer of sand (largest soil
particle) exists over a layer of clay (smallest soil particle) in
a glass beaker. If you separate the two soil types and pour water
over each, within which type will water move faster and why? Answer.
Now, what do you think will happen when water is poured over a layer
of sand above a layer of clay? Let’s watch…
As you can see, the water moved quickly through the sand and saturated
that layer. However, once all the pore spaces between the sand particles
were filled the water was backed-up by the smaller clay particles
which fit closer together and have less pore space. This caused the
water to perch on top of the sand layer. Eventually this water will
move through the clay particles at a slow rate similar to how water
eventually moves into the ground after a strong rain or flood event.
Most soil in Virginia is a mixture of sand and clay (not sand over
clay like in the example). However, where sand does occur over clay,
the soil is often not suitable for agriculture or building houses.
This is the situation on parts of the coastal plain where many of
our most important forests occur. One way to tell if your soil is
mostly sand or mostly clay is to perform a soil ribbon test. The first
step is to pack the soil in your first. Then, slowly and deliberately
press the soil with your thumb against your index finger. Keep moving
the soil up your hand and try to form a “ribbon.” If the
soil ribbon can be made longer than two inches you have a clay-based
soil but if the ribbon is less than one inch than you have a sand-based
soil. Ribbons may also be formed between one and two inches from silt
(medium soil particle), which is found mostly in mid-western states.
The mixture of the three soil particle sizes allows for plants to
have pore space to grow roots and allows a proper amount of water
to be available for the plants to take up. Some ‘hydrophobic’
(water-repelling) plants are capable of growing in arid climates with
sandy soils because they require very little water. Can you think
of an example of a hydrophobic plant? How about the
ocotillo ? Other plants, like the American
sycamore, are capable of growing in sandy soil but are mostly
found along stream banks where there is plenty of water. These plants
are referred to as ‘hydrophytic’ (water-loving).
Every state has designated a state soil similar to how each state
has designated their own bird, flag, or flower. More information on
state soils can be found at the Natural Resources Conservation Service
Most soils are designated because they are the most common
within that state.
- What soil properties affect root growth?
- Is a sandy or clayey soil better for agriculture and development?
- What is the purpose of conducting a soil ribbon test?