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.

Pictures of making ribbons (click any thumbnail to enlarge)
Ball of clay made in fist	Starting to form a ribbon	Clay Ribbon (>2")	Starting to make a ribbon from sand	Ribbons are hard to form with sand (<1")

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 (NRCS) website. Most soils are designated because they are the most common within that state.

Questions:

1. What soil properties affect root growth?
2. Is a sandy or clayey soil better for agriculture and development?
3. What is the purpose of conducting a soil ribbon test?