Getting Your Hands Dirty!

September 01, 2015 | 11:09 am

A visual activity on the properties of soil particles

By Amy Guevara, Florida Regional Representative

 

Different types of soil, like silt, loam, and clay, have different properties that make it unique. Generally, we do not think much about the soil. Sometimes, soil even has a negative connotation when we call it dirt and wash it off our clothes and bodies. In reality, soil is essential to our survival and that of nearly every organism on Earth. Our planet is mostly made of rock with an iron-nickel core. Plants and animals, including us, occupy a thin veneer on its surface. Our existence is possible because of the thin layer of soil that lies between the planet’s rocky interior and the surface.

The weathering of rock slowly produces soils. Constant exposure to wind and rain cause the rocky crust to break down slowly into smaller particles. The process can take centuries to produce fertile topsoil. As rainwater seeps into cracks in the rock, temperature extremes cause the water to freeze and later melt. The rock expands, contracts, and fractures. These weathering actions are helped along by organisms that live on and in the soil. Soils are composed of both inorganic material derived from rock and organic material derived from living and dead organisms. Both are important to support plant growth, which is vital to feeding the world!

The relative amounts of clay, silt, and sand present determines the ability of a soil to accept and retain water. Porosity refers to spaces in the soil that can hold either air or water. Scientists define permeability as the rate at which water can travel through soil. The table below lists properties of particle size relative to soils’ interactions with water. Scientists call soils with desirable properties for farming ‘loams.’ Loamy soils typically contain about 50 percent air space, which allows root systems to “breathe” (i.e. obtain O2 for respiration). The solid half of soils is about 90 percent minerals and 10 percent organic material. Usually, loamy soils have names that more accurately reflect their composition, such as clay loam or silt loam.

Although the organic fraction of most soils is small in volume compared to the mineral fraction, it plays an important role in supporting plant growth. The organic material is composed of living organisms, plant roots, and plant and animal residue. A single gram of healthy topsoil may contain 100 nematodes (small roundworms), 1 million fungi, and 1 billion bacteria.24 Present in smaller numbers may be earthworms and a wide variety of insects. Organic material contains a significant amount of nutrients and it, together with plant roots, helps

  • decrease erosion;
  • increase water infiltration and storage;
  • act as a pH buffer (to maintain an acid-base balance);
  • decompose organic material, releasing nutrients;
  • recycle carbon, nitrogen, and other nutrients; and
  • retain available nutrients such as metal ions.

The soil is a “bank” for nutrients taken up by plants, and these nutrients must be replenished for continued plant growth. Before the advent of modern agriculture, farmers relied solely upon tillage to break down existing organic material and release existing soil nutrients. This practice is still used in many less developed countries.

Properties of Soil

Objective(s): Upon completion of this activity students will be able to:

  • Describe the properties of different soil particle sizes.
  • Discuss the role of soil particle size properties play in plant growth

Materials:

  • Construction Paper
  • Ruler
  • Glue
  • Scissors
  • Pen
  • If possible, samples of clay, silt, and sand

Procedure:

  1. Students should draw a 4 X 4 square table on one sheet of construction paper with a ruler.
  2. Students label the top of the table with the following words: Property, Clay, Silt, and Sand. Then label the left row with Porosity, Permeability, and Water-Holding Capacity.
  3. Students should then examine the clay, silt, and sand samples, if available, and discuss their observations for each property.  Note: A great way for students to examine soil properties is by “running a ribbon”. See the “Running a Ribbon Procedures” below.

Questions:

  1. What is the clay sample’s permeability? Answer: The clay sample seems to have a slow permeability; water moves slowly through the clay.
  2. Question: What did you observe with the sand sample’s water-holding capacity? Answer: Sand has very limited water-holding capacity. It does not seem to retain very much moisture at all.
  3. Students should complete the table descriptors of each category.

 

Property Clay Silt Sand
Porosity Mostly small pores Mostly small pores Mostly large pores
Permeability Slow Slow to moderate Rapid
Water-holding capacity Large Moderate Limited

 

soil ribbons

Running a Ribbon Procedures:

  1. Place about 2 tablespoons of soil in palm of hand. Add water dropwise and knead the soil to break down all its aggregates. Soil is at the proper consistency when plastic and moldable, like moist putty.
  2. Place ball of soil between thumb and forefinger, gently pushing the soil with the thumb, squeezing it upward into a ribbon. Form a ribbon of uniform thickness and width. Allow the ribbon to emerge and extend over the forefinger, breaking under its own weight.

For more information, extensions and other lessons visit:

  • Nutrients For Life Foundation: nutrientsforlife.org
  • From the Ground Up: thescienceofsoil.com

References:

Herbert, J., Brown, R., & Hanlon, E. (2007, August 1). Land Judging and Homesite Evaluation in Florida. Retrieved July 31, 2015.

McGuire, J. (2013, August 5). Nutrients for Life Foundation. Retrieved July 31, 2015.