Tuesday, June 14, 2011

AgBC's: S is for Sensational Soil

Soil is so important, yet we hardly think about it! Soil anchors plant roots, holds water for plants, and even provides air spaces for plant roots to grow.

Image from the Extreme Pumpkin Store

But for all that, did you know soil is basically made of of 3 sizes of particles? Sand is the larges, clay is the smaller, and silt falls in the middle size-wise. The proportion of each of these particles varies from soil to soil, giving each soil unique characteristics. Some solid drain more quickly (sand), while others hold water or become saturated with water (clay). Many plants, trees and shrubs prefer to grow in a balanced mixture of these particles called "loam." Loamy soils hold a moderate amount of water, air and nutrients while supporting the plant.

A vineyard in Lake County, Ohio

Because characteristics of the soil can affect plant growth, farmers and gardeners often test soil to determine if it's good for growing specific plants. Two such simple tests include the ribbon test and the "soil shake" test. The results are a good predictor of whether or not the soil is good for growing specific plants. More accurate assessments can be sent to soil testing labs, and today it is increasingly common for farmers to have their cropland grid sampled and have the results of their soil testing GPS mapped to improve the efficiency of their farming operations.

Here are some simple instructions for conducting the ribbon and "soil shake" tests with your students:

  • Ask the students if they've ever heard of the ribbon test. The explain that is can help determine what kinds of particles are in their soil. 
  • Demonstrate the text by moistening a handful of garden soil or silt in the palm of your hand until it has the consistency of putty. Work the soil into a ball about 1/2" in diameter. Press the ball between your forefinger and thumb to form a ribbon (see below).
  • If the soil will not form a ribbon, it is sand. If it makes a ribbon 1-2 inches long, it is loam. Ribbons longer and 2 inches are clay. Here is a more detailed description.
  • Explain that each of the three types of soil particles forms a different type of ribbon. Repeat the test with clay and sand.
  • Ask the children to describe the difference between the three ribbons.
  • Pass out different kinds of soil without telling the students what soil type they have; let them each conduct their own ribbon test and hypothesize about what type of soil they have.
Making a soil ribbon.
Students can also conduct a "soil shake" test:
  • Fill a jar 1/3 full with soil.
  • Fill the jar 1/3 full with water.
  • Add a tablespoon of alum (alum speeds the settling process and can be found in the spice section of the grocery store.
  • Ask the children to predict what will happen when the jar is shaken. 
  • Divide the children into groups of 3-4. Ask them to collect soil from outdoors or give them some soil you have collected.
  • Tighten the lid and shake for 3 minutes. Make sure all the lumps have broken apart. Tell the children to observe the jar after 30 seconds, 1 minute, 2 minutes and 3 minutes and write down what they observe.
  • Have them report their findings to the class. Discuss how the soil separates by particle size and identify the particles int he layers they see. The larger sand particles will be on the bottom, with the silt in the middle and any clay particles on top.
For more fun, investigate what kinds of plants grow best in various kinds of soil.

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