Soil Secrets

By Roseanne Williby

Time Allocated:  2 days

Introduction

Soils are classified so that we can see their relationship to the environment and to one another.  By its classification, there is an assemblage of knowledge about each soil.  The current system of classification was adopted by the Cooperative Soil Survey in 1965 and is broken into six categories.  From the most general to the most specific these are:  Order, Suborder, Great Group, Subgroup, Family, and Series.

In this system classes of soils are defined in terms of observable and measurable properties. Families established in each subgroup are classified on the basis of the properties important to the growth of plants or properties significant in engineering.

Texture, reaction, and permeability are among the properties considered. 

Soil texture is the relative proportions of sand, silt, and clay particles in a mass of soil.

These are defined by the following relative sizes:

  • Clay < .002mm
  • Silt  = .002 to .05 mm
  • Sand  = .05 to 2mm

If the soil particles are greater than 2 mm they are not part of the soil texture and are considered gravel.  
The amount and kind of clay in a soil, influences the extent to which the soil shrinks as it dries out or swells as it gets wet.  The shrink-swell potential refers to the relative change in volume to be expected of soil material with changes in moisture content.  Shrinking and swelling of soils damages building foundations, roads, and other structures in places.  Soils having a high shrink-swell potential are the most hazardous.

Clay soils have more nutrient and water storage than sandy soils.  Clay soils hold more water but the water is harder to move through it. 

Soil reaction is the degree of acidity or alkalinity of a soil expressed in pH values.  A soil that tests to a pH 7.0 is neutral because it is neither acid nor alkaline.  Degrees of acidity or alkalinity are expressed in the following pH’s: 

  • Below 4.5 -----extremely acid 
  • 4.5 to 5.0 ------very strongly acid 
  • 5.1 to 5.5 ------strongly acid 
  • 5.6 to 6.0 ------medium acid 
  • 6.1 to 6.5 ------slightly acid 
  • 6.6 to 7.3 ------neutral
  • 7.4 to 7.8 ----------mildly alkaline
  • 7.9 to 8.4 --------- moderately alkaline
  • 8.5 to 9.0 ----------strongly alkaline
  • 9.1 to higher------ very strongly alkaline

Permeability is the quality of a moist soil that enables water or air to move through it.  This can be measured as the relative ease of fluid flow.  Permeability is measured within a depth of 60 inches.  Classes of soil permeability is measured in inches of water per hour as follows: 

  • Very slow-------------------------------- less than 0.06 
  • Slow---------------------------------------0.06 to 0.2
  • Moderately slow ------------------------ 0.20 to 0.6 
  • Moderate --------------------------------- 0.6 to 2.0
  • Moderately rapid ------------------------ 2.0 to 6.0
  • Rapid -------------------------------------- 6.0 to 20.0
  • Very rapid --------------------------------20.0 and over 

Purpose

To determine the texture, reaction, and permeability of two different soils for developing the concept of soil genesis or origin.

Procedure: Overview 

  • You will be given soils from two different sites within the same county.  One is a sample obtained from a densely vegetated area (more silt and clay than sand), and the other was obtained from an area that has vegetation present only slightly or not at all (more sand than silt and clay).  You will not be told which soil came from which location, vegetative or non-vegetative.  
  • Obtain both soils so that your group may test both samples.  Submit the two soils from different locations to the three tests.  Based upon these tests explain the results as well as identify each soil’s origin.  Compare your findings with the rest of the class.  Record final data calculations in the Data Collection Tables to follow.  
  • Identify the general locations for each of the soils. (for example: residential area, near a creek, in a field, on the side of a road)

General Location of Soil #1 ___________________________________ 
General Location of Soil #2 ___________________________________

Materials Per Group 

  1.  four 600 ml beakers
  2.  two different soils, a 500ml(cm) volume of each
  3.  2 cake pans or foil pie pans
  4.  1 100 ml-graduated cylinder
  5.  850 ml of distilled water
  6.  pH paper- two pieces
  7.  two 150 ml beakers
  8.  two large funnels
  9.  two pieces of filter paper
  10.  spatula or spoon 

1.  Teacher Helping Hints

Since this activity calls for a comparison of two soils, it is most helpful if the soils selected come from two different sites that will allow for some contrasts in the areas of soil texture, soil reaction or pH, and permeability.  For example: a clay, neutral pH soil obtained from a newly zoned area for residential use, contrasted well with a more sandy, alkaline soil obtained near a river or creek.

Also it was helpful to utilize the Nebraska Soil Map to show the soil classification scheme and general location for the soils selected as a means to stimulate discussion during the conclusion of the lab.

 

2. Modifications or Clarification

When students are finding total mass of the soil and the beaker in Day1-Part A -Texture, it may be necessary for students to find the masses of the soil and beaker separately if the total mass that the balance can measure is a maximum that is too low to accommodate the total mass.

On Part C- Permeability, students are told to pour 400ml of water over soil in a funnel.  Some funnels may not have a large enough capacity to hold the soil and the entire volume of water initially.  So begin timing the five minute period as students begin to the water and it drains through the soil.  They should continue to add water until the entire 400 ml is poured over the soil or five minutes elapses, which ever comes first.

Further explanation is needed for one column in A. Data Collection Table - Soil Texture, page 5.  The column headed Percent Change + or -, should be calculated by dividing the change from the previous column by the Soil & Water Mixture Density-Day 1. The purpose of this calculation is to compare swell to shrink difference that can occur within the same soil in period of two days.

For tight time constraints, it may be helpful to have half the lab groups test only one soil, and the other half of the groups test the other soil.   Results can be averaged for each soil and compiled as a class. 

3.  Benefits of the Activity

Since students seem to have a narrow knowledge base regarding soils, they learned a great deal and as a result could have a better conception of how ecosystems could develop with different plant communities.  Since soil forming processes differ with altitude and latitude, students could better understand the underlying importance of soils and their contribution to the shaping of the biotic factors within the ecosystem.

In the past, the teaching of this concept would have been simply presented through lecture and discussion without the benefit of a hands-on activity.  This activity provided an important link between the biotic and abiotic factors which have a relationship in the study of ecology.  Students have demonstrated the benefit of this activity by their above average performance on this activity and section tests within this unit.