Sampling Soils and Soil Characteristics

Soil characteristics (type, structure, chemistry, etc.) can have large effects on the distribution of and nature of the populations a given communities supports. A full understanding of the intricacies of measuring and studying soil characteristics and processes can take years to learn. However, even a basic introduction to some of these important soil characteristic and how they can be measured will provide you with a much greater appreciation and understanding of ecosystems and ecosystem processes.

Several physical soil characteristics that you can readily measure can potentially tell you a great deal about the soils you are working with, including:

  1. Soil Horizons
    The presence or absence, thickness and depth of different soil horizons can tell you a great deal about what soil forming processes are happening or not happening. (Read more about soil horizons) For example, in earthworm-free forests, there is often a thick O horizon while the A horizon may be absent or very thin. But in an earthworm invaded forest, the O horizon is often missing and the A horizon is very thick.

    You can sample soil horizons by digging a soil pit or by collecting soil cores. See Below.

  2. Soil Bulk Density
    Soil bulk density is a measure of how compacted the soil is and specifically refers to the weight dry soil divided by its volume. The lower the bulk density the less compacted the soil is and the more air space there is in the soil. As bulk density increases, the soil is more compacted and has less air space in it. Bulk density most often expressed as the grams of dry soil per cubic centimeter. In earthworm-free hardwood forests bulk density is often low, about .5g/cm3, after earthworm invasion soil bulk density can increase to .8g/cm3 or greater.

    You can sample soil horizons by digging a soil pit or by collecting soil cores. See Below.

  3. Soil Texture
    Soil texture refers to the relative proportions of sand, silt, and clay particles in a mass of soil (Read more about what differentials sand, silt and clay particles). The texture can tell you a great deal about the potential vegetation on a site. For example, sandy soils tend to be drier, have lower bulk density and nutrient availability than loamy soils. While clay soils tend to hold moisture better and the clay holds a lot of soil nutrients, but bulk density can be very high making it hard for plant roots to penetrate the soil and breath. As the physical properties of soils change, so do the plants and organisms that are typically found there.

    A simple “squeeze test” can tell you which major soil texture category your soil belongs to. Learn how.

  4. Soil Color
    Color is the most obvious of soil properties, and is easily determined.  It has little known direct influence on the functioning of soil, but it is useful because other more important characteristics that are not so easily quantified may be inferred from it. The importance of soil color is greatest within a local area since the base color of soils is largely determined by the parent material.  For example, if the soil texture is the same in a forest stand, but the color changes across a short distance (from an upland to a wetland, or across an earthworm invasion front) it can tell you that big changes may be occurring in the chemistry or organic matter content of the soil. Some important color changes you might see include:

    1. Dark color “values” (see below) indicate more organic matter than light color “values”. 
    2. Light gray or grayish colors commonly indicate reducing (low oxygen) conditions, either currently or in the past. This is common in soils that are usually saturated with water.
    3. In some environments, yellowish or reddish mottles or spots indicate alternating oxidizing (oxygen rich) and reducing (low oxygen) conditions. This is common in some wetland soils that are only seasonally or periodically saturated with water. Yellowish and reddish mottles are concentrations of material of which iron is one of the most important components. Read more about mottles.

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Physical soil characteristics

Several physical soil characteristics that you can readily measure can potentially tell you a great deal about the soils you are working with, including:

  1. Soil Horizons
    The presence or absence, thickness and depth of different soil horizons can tell you a great deal about what soil forming processes are happening or not happening. (To read more about soil horizons click here. For example, in earthworm-free forests, there is often a thick O horizon while the A horizon may be absent or very thin. But in an earthworm invaded forest, the O horizon is often missing and the A horizon is very thick.

    You can sample soil horizons by digging a soil pit or by collecting soil cores.

  2. Soil Bulk Density
    Soil bulk density is a measure of how compacted the soil is and specifically refers to the weight dry soil divided by its volume. The lower the bulk density the less compacted the soil is and the more air space there is in the soil. As bulk density increases, the soil is more compacted and has less air space in it. Bulk density most often expressed as the grams of dry soil per cubic centimeter. In earthworm-free hardwood forests bulk density is often low, about .5g/cm3, after earthworm invasion soil bulk density can increase to .8g/cm3 or greater.

    You can sample soil horizons by digging a soil pit or by collecting soil cores.

  3. Soil Texture
    Soil texture refers to the relative proportions of sand, silt, and clay particles in a mass of soil (Read more about what differentials sand, silt and clay particles). The texture can tell you a great deal about the potential vegetation on a site. For example, sandy soils tend to be drier, have lower bulk density and nutrient availability than loamy soils. While clay soils tend to hold moisture better and the clay holds a lot of soil nutrients, but bulk density can be very high making it hard for plant roots to penetrate the soil and breath. As the physical properties of soils change, so do the plants and organisms that are typically found there.

    A simple “squeeze test” can tell you which major soil texture category your soil belongs to. Learn how.

  4. Soil Color
    Color is the most obvious of soil properties, and is easily determined.  It has little known direct influence on the functioning of soil, but it is useful because other more important characteristics that are not so easily quantified may be inferred from it. The importance of soil color is greatest within a local area since the base color of soils is largely determined by the parent material.  For example, if the soil texture is the same in a forest stand, but the color changes across a short distance (from an upland to a wetland, or across an earthworm invasion front) it can tell you that big changes may be occurring in the chemistry or organic matter content of the soil. Some important color changes you might see include:

    • Dark color “values” (see below) indicate more organic matter than light color “values”. 
    • Light gray or grayish colors commonly indicate reducing (low oxygen) conditions, either currently or in the past. This is common in soils that are usually saturated with water.
    • In some environments, yellowish or reddish mottles or spots indicate alternating oxidizing (oxygen rich) and reducing (low oxygen) conditions. This is common in some wetland soils that are only seasonally or periodically saturated with water. Yellowish and reddish mottles are concentrations of material of which iron is one of the most important components. Read more about mottles.

    Soil color can be objectively determined using Munsell soil color charts. These charts provide plates of colors that you can compare your soils to. They are used to determine the moist soil color for each horizon described.  This system uses three elements of color: hue, value, and chroma.  Hue is the dominant spectral color of the soil and is related to wavelength of the light.  The most common hues in the Great Lakes region are 10R, 2.5R, 5YR, 7.5YR, 10YR, 2.5Y, and 5Y.  Value, is a number that refers to the relative lightness of color and is a function of the total quantity of reflected light.  Chroma, is a number indicating the relative purity of the dominant spectral color.  The notation is recorded in the form: hue, value/chroma.  For example, 5Y 6/3 means this soil has a hue of 5Y, value of 6 and chroma of 3. 

    There is one Munsell color chart for each Hue, with the variations in Value and Chroma for that Hue arranged vertically and horizontally on orderly scales of equal visual steps which are used to measure and describe color and accurately under standard light conditions.

Chemical Soil Characteristics

Soil Collection

Soil Pits

Scientists often use something called a soil pit to describe the soils in different habitats, since the characteristics of the different soil horizons can change dramatically as you move from the top to deeper in the soil pedon (pedon is a fancy word for an entire cube of soil from the parent material all the way to the surface). A soil pit may be very large and deep so that people can stand in it while conducting detailed examinations of the soil horizons, or it may be smaller and slightly more shallow if you are simply attempting to describe the upper soil horizons (O, A and E, click here to learn more about the soil horizons). Digging a soil pit is easy to do (if the soils are not to rocky) and you might be very surprised at what you find!

To dig a small soil pit to measure surface horizons and bulk density, it is best to use a shovel with a flat, square spade so the sides of the pit can be smooth and relatively undisturbed. Dig a pit at least 1/3 of a meter (~ 1 foot) on each side and at least 35 centimeters (~1 foot) or more deep. Don’t be to destructive, only dig a pit large enough so that you can see the layers of soil relatively easily. Take care so that at least one side of the pit is straight down and smooth, so that the natural layering of the soil visible and not disturbed by the digging. When you are finished, be sure to put the soil back into the pit, or mark it so people will see it and not fall into it!

soil profiler

UPDATE THIS PHOTO!  You can purchase specialized soil cores such as this for sampling soil horizons that provide a nice cross section making it easier to see the horizons, or use a sharpened bulb planter which also give a nice cross section since the diameter of the core is nearly 6cm.

Soil Cores

If you do not have the time, or strong back for digging soil pits, or simply do not want to cause such a large disturbance on the site, you can sample soil horizons and upper soil horizon bulk density by collecting soil cores. There are many types of soil corers (also called borers) that you can buy commercially. However, in many cases a simple bulb planter that you can purchase at any garden center may do just fine, at least for sampling the upper 10-15 cm of soil. To make a bulb planter and even more effective soil corer, sharpen the bottom edge with a file so it will easily cut through surface litter and plant roots as you push it into the soil. For heavy duty use you may want to purchase a professional soil corer, but for light duty, a bulb planter will work well and is much less expensive.

Measuring Soil Bulk Density

To measure soil bulk density you need to collect a known volume of soil and then determine the dry weight of the soil that was in that volume. The trickiest part of this, is preventing compaction of the soil when you collect it. If you are collecting samples from a soil pit, you can use a corer to take a sample out of the side of the pit from each horizon. If the horizon you are sampling in thin, you may have to be creating in finding an appropriate sampling tool. When collecting bulk density samples with a soil corer, you can avoid the very top of the core which is usually the most vulnerable to compaction and collect your samples from the mid-point of each horizon. For example,

To calculate bulk density

  1. Determine the volume of soil collected (cm³)

    For example, if your soil corer was 6 cm in diameter and you collected a 2 cm thick sample of soil, then the volume of that sample can be calculated using the formula for the volume of a cylinder, (π*radius²) * thickness, where the radius is ½ the diameter of your core. So in this example, the volume would be:

    (3.14 * 9cm²)*2cm = 56.5cm³

  2. Weight your fresh soil sample (grams)
  3. Place your sample in a drying oven at 60° C for 24-48 hours (or until it no longer loses weight) to drive off all remaining soil water content.
  4. Weight you fully dry soil sample (grams)
  5. Express your soil bulk density as the grams of dry soil per unit volume

     grams of dry soil collected
    volume of soil collected

    For example, if your dry soil sample weighed 45 grams and your sample volume is 56.5 cm³, then your bulk density is 45 grams/56.5 cm³ = 0.796, which you would round to 0.8 g/cm³.