Great Lakes Worm Watch

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Research Methods

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. (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:

    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.

    Munsell Color Munsell Color book

    Munsell color chart (left) and Munsell color book (above).

    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.

Several chemical soil characteristics can potentially tell you a great deal about the soil processes occurring in your soils, including:

  1. pH and total nitrogen, phosphorus and potassium content
    There are many over the counter kits available to easily measure soil pH and the total amount of the primary limiting nutrients (nitrogen, phosphorous, and potassium). While these are good basic measures to conduct, they may or may not tell you a great deal about what is happening in your soils over short periods of time. To know more about nutrient dynamics and cycling in your soil, you need to look at the amount of these nutrients that are actually available to plants in the soil… read more.

  2. Available nitrogen and phosphorous content
    Measuring the amount of a given nutrient that is actually available to plants roots is a much more difficult process than measuring the total amount of a given nutrient. This is the case because ‘available’ nutrients are in molecular forms that are transient, they don’t stay around very long. Once they are created, they are either taken up by plant roots, leached from the system or get changed into another molecule by the action of soil microorganism. Scientists study the nutrient availability using specially formulated resins that act like plant roots by absorbing the available nutrient molecules when they are present. Then later in the laboratory, these nutrient molecules can be extracted and measured to see how much of that particular nutrient or molecule was available during the time the resin was buried in the soil.

  3. Total organic matter content
    While soil color can give you an indication of the relative amounts of organic matter in a soil, to measure the actually amount you have to do a bit more. Since organic matter is made up of carbon compounds, you can use a process called “loss by ignition” (a fancy way of saying you burn up the carbon) to measure how much there is. The process if relatively simple but requires a muffle oven that will burn at 500oC . Here’s the process…

    1. weight your fresh soil sample (grams)
    2. 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.
    3. Weight you fully dry soil sample (grams)
    4. Place your sample in a muffle oven at 500° C for a minimum of 4 hours.
    5. Weigh the burned soil sample (grams)
    6. Subtract the final burned weight from the initial dry weight to get the total ‘”ost by ignition” which equals the total amount of organic material present in your sample.
    7. Express your total organic matter content as the percent of your soil sample

    equation

    For example, if your initial dry soil sample weighed 115 grams and your final burned soil sample weighed 93 grams, then the loss by ignition is 22 grams (115-93 = 22). Then the total percent organic matter for your soil sample is 19% [(22/115) x 100 = 19.1]

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