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Research Methods

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

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 Horizons

Starting at the top, for each horizon present, measure the total thickness and the depth (in centimeters). When using a soil pt you will usually be able to get to a deeper total depth than when using a soil corer. Whatever method you use, describe all the layers you can see, even if it is only a partial layer. For example, when sampling soil horizons with a soil corer, you often see only the top of the E horizon. If that is the case, indicate that it is there, the thickness visible and that you know you were not at the bottom (i.e. E horizon = 5+cm thickness at depth of 12-17+cm).

It is almost always the case that the thickness of the horizon varies somewhat from place to place. When sampling with a soil pit, be sure to take and average of the thickness across the entire width of the exposed face. When sampling with soil cores, it is important to collect multiple cores to get an average thickness for each horizon. A minimum of 3 (you need at least 3 samples to calculate and average), but the more variable the horizon thickness is, the more samples you would want to collect to ensure that you measure is as accurate as possible for your site.

measuring soil horizons

In this example there is a thick forest floor (O horizon), a thin topsoil horizon (A horizon) and a moderately thick mineral soil (E horizon).

soil horizons

Collecting soil for bulk density measurements.

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.

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