Soil Bulk Density - Physical Properties

The combined influence of soil texture and structure may best be described by the term "soil bulk density." Soils are composed of solids (minerals and organic matter), and pores which hold air and water. The bulk density of a soil sample of known volume is the mass (or weight) of that sample divided by the bulk volume.

In this section, you will learn the importance of bulk density and why high bulk density in the soil can be a concern. The role that texture and structure play in bulk density is explained. Human activities that alter soil bulk density are considered, as well as some guidelines for minimizing negative consequences. You will learn how to obtain an undisturbed soil sample and determine the bulk density of that sample. We will introduce simple mathematical equations to calculate bulk density, percent pore space, and the percent moisture and air at the time you sampled the soil.

Importance of Soil Bulk Density

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An ideal soil can be described as being 50% solids and 50% pore space, with half the pore space filled with air and half with water. This "ideal" soil would hold sufficient air and water to meet the needs of plants with enough pore space for easy root penetration, while the mineral soil particles would provide physical support and plant essential nutrients. Texture, structure and organic matter combine to influence the amount of pore space, as shown in the graphic below.

Most soil bulk densities fall between 1.0 g/cm3 and 2.0 g/cm3; root penetration is severely impacted at bulk densities greater than 1.6 g/cm3. As density increases, pore space decreases and the amount of air and water held in the soil also decreases. As you can see from the figure above, soils with granular structure (high percent organic matter) are higher in percent pore space regardless of the amount of sand or clay in the soil. Angular blocky structure has about the same percent pore space irrespective of sand or clay content. Platy structure, usually associated with compacted soils with low organic matter, has little pore space in sandy textured soils. Clayey soils with platy structure have little to no pore space.

Lower soil bulk density is desirable for plant growth, whether those plants are agricultural crops, trees, or turfgrass. Low bulk density soils have greater water infiltration rates which minimize runoff, improve water quality, and reduce stormwater flow. Activities such as plowing, timber harvesting or compaction of the soil during home construction can increase soil bulk density and reduce pore space. Subsequently, the soil contains less air and water, and high bulk density can impede root penetration.

Factors that affect Bulk Density

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Fine-textured soils such as silt loams, clays and clay loams generally have lower bulk densities than sandy soils. Finer textures, especially in the presence of organic matter, tend to have better structural organization. Even if soil aggregates are the same size as sand grains (see the figure below), there can be "extra" pore space within the aggregates called intraped micropores. This contributes to overall pore space and lowers the bulk density. In sandy soils, the percent organic matter and clay content is usually low resulting in less aggregation. Macropore space may be similar between well-aggregated fine-textured soils and sandy soils, but lack of micropores gives sandy soils higher bulk densities.

Particle density is a measurement of just the mineral solids without any pore space. It is strongly influenced by the mineralogy of the soil. Soils that are predominantly quartz, feldspars, micas and colloidal silicates have approximate particle densities of 2.65 g/cm3. Soils with large amounts of higher density minerals like magnetite or hornblende may have particle densities closer to 3.0 g/cm3, and soils formed in organic rich materials might have particle densities of 0.9 to 1.4 g/cm3.

One other factor associated with soil texture is packing ratios of the particles. Loosely packed, well-sorted sandy soils (or well-aggregated finer textures) will have more and larger voids than tight-packed aggregates (see the accompanying figure). If poorly-sorted (well-graded) particles are loosely packed, smaller particles will fill the voids created by the larger particles and again, bulk densities increase.

Soil structure (as discussed in a previous section of the tutorial) affects the amount of pore space as well as the size and orientation (tortuosity) of pores. Platy structures are densely packed vertically so pores have smaller diameters. Pores are very tortuous, winding back and forth along relatively long horizontal and short vertical distances. It can take a long time for air and water in the soil to move upward or downward due to the nature of pores in platy structured soils. Soils with granular and subangular blocky structure have many large pores between peds or aggregates and it is easy for air and water to move from one pore to the next; movement is rapid and in a fairly direct line.

Other factors that have some effect on bulk density are parent material and depth in the profile. Deeper in the soil profile, there is less organic matter and less aggregation, fewer fine roots and lower populations of soil microorganisms, all of which contribute to higher bulk densities. Compaction deeper in the profile may also occur from weight of the overlying soil. Soils formed in parent materials like glacial till can have extremely dense subsoils as a result of compaction caused by the enormous weight of glaciers. In some areas, glacier height may have reached 1 to 2 miles thick and evidence of compaction by these masses of ice is still present 10,000 years later.

Soil Sampling and Calculating Bulk Density

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Obtaining an undisturbed soil sample is necessary when determining bulk density. Click here to read how.

Once you have obtained your sample, weighed and dried it, you are ready to determine soil density, pore space, and percent moisture. This useful information can, for example, suggest reasons for the success or failure of a home garden area. If the soil bulk density is too high (and pore space too low), plants will have difficulty obtaining air and water; you may be watering frequently to make up for low water storage in the soil. The incorporation of organic matter via tilling will improve soil structure and lower bulk density, and also help the soil hold more water, air, and nutrients.

Calculating Bulk Density, Moisture and Pore Space

  1. To calculate bulk density you need the dry weight (in grams) of soil and the volume (in cubic centimeters) of soil. The volume of your tin can will be the volume of soil.

  2. Bulk density = dry weight (grams) ÷ volume (cm3)

  3. Now calculate percent moisture at time of sampling, based on weight of the soil (in grams).

  4. % moistureweight = [(wet weight – dry weight) ÷ dry weight] × 100

  5. Would you like to know how much of your sample is solids and how much is pore space? Let’s calculate percent solids first; we need to use particle density—the density of just the solids—and bulk density. Assume the density of quartz, 2.65 g/cm3, for particle density.

  6. % solids = 100 × (bulk density ÷ particle density)

  7. Since we know that the soil sample is solids and pore space, which total 100%, we obtain:

  8. % pore space = 100% - % solids

  9. Would you like to know how much air was in the soil when you sampled it? To calculate this we need to convert the percent moisture on a weight basis (number 2, above) to a volume basis; this tells us the volume of water that was in the soil when sampled. The conversion factor will be our soil bulk density, a value that has "volume" as a component.

  10. % moisturevolume = % moistureweight × (bulk density ÷ density of H2O)

  11. Since you know that soil pores contain only water and air, the volume of pores minus the volume of water should be equal to the volume of air in the pores.

  12. % air-filled pores at time of sampling = % pore space - % moisturevolume

Structural Management

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Bulk Density and Soil Strength

High bulk densities may occur as a natural feature in the soil, such as a fragipan or some type of cemented horizon. These horizons are so dense that it requires great effort to excavate soil with a shovel or backhoe. It is impossible for roots to penetrate for several reasons. Pore sizes are extremely small and there are very few pores, if any to accommodate root penetration and growth. Water movement is very slow, and it may take days for water to move downward a few inches. Soils may be poorly aerated and toxic gases (methane and ethylene) may build up and impede root growth. And soil strength is too great for roots to push through.

Roots penetrate the soil by pushing through soil pores. If the pore is not large enough, the root must push aside soil particles to enlarge it. In part, the actual density of the soil can restrict root penetration. But penetration is also limited by soil strength, a property of the soil that causes it to resist deformation. Soil strength increases with higher bulk densities and decreases with higher moisture content. Compaction of the soil usually increases bulk density and soil strength. Soil water content and soil texture are two factors that must be considered to determine the effect of bulk density on the ability of roots to penetrate soil.

Human Activity

Many human activities conducted at the Earth’s surface can impact soil bulk density. As discussed in the section on Soil Structure, activities that compact soil might result in platy structure with diminished pore space and size, therefore higher bulk densities. Soil characteristics such as inherent bulk density, texture and organic matter content, and site vegetative characteristics may influence the resultant bulk density after compaction. But any compaction causes a reduction in pore space and the amount of air and water a soil can hold.

Most people would agree that tilling the soil before planting crops is beneficial. It aerates the soil and breaks up clods, creating a more favorable seedbed. While this may be true, there may also be some undesirable consequences of repeated tillage. Aerating soil increases the rate of organic matter decomposition which may negatively affect aggregation. Tillage can weaken existing structure and increase bulk density. The result is known as a plow pan, a layer of soil with higher bulk density than layers above and below it.

Techniques that may minimize compaction include conservation tillage, addition of organic matter to promote good structure, tilling when the soil is dry, and varying the depth of tillage.

Soil compaction is also an issue in commercial forest operations. Undisturbed surface horizons in forested soils are often high in organic matter and have relatively low bulk densities. The root system of many trees is relatively shallow, and nearly all of the finer roots (responsible for air and water uptake) are in the upper 12 inches of the soil. Any type of ground disturbance will impact these shallow roots, including soil compaction. During commercial timber harvest operations, one-fourth to one-half of the ground surface may be disturbed. With mechanical harvest techniques and dragging cut logs on skid trails, logging operations compact the soil and disturb the protective layer of organic matter and debris. One practice that can minimize total disturbance on a site is to reduce the number of skid trails and confine traffic to the same area. Seventy-five percent of total compaction on a soil occurs with the first pass; an additional 10% occurs with the second pass and only 5% more with the third pass.

Another activity in our forests is recreation. Hiking trails and campgrounds can have very compacted soils, resulting in little to no vegetation growth in some locales. Recreational vehicle use will compact the soil, but if areas are muddy the soil structure may be completely destroyed; these “puddled” soils become dense and massive with little pore space. Surface runoff and soil erosion increase because these soils have lost the ability to absorb rainfall. Some guidelines to minimize the damage caused by compaction include confining traffic to designated paths, as on hiking trails; water bars or grade dips can also help control erosion. Careful design of campgrounds should be considered to minimize erosion that occurs on compacted soils, for example keeping locations on flat ground.

Summary

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Bulk density is a measure of percent pore space and solids in a soil. It affects root penetration and the amount of air and water the soil can hold. Soil strength, an important engineering property, is closely connected to bulk density. Texture and structure are two inherent soil characteristics that govern bulk density. Mineralogy, soil chemistry, parent material and depth in the profile are also influential.

Bulk density is susceptible to change stemming from human activity. Many activities such as tillage, timber harvesting and recreation have the potential to compact the soil to such an extent that plants can not grow. Alternatively, adding organic residue to the soil can lower bulk density by improving soil structure and increasing pore space.

Activities

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  1. Try this simple experiment to determine the particle density of sand. You need to know the volume of a known weight (or mass) of sand, and we will measure this through the technique of “volume displacement” in water.

    • Obtain a pure sand sample, for example a bag of sand suitable for a backyard sand box. You will weigh a small quantity of sand, no more than a few tablespoons.

    • Add water to a container with volume markings—a graduated cylinder from chemistry lab would work. Or you can make your own graduated cylinder, see how.

    • With a known amount of water in your graduated container, add the known weight of sand. Stir it briefly to ensure that no air bubbles are trapped in the sand.

    • After a minute, read the new level of water in your container. The difference between your new level of water and the original level is the volume of sand.

    • If you divide the weight of sand by the volume, you will obtain particle density (the density of a mineral material without pore space).

    • The formula is: weight of sand (grams) ÷ volume of sand where volume is measured by taking the new level of water in your container and subtracting the original level, with 1 ml = 1 cm3.

    Another alternative for determining volume of the sand is to use a tin can, and use the formula π × r2 × h to calculate the volume of the tin can. This works quite well for samples such as sand, but would not be suitable for obtaining the density of a mineral specimen, below.

  2. Another experiment is to measure the particle density (or "specific gravity") of actual mineral samples. Try several different types of minerals such as quartz, obsidian, hematite or pyrite. Any mineral will be fine. Weigh your mineral. With a known volume of water, add the mineral specimen and read the new volume of water to obtain the volume of the mineral.

  3. Remember the activity in the Soil Structure section with a trail and structure changes? We mentioned measuring bulk density. Using the techniques described in this section for taking an undisturbed soil sample, take one sample from the middle of a trail (be sure to obtain appropriate permission if the land is not yours) and a second sample 5 or 10 feet off the trail. Determine the bulk density of each sample. Remember that "root limiting" bulk density is about 1.60 g/cm3.

  4. Archaeologists sometimes need to know about the organization of abandoned Indian villages like the Double Ditch State Historic Site in North Dakota. Features that are no longer visible on the surface can be "seen" with technology, such as Magnetic Gradiometry or Electrical Resistivity, and provide important information about the lives of former inhabitants. To learn about how archaeologists can use changes in soil bulk density to "map" an archaeological site with modern technology, visit the following sites:

  5. http://www.cast.uark.edu/~kkvamme/geop/double.htm

    http://www.cast.uark.edu/cast/serdp/proj_desc/AppendixB1_RemoteSensingInArch.pdf [PDF]