SOIL QUALITY – URBAN TECHNICAL NOTE                                     No. 2

                                        Urban Soil Compaction
                          Introduction                             ! Bulk density increases with
                                                                     compaction. Bulk density is the
                          Soil is a crucial component of rural
                                                                     weight of soil per volume. It is
                          and urban environments, and in both
                                                                     commonly reported as grams of
United States             places land management is the key to
                                                                     oven dry soil per cubic
Department of             soil quality. This series of technical
Agriculture                                                          centimeter.
                          notes examines the urban activities
Natural                   that cause soil degradation, and the     ! Porosity decreases with
Resources                 management practices that protect          compaction. Porosity is the ratio
Conservation              the functions urban societies demand       of the volume of pores to the bulk
                          from soil. This technical note will        volume of the soil.
                          focus on urban soil compaction.
Soil Quality Institute                                             ! With compaction, the distribution
411 S. Donahue Dr.        Healthy soil includes not only the         of pores shifts toward smaller
Auburn, AL 36832          physical particles making up the soil,     pore sizes. Pore size distribution
X-177                     but also adequate pore space between       is the array of pores, from very
Urban Technical           the particles for the movement and         small to large, making up the
Note No. 2
                          storage of air and water. This is          soil’s overall porosity.
March, 2000               necessary for plant growth, and for a
                                                                      These changes influence the
                          favorable environment for soil
                                                                      movement of air and water in the
                          organisms to live. Compaction
                                                                      soil, ease of root growth, and the
                          occurs when soil particles are
                                                                      biological diversity and activity in
                          pressed together, thereby reducing
                                                                      the soil. For proper plant growth,
                          the amount of pore space. Examples
                                                                      void space must be available for
                          of compaction in urban settings are
                                                                      air and water movement.
                          traffic pans resulting from repeated
This is the second note                                               Typically a medium textured soil
in a series of Soil       trips across lots with trucks and
                                                                      has about 50 % solids and 50 %
Quality-Urban technical   machinery and excessive trampling
notes on the effects of                                               pore or void space. Compaction
                          by people, bicycles, etc. Soils are
land management on                                                    increases bulk density and reduces
soil quality.             particularly susceptible to
                                                                      the number of large pores in the
                          compaction if these activities occur
                                                                      soil. (Schuler et al., 2000).
                          when the soil is wet. The primary
                          impacts of soil compaction are           Compared to agricultural land,
                          changes in the soil’s physical           compaction in urban areas can be
                          properties (Schuler et al., 2000):       more permanent because of the
                                                                   difficulty in bringing in equipment to
                          ! Strength increases with
                                                                   loosen the soil, due to the presence
                            compaction. Soil strength is the
                                                                   of utilities and the prevalence of
                            ability to resist penetration by an
                                                                   perennial vegetation.
                            applied force and is desirable
                            under roads and buildings.
Causes of Soil Compaction in Urban            ! Reduced soil biological activity.
Areas                                         For communities, excessive levels of soil
Causes of compaction in urban areas are       compaction lead to environmental problems
generally of two types:                       due to:
1. Deliberate compaction during               ! Increased storm water runoff as a result of
   construction activities.                     low infiltration rates of compacted soils.
   !   Compacting of entire areas in order    ! Increased flooding due to runoff.
       to increase strength for paving and
                                              ! Increased erosion from construction sites.
       housing foundations without
       consideration for leaving non-         ! Increased water pollution potential,
       constructed areas (landscaping areas     especially nitrates and phosphorus, in
       and lawns) in a more natural state.      local rivers, streams, lakes, and ponds.
   !   Use of heavy equipment for
       reshaping and sloping banks along      Detection of Soil Compaction
       roads and hillsides.
                                              Generally compaction is a problem within
   !   Grading lots and placing sod on hard   the top 12 inches of the soil surface.
       soil or soil denuded of topsoil.       Detection of compaction can be by:
2. Unintentional compaction of the soil       ! Observing discolored or poor plant
   after construction is completed.             growth.
   !   Allowing uncontrolled traffic (both    ! Probing with a firm wire (survey flag) or
       vehicles and foot traffic)               welding rod (18” in length) into the
   !   Allowing vehicles on lawn areas          compacted area.
       around homes or businesses,            ! Digging down to plant roots and finding
       especially when the soil is wet.         lateral root growth with little if any
                                                penetration of compacted layers.
Impacts of Soil Compaction                    ! Taking bulk density samples (Table 2).
For individual homeowners and businesses,     ! Using commercially available cone
soil compaction makes it difficult to           penetrometers that indicate force required
establish and maintain lawns and                to penetrate the soil in terms of pressure
landscaping due to:                             (pounds per square inch). Roots are
                                                unable to penetrate soil compacted to 300
! Restricted root growth.
                                                psi or more. This varies with soil type and
! Reduced plant uptake of water and             moisture content of the soil when tested
  nutrients.                                    (Schuler et al., 2000).
! Reduced available water capacity.

    Table 2. General relationship of soil bulk density to root growth based on soil
    texture (NRCS Soil Quality Institute, 1999).
      Soil texture            Ideal bulk         Bulk densities that Bulk densities that
                              densities          may affect root      restrict root growth
                              (g/cm )            growth               (g/ cm3)
      Sands, loamy sands      <1.60              1.69                 >1.80
      Sandy loams, loams      <1.40              1.63                 >1.80
      Sandy clay loams,                          1.60
      loams, clay loams       <1.40                                   >1.75
      Silts, silt loams       <1.30              1.60                 >1.75
      Silt loams, silty clay
      loams                   <1.10              1.55                 >1.65
      Sandy clays, silty
      clays, some clay loams     <1.10             1.49                  >1.58
      (35-45% clay)
      Clays (>45% clay)          <1.10             1.39                  >1.47

Prevention of Urban Soil Compaction                  traffic or involving high volume of people in
                                                     concentrated areas. Mesh elements have
Compaction problems during urban
                                                     been used for sporting fields (Beard and
development can be avoided by proper
                                                     Sifers, 1990).
planning. Working with local governments
may help prevent total compaction in                 These measures may take a little more time
development areas. Divide large areas into           initially, but will pay dividends in the long
sections to be consciously compacted for             run. The benefits of planning and wise urban
roads and foundations, and sections for              development are:
lawns and landscaping. Disturb only areas
                                                     ! Satisfied buyers of homes with soils that
needed for construction. Also, only
                                                       function well
manipulate soil when dry (less than field
capacity).                                           ! Soils that have good infiltration rates (less
                                                       frequent irrigation)
Soil that will support lawns can be protected
by subsoiling, and by stockpiling topsoil that       ! Reduced run-off (less chemical and
will be returned to the site after construction.       fertilizer loss to water bodies)
These two measures can restore water flow            ! Lower mortality rates of perennial
functions to near natural conditions.                  vegetation (lawns and trees)
Establishing sod or seeding a lawn is much
more successful on a loose soil with topsoil         ! Better plant growth and quality for
than on a compacted soil without adequate              shrubs, flowers, trees, gardens, and lawns.
In parks and recreation areas, specific areas        Management Practices for Compacted
can be designated for heavy traffic (paved           Urban Soil
areas or trails). The remaining vegetated
                                                     Although prevention is more effective, the
areas will benefit from less compaction
                                                     detrimental effects of compaction can be
because of controlled traffic. During special
                                                     lessened after soils are compacted.
events, lay down metal or wood mats for
                                                     Management practices to reduce the effects
better distribution of weight for vehicular
                                                     of urban compaction are:
! Subsoiling to alleviate compacted soils.                              healthy plants, higher plant mortality rates,
  Always have underground utilities and                                 and higher pollution potential from storm
  other underground plumbing or wires                                   water runoff. Urban soil compaction is more
  located and marked.                                                   complicated than in an agricultural setting. It
                                                                        is less convenient to alleviate urban
! Partial or total soil replacement. Replace
                                                                        compaction because soil cannot be disturbed
  dense soil with loose soil or haul in
                                                                        easily around perennial vegetation,
                                                                        underground utilities, buildings, drive ways,
! Increasing organic matter. In gardens, go                             etc. Planning will prevent many problems
  to residue management/no-till systems                                 with compaction in developments and
  and/or cover crops.                                                   subdivisions. Preventive practices, including
! Use of mulch, compost, manures, and                                   limiting the extent of disturbed areas,
  amendments.                                                           manipulating soil only when dry and
                                                                        restricting traffic, are more effective and less
! Annual aeration of turf grasses to improve                            expensive than practices to alleviate
  infiltration.                                                         compaction after it occurs. Preventing and
! Aeration of soil using a metal tube and air                           managing compaction results in soils that
  compressor. This is usually used around                               function well and that benefit all of society.
  tree roots. (Personal communication with
  John Lesenger. Used at the Alabama
  Shakespeare Festival.)                                                References
! Irrigation management. Frequent, low                                  Beard, J.B., and S.I. Sifers. 1990. Feasibility
  rates of water are necessary because                                     assessment of randomly oriented
  compacted soil holds little water. Over-                                 interlocking mesh element matrices for
  irrigation wastes water and may lead to                                  turfed root zones. p. 154-165. In R.C.
  environmental pollution from lawn                                        Schmidt et al. (ed.) Natural and artificial
  chemicals, nutrients, and sediment.                                      playing fields: Characteristics and safety
                                                                           features. Amer. Soc. Testing & Materials,
! Cutting grass at higher heights, which                                   Standard Tech. Pub. 1073.
  reduces evapotranspiration losses (see
  local turf grass recommendations–                                     NRCS Soil Quality Institute. 1999. Soil quality
  Extension Service).                                                     test kit guide. United States Department of
                                                                          Agriculture, Agricultural Research Service
Summary                                                                   and Natural Resource Conservation
Compaction changes important physical                                     Service. Kit guide can be obtained at
properties of the soil. Soils with higher                       
strength, higher bulk densities, and                                    Schuler, R.T., W.W. Casady, and R.L. Raper.
decreased pore space have lower infiltration                               2000. Soil compaction. Chapter 9. In R.C.
rates, reduced water holding capacity, and                                 Reeder (ed.) MWPS special publication:
more runoff. This degradation of soil quality                              Conservation tillage systems and
results in the need for more irrigation, less                              management. (In Press).

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