A previous blog entry, Healthy Soils for Healthy
Trees, discussed the importance of preserving soil structure
from being destroyed by compaction. Together, soil texture and
soil structure have the greatest influence on pore space in soil,
and how easily air, water, and roots can move through a soil.
Many people are aware of what soil texture – proportions of
sand, silt and clay – they are dealing with on a site. Few people
consider a soil’s structure, though, even though in most soils,
the structure is just as important as the texture. Two soils with
the same texture can behave very differently depending on their
structure. A clay soil, for example, can be easy for air, water,
and roots to move through with good structure, or be almost
impenetrable by roots, air, and water when its structure has
been destroyed by compaction.
How soil structure develops
Soil structure refers to how particles of soil are grouped together
into aggregates (also called peds). They are cemented or bound
together by physical, chemical, and biological processes.
Physical-chemical processes that build soil structure include:
• Polyvalent cations like Ca2+, magnesium Mg2+, and
aluminum Al3+ bind together clay particles
• Soil particles are pushed closer together by freezing and
thawing, wetting and drying, and by roots pushing through
the soil as they grow in length and width.
Biological processes that build soil structure include:
• Soil particles are cemented together by humus, by organic
glues created by fungi and bacteria decomposing organic
matter, and by polymers and sugars excreted from roots.
• Fungal hyphae and fine roots stabilize aggregates
(University of Minnesota Extension 2002.)
Organic matter and plant roots are therefore key to soil
structure.
How soil structure deteriorates
Factors that can deteriorate or destroy soil structure include, for
example:
• Compaction
• Cultivation
• Removal of vegetation
• Excessive moving and handling of soil
• Screening
• Excessive sodium
A high proportion of sodium to calcium and magnesium causes
clay particles to repel each other when wet, so aggregates are
dispersed and the process of soil structure formation is
reversed. Soils with too much sodium become almost
impermeable to water because the dispersed clay and small
organic particles clog up remaining soil pores (Donahue et al
1983). Excessively high sodium levels can result from irrigation
and salting roads.
Different types of soil structure
Soil structure is classified by type (shape), class (size) of peds,
and grade (strength of cohesion) of aggregates. The shape,
size, and strength of aggregates determine pore structure, and
how easily air, water, and roots move through soil (Donahue et
al 1983).
Figure 1 shows the different types of soil aggregates, and how
easily water typically moves through each of these types.

Granular structure is the most common in surface soil layers,
especially those with adequate organic matter. Granular
structures offer the most pore space of any structure
(Cooperative Soil Survey, no publication date given)

Columnar structure is often found in soils with excessive
sodium, due to the dispersing effects of sodium, which destroys
soil structure, rendering the soil effectively sealed to air and
water movement (Cooperative Soil Survey, no publication date
given).

Platy structure has the least amount of pore space and is
common in compacted soils (Cooperative Soil Survey, no
publication date given).

Some soils have no true structure, like single grain soils (like a
loose sand with little to no attraction between the grains of
sand), and massive soils (large cohesive masses of clay).

For more information on soil structural classification, see the
resources listed in the references section below.
Ways to preserve desirable soil structure
As the USDA Natural Resources Conservation Service (2008)
explains: “practices that provide soil cover, protect or result in
the accumulation of organic matter, maintain healthy plants, and
avoid compaction improve soil structure and increase
macropores.”
Other key practices to preserve soil structure include eliminating
soil screening and minimizing handling, and avoiding the use of
sodium salts.
Implications for bioretention
Preserving soil structure may increase the range of soil textures
acceptable for bioretention. Bioretention soils are often sandbased, primarily to ensure adequate infiltration rates. Clay and
silt content is often limited to a maximum of only 3 to 5 percent,
which is very, very low, limiting soils to sands according to the
soil textural triangle. While a clay soil that has been screened
and has no structure will have a very low infiltration rate, with
proper structure, many soils with more clay can also have
adequate infiltration rates. Increasing clay content above the
very low maximum of 3 to 5 percent could provide important
benefits, including increased soil water holding capacity and increased cation exchange capacity, which increase potential
pollutant removal. When increasing clay content, however, keep
in mind that the higher the clay content, the more crucial it
becomes to protect soil from compaction and from excess salt,
as clay soils are more prone to compaction and loss of structure,
and unacceptable decrease of infiltration rates due to dispersion
from sodium ions.
References
Cooperative Soil Survey. No publication date given. Soil
Structure – Physical Properties.
Donahue, Roy L., Raymond W. Miller, and John C. Shikluna.
1983. Soils: and introduction to soils and plant growth. Fifth
Edition. Prentice-Hall, Inc.: Englewood Cliffs, N.J.
Plaster, Edward J. 1992. Soil Science and Management.
Second Edition. Delmar Publishers, Inc.: Albany NY.
University of Minnesota Extension. 2002. The Soil Scientist.
USDA Natural Resources Conservation Service. 2008. Soil
Quality Indicators.
Nathalie Shanstrom is a landscape architect with The Kestrel
Design Group.

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