Today’s post will focus on soil structure. Previously, soil texture was defined as the relative proportion of soil particles (sand, silt, and clay). Soil structure is the arrangement of those soil particles into aggregates, or peds. Soil structure is important because it has a direct impact on many soil physical properties, but especially on the rate at which water flows through the soil. This impacts water availability for crops, site suitability for various buildings and septic systems, and more.
Soil scientists describe soil structure based on three things – type, size, and grade.
The structure type is simply the shape of the chunks of soil, referred to as “peds”. The different types are shown here:
Granular soil structure is roughly spherical in shape. It is similar to the shape of granola and/or Grape Nuts (the cereal). Both start with “g-r-a-n”, so think of those two foods when you think of granular. This type of structure is common where there is a high amount of organic matter present – often in the A horizon, or surface horizon of the soil. It allows for a fast infiltration rate (the rate at which rain water can enter the soil profile), thus maintaining organic matter in your soil profile can benefit your garden/crop by allowing more rainwater to enter the soil profile.
Blocky soil structure is shaped just as it sounds – like little blocks. The blocks are nearly the same size in all dimensions. Blocky structure is most common in the B horizon where clay content is higher. It can also occur in Histosol soils that have been drained. There are two types of block structure – angular, and subangular. Angular has sharp edges, while subangular has more dull edges. The type of block structure depends on the clay minerology, and the amount of shrink-swell clay (more on that in a future blog post). Shrink swell clay can expand upon wetting, thus causing slow movement of water. Good subangular blocky structure can have very rapid movement of water.
Prismatic soil structure is similar to blocky, except that the blocks are more tall than they are wide. Prismatic structure usually occurs in the B horizon. It is common to have prismatic structure that breaks into smaller, blocky structure when the prismatic ped is handled. This is called “compound structure”, meaning there is one structure type within another.
Columnar soil structure looks very similar to prismatic structure, except columnar has a rounded top. This structure occurs in arid climates where there is a higher concentration of salts in the soil.
Platy soil structure consists of soil peds that are wider than they are tall. It often occurs in compacted soil. Platy structure causes slower movement of water down through the profile. If it formed due to compaction, root growth through that soil layer will also be hindered.
Wedge soil structure is a shape similar to that of a football (North American). It occurs in soils that have a high amount of shrink-swell clays like in the Vertisol soil order (Soil Taxonomy). As the peds wet and dry, the peds slide over each other elongating the peds into a wedge-like shape. Where ped faces slide over each other, slickensides (smooth films of clay) line the outside of the peds.
Structureless – single grained soil structure means that there is no structure, only individual soil particles (sand) that are not aggregated into any other soil structure. Beach sand is a good example.
Structureless – massive soil structure means, again, that there is no structure. The soil does not easily break along plains of weakness because there are none. This is common in C horizons where it is only parent material and no (or little) soil development has occurred.
Structureless – massive, rock controlled fabric (a term used by the NCSU soil science folks, but not commonly used by others) occurs in saprolite (rotten, weathered rock) where the soil does have structure. However, that structure is not due to soil forming processes, but is due to the inherited structure of the parent material (what was once a rock).
In addition to structure type, structure is also described by soil scientists based on ped size, and structure strength. Ped size is simply the average size of peds within different size categories. The picture below shows the use of the USDA’s Field Book for Describing and Sampling Soils to determine the ped size for angular and subangular blocky structure. The sizes are grouped into different ranges of diameter, such as fine, medium, coarse, etc. The range “descriptor” may change as well, such as thick, medium, and thin for platy structure.
The final characteristic used to describe soil structure is Grade. Numbers and words (associated with strength) are used to describe grade. The numbers 0 through 3 are used for grade. A grade of 0 indicates that no soil structure is present (ie, the soil is massive). A grade of 1 indicates weak soil structure, 2 indicates medium structure. Lastly, 3 indicates strong structure. Strong structure can easily be seen in a soil pit, as the soil peds will have large gaps in-between the peds. Weak soil structure will not be easily seen in a soil pit, but peds may break into a discernible shape when they are manipulated or squeezed. Medium is somewhere in-between strong and weak.
Implications and Summary
As I stated in the beginning, soil structure is very important for a variety of reasons. Most of those reasons boil down to how fast water can flow through the soil, and how easily plant roots can penetrate the soil. The stronger the structure, the faster water and roots can penetrate the soil. A diagram from Colorado State University (below) does a great job of sumarizing how structure can affect water flow:
Soil structure is created through pedogenic (soil forming) processes over long periods of time, but can be destroyed quickly through compaction. In your field, garden, or yard, you can encourage good soil structure by limiting traffic on the soil, avoiding tillage when the soil is wet (when it has the least strength), and adding organic matter either manually, or through agricultural practices.
Soil structure is a topic that usually takes several lectures to cover in college-level soils courses, so we won’t be able to cover all of it in one blog post here. However this post should give you a thorough understanding about how structure is described, and some potential impacts of soil structure.