While soil nutrients are important, understanding soil structure is also a critical factor to knowing and improving soil. Soil structure determines permeability, aeration, and root growth. Although soil structure takes millennia to develop, it can be destroyed in moments yet also improved relatively quickly with proper management techniques. Read all about soil structure from Delaware Valley University Soil Science Adjunct Professor and BCFA Director Steve Dadio.
Soil Texture
In thinking about the physical properties of soil, the first property to be evaluated is texture, how does the soil feel? This property describes the physical composition of the mineral (not organic) soil materials: rocks, sand, silt, and clay. The soil textural composition is related to the environment of deposition of the various parent materials present. If the native bedrock is sandstone, the soils that formed from those rocks are sandy. If the native bedrock is siltstone, the soils are silty. If the native bedrock is mudstone, the soils are clayey. In addition to the soil that formed from weathered bedrock, there are layers of silt that cover much of Bucks County, resulting from wind-blown loess along the banks of the Delaware River that was deposited over most of the County during the Pleistocene Epoch (appx. 20,000 years ago). There are no easy ways to modify soil texture as that was largely determined long ago and the addition of soil mineral materials is cost-prohibitive.
Soil texture sizes from largest to smallest: rock, sand, silt, clay
Soil Structure
The second physical property to be evaluated is the soil structure. When you are at the beach, you can see individual sand grains, but most of the time (especially in Bucks County), the individual soil particles are clumped into defined larger units called aggregates. Soil structure describes the shape and grade (strength) of the soil aggregates. While soil structure is a physical property, it is largely influenced by soil chemical and biological properties to develop these aggregates. The interaction of soil macrofauna (e.g. earthworms) and microfauna (e.g. actinomycetes) with the organic material in the soil (e.g. roots) help develop these aggregate shapes and strength.
While it can take hundreds to thousands of years for soil structure to develop, it takes far less time to destroy soil structure from poor management practices.
Structural Shapes and Sizes
In terms of structural shape, we will focus on three shapes that are most common in this area: granular, subangular blocky, and platy. These different structural shapes are described in greater detail below:
Grade refers to the strength of the soil structural aggregate (aggregate stability) and is closely tied to the amount of organic matter in the soil. The ability for a soil aggregate to maintain its shape when wetted is dependent upon the amount of organic matter in the soil, the greater amount of organic matter, the stronger the soil structure.
How can I improve soil structure?
Soil structure can be improved through several management practices.
Utilization of permanent vegetation (permaculture) – perennial plants generally have deeper and more extensive root systems than do annual vegetation. These root systems facilitate greater populations of soil fauna (microorganisms) that will help to create and strengthen soil structure.
- Addition of organic materials – the addition of organic matter (e.g. composts and manures) is essentially feeding the soil fauna (microorganisms) that will help to create and strengthen soil structure.
- The use of intercrops or cover crops – any practice that expands the living materials being grown in the soil will increase the organic matter content of that soil.
- Compaction awareness – efforts to reduce compaction include reduced tillage (conservation tillage or no-till), refraining from walking/driving on wet soils (wet soils are structurally weaker than drier soils).
As previously stated, soil structure takes a long time to form but can be damaged in a short time period. Soil structure is the one physical property that can be improved through management practices, primarily through the incorporation of organic matter content in the soils.
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