Editor’s Note: You may have noticed bands in soil along roadsides, or on hikes. Soil scientists refer to these bands as lamellae [luh–mel-ee]. They are interesting areas of soils, formed by various processes, and are quite beautiful. Knowing how they form and how that might impact the area is important to growers, homeowners, road builders, and more.
If you’re a regular reader of Soils Matter, you know that soil is a complex mixture of soil particles of varying sizes. Most soils are a mixture of sand, silt and clay. The lamellae we see in soils occur when clay particles bond to the top layer – making a temporary or permanent “sculpture” in the landscape. Read on to learn more about lamellae!
Just like a band, lamellae are defined by horizontal-to-wavy stripes or elongated areas of a different texture or composition compared to its surroundings. The formation of these thin (< 7.5cm), relatively clay-rich zones within a sandy soil are not fully understood. This is because they may have developed in different ways but have the same final form.
How do lamellae form?
The most common explanation of their occurrence is the movement of clay and iron oxides. This soil-forming process forms lamellae as percolating soil-water carries small amounts of clay down a sandy soil profile (to review soil-forming processes, visit here). Water infiltrating the soil will work its way down following the least resistive pathways. The boundary between soil that has been wetted and the unchanged soil is known as the wetting front. If the clay in suspension is near the edge of the wetting front, the clay will be deposited in a contorted initial lamella. This deposited lamella may act as a filter for future wetting fronts, stripping away clay particles and thickening as it does so.
Where the water front stops moving down and forms the initial band in the profile can be attributed to different reasons. It may also perhaps be random. A texture change or different parent material can cause a wetting front to stop and form lamellae due to the subtle differences in pore size discontinuities.
Clay also may be deposited as the wetting front is still moving. As more soil particles accumulate in the water front, it may reach its maximum carrying capacity and force the clay out of the solution and be deposited. Clay particles may also form a clay band due to the presence of free iron oxides or carbonates which will promote the clay to clump.
Other hypotheses for the formation of lamellae include features originating during the deposition of the parent material. Or, they could form from a mix between processes controlled by the properties of the inherent parent materials.
Where can you find lamellae?
Parent material refers to the general physical, chemical, and mineralogical composition of the materials in which soil forms. The parent material largely determines the presence or absence of lamellae. Stratified sandy materials and soils with textural discontinuities (e.g. finer, clayey textured soil directly above coarser, sandy material) are where you will most likely find lamellae. Nearly two-thirds of the lamellae described in soils in the U.S. have sandy or sandy-skeletal soil textural classes with excessively or somewhat excessively drained soils. None have been described in the U.S. with an aquic (wet) soil moisture regime. This suggests that downward movement of water is essential for its formation. In addition, poor drainage conditions do not favor lamellae formation.
The soil on the right formed in the Mississippi Embayment along the ancient shorelines and river deltas of the Gulf of Mexico. As the oceans lowered, the mixed marine/river coastal plain sediments were exposed and weathered. Following the retreat of the ocean, finer soil particles (i.e. clay and silt) blew in, forming the thick, brown layer of soil. Over time, the clay particles moved downward via wetting fronts into the white sand material, forming the lamellae bands. The oldest lamella is the band closest to the soil surface while the lowest lamella is the youngest. Lamellae in the middle tend to have the highest clay content in the top of the band. This is because clay is periodically moved from an upper lamella band to a lower lamella band, thickening as clay is added to their top of the band. Since this process is not uniform, lamellae are wavy and can develop branches (seen in the red box above).
How long does it take for lamellae to form?
Soils form and change (generally) on geologic time scales. Lamellae have been described in soils dating less than one thousand years old, suggesting that they can form in hundreds of years.
Why should you care?
Lamellae dramatically increase the water-holding capacity of sandy soils. The clay particles in the lamellae also increase the cation-exchange capacity of sandy soils (read more about that here). This has been shown to enhance site productivity – like increasing the biomass in forests. These soils can be poorly suited for septic tank absorption fields and sewage lagoons. That’s because although the sandy interlamellae readily absorbs water, it doesn’t adequately filter the effluent. The poor filtering can result in the pollution of ground water supplies.
No matter how lamellae form or their impact on land management, soils containing lamellae can be some of the most beautiful soil you will ever see!
By Mike Badzmierowski, Virginia Tech