Substrates …

What is the track?

The sand it’s made in? The shapes held by the sand? The record of foot forces applied? All of the above? Seem like too simple of a question?

To avoid missing critical information: a tracker needs to know something about substrates, where every track is born, lives, and weathers. While not necessary, a little knowledge of physics and chemistry may give deeper insight into what’s really going on in a substrate, and why the track takes the form it does.

There are many substrate types: each with their own physical characteristics. Some substrates are a complex physical system recording fine details of the history of forces employed as a track is made, as well as the effects of time and weather. The physical and chemical properties of a substrate have much to do with which information is recorded, the level of detail, how the recorded info alters with time, and how easily it can be read.

A brief example of what happens as a track is made: the foot contacts the substrate, lands with some force, exerting pressure, and employing friction as it rolls, sinking to some degree (in particulate substrates), then applies yet more force and employs friction again to push off. During the entire process, the internal forces within the substrate are continually re-balancing themselves as they respond to the external forces. After the foot has left, the final re-balancing of the substrate may last until well after the foot has moved on.

Complexity? Do the math for yourself … multiple forces with varying levels, internal temperature and moisture levels constantly changing, organisms living between and even within the particles, external temperature varying in daily cycles, sun and shadow, all influencing the USDA’s 12 soil orders, and 64 suborders. (Source: nrcs.usda.gov/wps/portal/nrcs/detail/soils/edu/?cid=nrcs142p2_054278 )

Composition of a common substrate:

“Soil texture refers to the proportion of the soil “separates” that make up the mineral component of soil. These separates are called sand, silt, and clay [and have] … the following size ranges:

  • Sand = <2 to 0.05
  • Silt    = <0.05 to 0.002 mm
  • Clay  = <0.002mm”

(Source:  nrcs.usda.gov/wps/portal/nrcs/detail/nj/home/?cid=nrcs141p2_018993)

“Soil water is the term for water found in naturally occurring soil. Soil water is also called rhizic water. There are three main types of soil water … defined based on the function of the water in the soil …”.  (Source: hunker.com/13427998/different-types-of-soil-water)

“Gravitational water is free water moving through soil by the force of gravity. It is largely found in the macropores of soil and very little gravitational water is available to plants as it drains rapidly down the water table in all except the most compact of soils.”   (Source: hunker.com/13427998/different-types-of-soil-water)

“Capillary water is water held in the micropores of the soil, and is the water that composes the soil solution. Capillary water is held in the soil because the surface tension properties (cohesion and adhesion) of the soil micropores are stronger than the force of gravity. However, as the soil dries out, the pore size increases and gravity starts to turn capillary water into gravitational water and it moves down. … Capillary water is the main water that is available to plants as it is trapped in the soil solution right next to the roots if the plant.”   (Source: hunker.com/13427998/different-types-of-soil-water)

“Hygroscopic water forms as a very thin film surrounding soil particles and is generally not available to the plant. This type of soil water is bound so tightly to the soil by adhesion properties that very little of it can be taken up by plant roots … hygroscopic water is found on the soil particles and not in the pores”   (Source: hunker.com/13427998/different-types-of-soil-water)

“… the physical nature of a typical mineral soil … usually contains about 50% solid particles and 50% pores on a volume basis … Soil particles are the building blocks of the soil skeleton. But the spaces (pores) between the particles and between aggregates are just as important as the sizes of the particles themselves. The total amount of pore space and the relative quantity of variously sized pores—large, medium, small, and very small—govern the important processes of water and air movement.”  (Source: sare.org/Learning-Center/Books/Building-Soils-for-Better-Crops-3rd-Edition/Text-Version/Soil-Particles-Water-and-Air )

Soil pore space can be filled with either water or air, and their relative amounts change as the soil wets and dries..”  (Source: sare.org/Learning-Center/Books/Building-Soils-for-Better-Crops-3rd-Edition/Text-Version/Soil-Particles-Water-and-Air/Water-and-Aeration )

Behavior of a common substrate:

Internal forces … “Soils are particulate materials. Therefore, the behavior of soils is determined by the forces particles experience. These include forces due to boundary loads (transmitted through the skeleton [of the soil]), particle-level forces (gravitational, buoyant, and hydrodynamic), and contact level forces (capillary, electrical and cementation- reactive).    Also including external pressures … (Site author). “… The relative balance between these forces permits identifying various domains of soil behavior … Generally accepted concepts gain new clarity when re-interpreted at the level of particle forces.”.    (Source: Santamarina, J. Carlos. “Soil Behavior at the Microscale: Particle Forces.” Soil Behavior and Soft Ground Construction (2003). Print.)

Soil Breathes … “Soil pore space can be filled with either water or air, and their relative amounts change as the soil wets and dries…”.  (Source: sare.org/Learning-Center/Books/Building-Soils-for-Better-Crops-3rd-Edition/Text-Version/Soil-Particles-Water-and-Air )

Soil Compaction … “In geotechnical engineering, soil compaction is the process in which a stress applied to a soil causes densification as air is displaced from the pores between the soil grains.”   (Source: en.m.wikipedia.org/wiki/Soil_compaction)

“Normally, compaction is the result of heavy machinery compressing the soil, but it can also occur due to the passage of (e.g.) animal feet.”   (Source: en.m.wikipedia.org/wiki/Soil_compaction )

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Concept Illustrated: Re-expansion of dry-ish fine-grain sandy soil after a track has been created. In dry conditions a substrate composed of silt-sized sand particles generated from this soil type (Breece) may compress under the foot and later re-expand enough to make the change noticeable. Why does that re-expansion happen, how long does it take, and how is it useful to a tracker?

The answers to those questions are complex and worth the effort to understand because they yield a clearer conception of how both soil and tracks behave. The physics forces involved in the process are listed above under, “Behavior of a common substrate … Internal forces

Why does a soil sometimes re-expand after a foot has compressed it?

Compression – When that soil is stepped on, it compresses to some degree, air in the soil near and under the foot also compresses or moves, and the capillary water between the grains can be forced to flow to a small degree. A drastically overstated example of this would be what happens when a wet sponge that has absorbed a little soapy water is stepped on.

Re-expansion – As the foot leaves contact with the ground, compression ends and the internal soil forces mentioned above under “Behavior of a common substrate … Internal Forces“, begin acting, with one result being a slight expansion of the compressed soil.  

It must be understood that this expansion is slight, slow, and usually visually apparent only an hour or more after the track was created.  Author hasn’t attempted to duplicate this in a track box.

The Idea: Illustrate how a soil may compact beneath a foot as weight and pressure are added, and how this soil may gradually re-expand after the foot has pushed away..

The Take Away: Soil is particulate: it holds air and water, and living organisms, allowing it to breathe, contract, compress, and expand. The re-expansion of soil in a track, after the foot has left, is one contributor to track weathering and can also be a useful aging tool. A problem is that the process is so gradual the mind can’t easily notice it. To notice it one must observe a fresh track, then return to re-observe some hour/s later. This re-expansion is slight, slow, and likely to happen at a variable rate depending on soil type and weather conditions.

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