Substrates

Every track is born, lives, and fades in or on some kind of substrate. The substrate records the history of forces employed by the animal, and the effects of time and weather.

An example of what happens in a soil as a track is made: the foot contacts the substrate, sinks (in some substrates), rolls, then pushes off and leaves. The substrate is active during the whole process. Each component of a substrate pushes back with its share of the force the foot is employing (Newton’s third law: for every action there is an equal and opposite reaction).

According to the USDA there are 12 orders in their soil taxonomy, and a total of 64 suborders. That’s quite a few different kinds of soil … then there are all the variations that come with temperature and moisture content.

(Source: nrcs.usda.gov/wps/portal/nrcs/detail/soils/edu/?cid=nrcs142p2_054278 )

A tracker needs to know something about soil types to avoid missing significant information. If someone made the identical track in damp sand, freshly tilled garden soil, and wet clay; what differences might be observed?

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What is one common substrate composed of?

“… the physical nature of a typical mineral soil. It 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 )

How does that substrate behave?

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 )

Soil Consolidation … “When stress is applied that causes densification due to water (or other liquid) being displaced from between the soil grains … consolidation, not compaction, has occurred.”

(Source: en.m.wikipedia.org/wiki/Soil_compaction )

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Concept Illustrated: Because only some soils will compress and re-expand enough to notice, this example simulates a one that will; a sandy substrate, with a small amount of finer mineral material and just a bit of fine organic material mixed in. There is air between the grains, and a film of water coating them.

When that soil is stepped on, it is compressed to some degree, because air compresses or moves, and the water coating the grains can flow to a small degree. An overstated example of this would be what happens if a wet sponge that has absorbed a little soapy water is stepped on.

The subsequent re-expansion of a substrate after a foot has compacted it is part of the weathering process, and another way to age tracks; how and why does it happen?  The answer includes air, water, and pressure.

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

The Take Away: This expansion is one contributor to track weathering and so can be a useful aging tool. A problem is that the process is so gradual the mind can’t easily notice it. If one observes a fresh track, then returns some time later, it is sometimes possible to notice the re-expansion. How useful would it be to have some idea of how fast this happens, how great the re-expansion is, and what substrate types will demonstrate this behavior?

Illustration in production