Soil Structure revisited

First, flow in a composite column:

2 equations: overall flux and pressure drop, AND

conservation of mass (flux 1 = flux 2)

A consequence of this, is that the larger pressure gradient is put onto the lower conductivity portion so as to have conservation of mass.

In other words, the region of lowest conductivity limits the flux through the whole.

What we’ve seen of soil structure so far has involved structure of the solid phase. Now I want to look at structure of the voids, the pore space.

Highway (Transport) map of Iowa as a thin section:

Pore size

Pore orientation

Pore continuity

Pore spacing

Pore connectivity

(Almost) everything conducts, but the actual flux through a given pore has to do with context as well as properties of the pore itself: orientation, connectivity, etc.

In particular, the largest pores tend to connect with other largest pores, like an interstate being continuous with itself as well as with county roads. This optimizes transport: you can cross the whole sample on one big pore. And while the transport that takes place through smaller pores is important, the fastest takes place through the big, continuous routes.

This preferential interconnection of the largest pores is what I would call soil pore structure (to distinguish it from soil solid structure – cementation, aggregation, and the like).

In the field, excavations have shown that earthworm holes and desiccation cracks interconnect. This interconnectivity is not “accidental” or random. When dealing with flow, connectivity and continuity are often the keys.

However, most of what we measure is geometry – pore sizes, for example – rather than connectivity (mathematically speaking, the topology).

So we return to the question, how can you figure out the K_sat of a soil when you only know what’s in the soil survey?

Look it up.

Look it up for a similar soil

Similar must mean in texture, bulk density, and organic matter

Note that texture is about the smaller pores, but density and organic matter tell you something about structure and structural pores – connectivity. Unfortunately, the structure of the largest pores (the so-called “structural pores”) is the single greatest determinant of the saturated hydraulic conductivity. This information is not contained in the soil survey. So for now, the answer is that you can’t reliably predict K_sat from the information we find in the survey.