Why do scientists care how water moves through soil?

In the May 15^th Soil Matter blog post, I covered the basics of soil hydrology – how water moves through the soil system (https://soilsmatter.wordpress.com/2016/05/15/how-does-water-move-through-soil/).

Some of the pores in soil create “preferential flow channels” that direct water movement in various ways. Knowing about preferential flow is important for soil scientists. It helps them predict and solve problems seen by growers, researchers, developers, and conservationists.

Roots make channels through soils that help water flow through and be absorbed. Credit: Daniel Giménez

Preferential flow channels form by many natural processes.

• Cracks and fissures can develop in soil profiles based on the soil texture and when the soil experiences wetting and drying cycles.
• Burrows created by earthworms, ants, and other organisms create flow channels that can extend from the soil surface down to lower soil layers.
• Roots growing down into soil profile –in search of nutrients and moisture – create flow channels.
• As the parent plants die, the roots also die, leaving open channels that allow for further preferential flow in that soil system.

These preferential flow channels can have a large impact on water flow. For a little math, the rate at which water flows through a pore is proportional to the 4^th power of the radius of the pore. As an example, consider a 0.1 mm radius crack made when the soil was very dry. The flow rate through this channel is 1. Then, consider a channel created by a dead plant root that has a 1 mm radius. The amount of water that can flow through that channel is 1 x 10⁴…that’s 10,000 times!

Often water movement through these channels is rapid enough to bypass wetting the bulk of the soil matrix. You might notice that after a rainfall, the soil doesn’t seem so wet…this could be one reason.

Water can erode soil and cause loss of valuable topsoil and nutrients. Credit: J. Sebastián Silva O.

Preferential flow has its advantages and disadvantages. On one hand, these channels can quickly drain the soil surface after a heavy rain event. This reduces erosion potential and the loss of valuable topsoil. On the other hand, chemicals can rapidly move through the soil profile via these channels. Preferential flow is often blamed for polluting groundwater for this reason.

How might a soil scientist apply this concept in the field? Think of a plot of agricultural land that seems to have ponding issues (standing water after rains). We might suggest the grower incorporate plant species with large root channels – something like radishes over the winter. Their roots will provide a drainage system for that soil, and alleviate the ponding.

By James Hartsig, Duraroot

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