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On this rainfall simulator, the jars on the front show runoff, and the second row of jars show water infiltration. This is for four different management approaches on the same soil type. (Abbey Wick/Special to Agweek)

Understanding water movement in soils

This past week, water infiltration and runoff were topics in the Soil Health Minute segment on AgWeekTV. These terms are used often when talking about soil health because managing water (increasing infiltration) and minimizing erosion (reducing runoff) are common goals mentioned when farmers start using soil health building practices. Also, infiltration and runoff are incredibly visual amongst different management practices. I'll explain the terms infiltration and runoff and the role management plays.

Infiltration

Infiltration is a term typically used in reference to water movement into the soil and is often talked about in terms of "rates," meaning how fast is water moving into the soil profile. The major factors contributing to water movement into the soil are how wet the soil is to start, soil texture, soil aggregation and structure and uniformity of the soil profile (for example, is there a clay or sand layer in the soil that might hold up water movement?).

I also think of continuous pores from the surface into the soil (as a result of root channels or earthworm movement) as being important, too, in how quickly water moves into the soil. Reducing tillage is a great way to maintain aggregation and continuous pores, but should be used in combination with crop rotation and cover crops to build aggregation and increase continuous root channels in the soil.

Here's an important tip when determining infiltration in your fields: The initial water content of soil is often overlooked. I've made this mistake before when a farmer and I set up infiltration rings from a soil quality test kit in his field. If you're not familiar with this test, you dump water into a ring to simulate 1-inch of rainfall, then use a stopwatch to time how fast that water moves into the soil. We dumped water in the ring and nothing happened. Bummer! What I should have done is dug a hole first because then I would have seen that the soil was saturated at 3 inches and doing the infiltration test would tell us absolutely nothing. There was nowhere for the water to go despite his use of no-till and cover crops — but at initial observation, it looked like we were failing. This is why initial soil water content is important when evaluating infiltration. Learn from my mistakes.

Runoff

Raindrop impact is one of the most important mechanisms for detaching soil, entraining/holding it in water and then transporting it off of a field. Reduce the effects of raindrop impact by building soil aggregation and protecting the soil surface with residue. Aggregated soil (compared to individual soil particles like sand, silt and clay) is more difficult for a raindrop to detach from. Also, residue on the surface can reduce the impact from each raindrop — less energy means less soil detached. So, build aggregates and keep the soil covered to reduce runoff from rain events. By default this will help with wind erosion events, too.

For more information, join us on the NDSU Soil Health Bus Tour July 25-26. We will be talking about both of these concepts in the field. To register, go online to https://www.ndsu.edu/soilhealth/ (information is posted under the "Events" tab).