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Rain Simulator Helps Understanding of How Water Flows

Dig a hole. Shovel the soil back in. It never quite fills up level to the surrounding soil. The reason, of course, is there are air spaces, and they are disturbed when you dig a hole or till a field.

Those air spaces – micropores (small holes) and macropores (larger holes and channels) – are necessary to create a healthy environment for good soil structure. More important, considering the wet springs common in the Midwest lately, is allowing water to infiltrate instead of sitting on the surface or running off the field.

To help producers visualize where water flows in fields with different types of tillage, soil scientists and agricultural agency staff have been setting up rainfall simulator demonstrations developed by the Natural Resources Conservation Service (NRCS) at field days and educational events.

Chris Augustin, an area Extension specialist in soil health with North Central Research Extension Center-Minot, North Dakota State University, led a session at a Soil Health Field Day in Mooreton, North Dakota, last summer. He says the demonstration illustrates three things: detachment (soil particles breaking off), transport (erosion/runoff), and deposition (soil being deposited).

The setup includes a sprinkler system to put down 1 inch of water, gallon jugs to collect water, and trays of soil. Each tray replicates different tillage methods: conventional, tilled with stubble, stubble, cover crop, and tilled high-saline soil, which is a big issue for North Dakota producers.

Predictably, the saline and tilled soils had the least infiltration and the most runoff.

“Keep in mind that you’re not just losing soil, you’re also losing fertilizer and pesticides,” notes Susan Samson-Liebig, soil quality specialist with NRCS. “Soil temperature is also important with soil biology. When the soil is left black, soil temperatures can be up to 130°F. to 150°F. That’s too hot for microbes trying to live in your soil.”

When soils are covered, temperatures only rise to about 90°F.

“In no-till, the soil hasn’t been disturbed and allowed to have structure,” Augustin says. “That allows water to infiltrate. You get micropores and macropores that are caused by different plant roots, insects, earthworms – all the stuff that’s good for the soil. Every time you go out and till that soil, you destroy the structure.”

The cover crop and residue-covered soils also did a fair job of preventing runoff and holding water. The saline soil fared the worst for runoff and illustrated one of the main messages of the field day.

“With saline soils, it comes down to water management,” Augustin says. “You can manage with subsurface drainage; you can do it with cropping systems. When you leave that soil barren and black, though, you leave it to evaporation. When evaporation happens, it wicks those salts to the surface. Water evaporates and salts get left behind. So at least having something growing out here will reduce the evaporation, and the roots will use the water and draw that water table down. As that goes down, so will the salts.”

He notes that no-till isn’t an option for producers of crops such as sugar beets and potatoes that require disturbing the soil. There is time after harvest, until about October 15 in North Dakota, to plant some kind of cover crop.

“I’ve seen radishes green through November and sometimes in December,” Augustin says.

With a wet cycle since the 1990s, cover crops take up excessive moisture and help you get in the fields sooner in the spring. Whether it’s using no-till or planting cover crops, Augustin and Samson-Liebig emphasize the importance of creating pore spaces to collect nutrients and deliver water where it is needed.

Macropores are like the interstate; micropores are like dirt roads, Augustin says.

“Pore spaces are where biology is at. Having plenty of them is important,” Samson-Liebig concludes.

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