Rainfall Simulator Compares Soil Health

The rainfall simulator is a great tool that demonstrates runoff generation, infiltration rates and topsoil loss. This trial simulates the effects an inch of rainfall has on three surface soil samples collected during the summer of 2016.

In this experiment, there is soil managed organically, conventionally and that of a perennial buffer from the farm. All of the soil samples are from the no-till plots in the Farming Systems Trial at Rodale Institute. The sample on the left side is specifically from an organic manure treatment, while the middle sample is from a conventional treatment where herbicides are used. On the right side, the perennial soil sample is from a grass-based buffer.

The shallow two-inch depth pans hold approximately the same amount of soil in them.

Underneath the pans are trays that have holes in them that allow for water to flow through. As the oscillator moves back and forth, each of the three samples receives the same amount of water. Every soil sample receives two passes as the oscillator reaches it peak and returns back down.

The front jars capture the water that will potentially runoff the surface of the soil. This water may contain soil particles, debris, organic matter, synthetic fertilizers and any other types of contaminants that have been applied to the soil. The greater the water collection in the front jar means more topsoil and organic matter will be lost due to runoff. Water collection in the front jar shows how organic matter, nutrients and fertility are more easily washed away with conventional soil. However, no water collection for the organic sample demonstrates that rainfall was not lost to runoff.

The back jars represent how much water holding capacity each of the soils have. These jars indicate the amount of water that will be infiltrated and potentially held in the soil. Infiltration measures the rate at which soil absorbs rainfall. In the back jar, the amount of water shows that the soil from the perennial buffer does not have as much water holding capacity as the soil from the organically managed system. The conventional soil does not have sufficient water holding capacity, as shown by the significant amount of water collection in the back jar. This water loss can cause plants to become water stressed and exhibit drought stress between rainfalls. The soil from the organic manure treatment has the most water holding capacity due to no water collection. Therefore, the organically managed system has greater soil health because rainwater is able to be held within the soil rather than running off. It is crucial that water is able to be held in the soil, so it is available to the roots between rainfalls.

The time leg from July through August is when temperatures are highest, and there are typically less rainfall events. If an inch of water cannot be held in a 2-inch depth of soil that water flows through, there will be less water available to the plants between rainfalls. After it rains, it is important that water efficiently flows into the soil so water can be stored where the roots are.

Residue seen primarily on the surface of the organic treatment and perennial buffer is due to the plot management and the various crops that are grown in the rotation. The organic sample came form a green manure treatment that has five different cash crops in it, thus producing more residue. Whereas the conventional soil is from a predominantly corn and soybean rotation, which are both crops that cause minimal to no residue. The plot management and rotation explains why the conventional soil appears to be bare soil. With the perennial buffer, there was a good accumulation of residue on the surface. As plants die that residue builds up on the surface and then is cycled by the biological communities and broken down over an annual basis.

Despite the conservation programs for agriculture here in the United States, about two billion metric tons of topsoil is lost every year. Much of that ends up in the Mississippi and Missouri Rivers which can lead to detrimental clogging. This water runoff may also contain various types of contaminants that can cause serious issues for other organisms.

Overall, this experiment demonstrates the roles runoff and infiltration play in soil health. Of the three samples, the organically managed soil had no water collection in either jars which thereby proves its ability to absorb rainfall efficiently without substantial water loss due to runoff.

This is a guest post by Communications Intern Amanda Bialek.

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