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PFR Report: Why Water Management?

What Three Years on a Clay Pan Soil Has Taught Us

Published on Tuesday, January 8, 2019

What has three years of implementing water management practices at our Practical Farm Research (PFR)® facility in southern Illinois (SIL) taught us? That in environments that struggle with drainage, tile has payed dividends. Despite the common belief that tile does not work on clay pan soils, our data has proved otherwise.

So, the question is, why did tile work? To gain a better understanding, we must first dig deeper into the soil moisture and temperature data we have gathered over the past three years with CropX LLC moisture sensors.

When we traditionally think about tile depth, it’s typically placed 30 to 36 in. deep. However, South Dakota State University has shown that the spacing and depth of tile is influenced by the restrictive layer (Figure 1).

Figure 1. Tile drainage diagram from South Dakota State Publication.

This led us to question whether we should we rethink tile spacing and depth in clay pan soils. If the soil profile is more restrictive, should tile be placed shallower and narrower? Our three-year data at Beck’s PFR in SIL says yes! The soil moisture data we have gathered provides a better understanding of how water moves through the profile with the various treatments and if the clay pan or restrictive layer has any influence.

In 2016, we compared the soil moisture over the tile line vs. between the tile line. By looking at soil moisture between the tile line and over the tile line, we gained a better understanding of how water moved within the profile. At the 8-in. depth, the soil moistures averaged across the two spacings increased by 10.6% volumetric water content (VWC) as shown in Figure 2. However, after we moved deeper into the soil profile, the opposite occurred. At the 16-in. depth, the soil moistures averaged across the two spacings increased by only 4.3% VWC over the tile line compared to between the tile lies.

Figure 2. CropX LLC soil moisture over the tile vs. between the tile at 8 and 16 in.

How? Figure 1 illustrates that we need narrower tile when we have a restrictive layer such as a clay pan due to the lack of horizontal movement. Not only do we struggle with horizontal movement, but we also struggle with vertical movement of the water. If you think of a tile plow, it acts as an in-line ripper that fractures the hard pan or restrictive layer. Because the restrictive layer has been fractured, it allows the water to move deeper into the soil profile. Since horizontal movement can be restricted, we see greater movement of water downward vs. horizontally, resulting in increased soil moisture deeper in the profile directly over the tile line.

In 2017, CropX LLC sensors were used to compare soil moistures across various tile spacings and depths. Averaged across all three spacings, the 24-in. tile depth had a 4.2% VWC increase in soil moisture at the 8-in. depth as show in Figure 3. However, at the 18-in. depth, the soil moisture increased by 2.1% VWC when the tile was placed at 36 in. vs 24 in.

Figure 3. Influence of Tile Depth on Soil Moisture at 8 and 18 in.

Once again, tile influenced how the soil moisture moved throughout the profile. In 2016, the data showed that the tile influenced where soil moisture was in the profile. We had less moisture in the upper part of the profile where we had tile, but greater moisture deeper into the profile. In 2017, we learned that the deeper the tile was placed, the greater the soil moisture would be in the lower portions of the profile. So, what is the right answer when it comes to determining tile depth? And how does tile depth and spacing influence yield?

In 2018, our SIL PFR site experienced one of the wettest growing seasons. However, because it was dry at the time of planting, we had uniform stands so we did not see the differences in emergence we saw in 2016 or 2017. Despite the fact that our SIL PFR site did not experience a wet spring in 2018, we still saw a 6.1 Bu./A. yield increase in corn and a 10.3 Bu./A. increase in soybeans with the use of tile. When looking at the three-year average, we see that the 24-in. depth resulted in a 2.3 Bu./A. yield increase in corn and 0.6 Bu. /A. yield increase in soybeans (Figure 4A and 4B).

This data further validates the suggestion in Figure 1 that on restrictive soils, tile should be placed both narrower and shallower. Why have we seen less variance in yields at the 24-in. depth vs. the 36-in. depth? One possibility could be the placement of the soil moisture as the 36-in. depth had greater soil moisture deeper into the profile. However, since the soil moisture is deeper into the profile more of the moisture remains in the restrictive layer. This could potentially hinder water uptake at the 36-in. depth.

In 2016 and 2017 the 36-in. tile depth at 15-ft. spacing tasseled two to three days before the 24-in. depth. This could possibly be a signal of drought stress. In 2018, we did not see a difference in tasseled timing which was most likely because of the abundance of moisture.

Figure 4A: Beck’s PFR SIL 3-Year Tile Spacing and Depth Yield Advantage
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Figure 4B: 2018 Beck's PFR SIL 2-Year Water Management Yield Average 
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This study has shown the importance of better drainage as well as irrigation in areas with a clay pan. Our three-year data highlights how tile and irrigation should be managed differently on clay pan soils vs. soils with deeper profiles as the restrictive layer can affect the placement as well as depth of the tile. As we continue to look at soil moisture with these various treatments, we hope to further expand our knowledge of why we see the response we do with clay pan soils.


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