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PFR Report

Soil Moisture - Where Does It Go?

Published on Wednesday, March 07, 2018

Over the last two years, Beck’s Practical Farm Research (PFR)® team in SIL has tested various water management systems. These studies included drip-fertigation and sub-irrigation studies as well as a tile spacing and tile depth studies. These different water management systems have resulted in some of the highest yields ever recorded at the SIL PFR site to date. In the February 2018 PFR report, we discussed the yield advantages we have seen from tile drainage and irrigation, and many of you asked why we saw such a yield increase.

To help explain the why, we must first understand how soil moisture moves throughout the soil profile within these various treatments. We have been evaluating soil moisture and temperature over the last two years to help explain why we’ve seen such yield increases. In 2016 and 2017, CropX Inc. sensors were used to monitor soil moisture. In 2016, these moisture sensors were placed in the tile spacing study directly over the tile and in between the tile lines. When placed at an 8 in. depth, soil moisture increased more between the tile lines than directly over the tile lines. However, when placed at a 16 in. depth, soil moisture was observed to be greater directly over the tile lines compared to in-between the tile lines. Why? One reason could have been the increased water infiltration. A tile plow acts as a ripper, fracturing the hard pan and improving soil aggregation. This results in greater water infiltration which allows water to reach deeper into the soil profile. When it comes to irrigation, the soil moisture was higher at the 16 in. depth in the drip-fertigation section when tile was placed under the drip. The reasoning behind this could be the same as with the tile spacing study. The tile allowed for better infiltration, helping the water to move deeper into the profile. Also, the treatments that had irrigation experienced more consistent soil moisture throughout the season. The soil profile in the non-irrigated treatments would recharge after a rain, but took much longer compared to irrigated treatments.

Soil Moisture Tile Spacing and Depth

In 2017, moisture sensors were used to compare soil moisture levels between the 24 in. and 36 in. tile depths. We did not observe a substantial difference with the 24 in. tile depth compared to the control when evaluating the sensors both mid-season and late-season. However, early in the season there was a substantial visual difference in soil moisture levels across all tile spacings as well as the control. What we observed was that the narrower the tile spacing, the faster the soil dried out early in the season. The sensors were not placed until later in the season and at this time, there was less of a difference between the control and the tiled treatments.

When averaging the sensor readings from mid-July to September, tile spacing at 15 ft., 30 ft., and 60 ft. at a 24 in. depth, the 8 in. soil moistures averaged 14.5%, 15.9% and 14.9% volumetric water content (VWC) respectively (Figure 1A).

When compared to the control, that averaged 14.8% VWC across the two tile depths. This could be explained by the dryer soil profile once we reached July. Though there was not much difference between 24 in. tile spacings, larger differences were seen with the 36 in. depth. The 36 in. tile depth averaged 8.0%, 15.0%, and 9.8% VWC on 15 ft., 30 ft. and 60 ft. tile spacing, respectively. Overall, the 36 in. tile depth 8 in. soil moisture was much lower compared to tile placed at a depth of 24 in. Averaged across spacings, the 24 in. tile depth averaged 15.1% VWC and the 36 in. tile depth averaged 10.9% VWC (at an 8 in. depth). Throughout the season, the 24 in. depth never resulted in greater than 3.6% VWC difference compared to the control. However, as much as a 9.9% VWC decrease was seen with the 36 in. tile depth. Why? One possibility could go back to water infiltration. Since the tile is deeper at the 36 in. depth, it may allow more of the water to move deeper into the profile, thus moving more water out of the 8 in. range. One visual difference we did see between the 24 in. and 36 in. tile depth was the corn in the 36 in. tasseled sooner than the 24 in. (Figure 2). Could this be why? More research will need to be done to prove this theory.

In 2017, the lower soil moisture reading was changed from 16 in. to 18 in., as the soil moisture was much more consistent across treatments (Figure 1B) at this depth.

At an 18 in. depth, the tile placed at 24 in. averaged 22.7%, 30.5% and 29.3% VWC soil moisture on 15 ft., 30 ft., and 60 ft. tile spacing, respectively. However, the tile placed at a 36 in. depth averaged 30.2, 28.6 and 30% VWC soil moisture on 15 ft., 30 ft., and 60 ft. tile spacing, respectively. Averaged across all spacings, the 24 in. tile depth soil moisture averaged 27.5% VWC and the 36 in. tile depth soil moisture averaged 29.6% VWC (at an 18 in. depth). For the 8 in. soil moisture readings, the 36 in. tile depth was lower compared to the 24 in. tile depth. The inverse was true at the 18 in. moisture reading. Some of this is explained by the variation of the 15 ft. spacing tile at the 24 in. tile depth. All tile spacing and depths were within 1.7% VWC from the control except for the 15 ft. spacing at the 24 in. depth, which was up to 14.8% VWC lower compared to the control. This can be explained by the narrow tile spacing and shallower depth. Since the 15 ft. tile spacing at a 24 in. depth is much narrow and shallower, the 18 in. moisture reading was much closer to the depth of the tile. However, once the late-season rains started, the soil moisture increased for both the 15 ft. spacing 24 in. tile depths. Though we can achieve better drainage early on with 15 ft. spacing, the soil moisture is more limited at the 18 in. depth late in the summer when the tile was placed at a depth of 24 in.

Soil Moisture-Irrigation

In the drip-fertigation portion of this study, drip-fertigation with and without tile soil moisture (at an 8 in. depth) averaged 16.2% and 13.5% VWC, respectively (Figure 3A).

Non-irrigated treatments such as the 30 ft. tile averaged 12.5% VWC and spent the later part of grain fill below 10% VWC. However, drip-fertigation both with and without tile never fell below 14.7% and 12.1% VWC respectively in the later part of grain fill. Though the 8 in. soil moisture may have averaged lower compared to the irrigated treatments, we must remember the benefits we saw during the planting season. For the sub-irrigation portion of the study, the sensor based sub-irrigation soil moisture averaged the highest compared to all treatments with a 16.5% VWC at an 8 in. depth. Just like drip-fertigation with tile, the sensor based sub-irrigation soil moisture remained higher and more consistent later in the season. Our 8 in. sub-irrigation soil moisture never fell below 14.8% VWC all season long. The manual sub-irrigation soil moisture was lower the entire time the sensor readings were taken. Manual sub-irrigation was up to 2.8% VWC drier compared to the sensor-based sub-irrigation. As for treatments without irrigation, just like the 30 ft. tile the contour drainage was much dryer at an 8 in. depth compared to irrigated treatments. The contour drainage averaged 2.7% VWC lower soil moisture (at an 8 in. depth) compared to sensor and 0.5% VWC lower compared to manual sub-irrigation, respectively. Just like in 2016, the 30 ft. tile had greater water removal compared to contour drainage later in the season. Averaged throughout the summer, the contour drainage soil moisture at an 8 in. depth was 1.3% VWC higher compared to the 30 ft. tile. And similar to 2016, all irrigated treatments resulted in more consistent soil moisture compared to non-irrigated treatments late season in 2017. This can be critical for shallower rooted crops such as soybeans especially during grain fill.

Across all treatments, the soil moisture was more consistent at an 18 in. depth (Figure 3B).

All treatments except the 30 ft. tile were within 4.3% VWC soil moisture at the time the readings were taken. The drip-fertigation soil moisture at the 18 in. depth averaged 29.9% and 30.9% VWC both with and without tile, respectively. Averaged across the summer, drip-fertigation with and without tile resulted in an 8.3% and 9.3% VWC increase in soil moisture at an 18 in. depth compared to the 30 ft. depth, respectively. Though soil moisture was much lower compared to irrigated treatments, on July 26, 2017 the 30 ft. tile was within 2% VWC compared to drip treatments after rainfall partially recharged the soil profile. However, soil moisture in the 30 ft. tile fell quickly below all irrigated treatments once we hit August. Sensor based sub-irrigation once again had the greater soil moisture and averaged a 2.7% increase in soil moisture compared to manual sub-irrigation. Contour drainage soil moisture was less compared to all irrigated treatments except the manual based sub-irrigation. Averaged across readings, contour soil moisture averaged a 6.6% VWC increase in soil moisture compared to the 30 ft. tile. Once again irrigation resulted in more consistent moisture throughout the season.

We have always assumed that improved driange contributed to the increase in yields in this tile study. Over the past two years, we have started to gain a better understanding of how water moves throughout the profile with various tile treatments. This has in return helped explain why we have see yield increases with tile. As for irrigation, it paid dividends this year and resulted in less variablity in soil moisture all season long. We look forward to further testing in 2018.

Joe Bolte, CCA | PFR Operator and Herbicide Specialist

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