Soil health is moving forward so quickly that much of the effectiveness of practices is based on observation. Don't get me wrong, I spend a lot of time observing to see how practices might or might not work. But, I also have the important job of understanding the science behind the observed practices so that statewide recommendations can be made. Over the next couple months, I'll take a look at observations and hopefully relate what we are observing with scientific results from North Dakota State University soil health research projects.
Observation: Tiling is used as a tool to manage salinity. I've had several conversations with individuals who have tiled their fields and seen great benefits on the yield monitor. I can see some of these benefits from the many fields I've gone into and aerial photos we've taken of different projects. I have also seen fields where tiling has not been as effective as hoped.
Here's what some of our data tells us.
One project where we've been evaluating tiling is the Soil Health and Agriculture Research Extension Farm - also called the SHARE Farm. This project is located on high clay, shrink-swell soils in the southern Red River Valley (Mooreton, N.D.) and addresses questions related to salinity management, conservation tillage, use of cover crops and evaluation of soil health. Keep in mind, the results from the studies at the SHARE Farm are specific to conditions at this site.
Leaching salt from intact cores: We removed large intact soil cores (8 inches diameter, 4 feet long) from "high salinity" (EC from saturated paste method: 4.5 mmhos/cm) and "low salinity" (0.7 mmhos/cm) impacted areas at the SHARE Farm (Figure 1). The goal was to run water through (or leach) these cores in the lab to see how much water is required to remove salts from the "high salinity" areas so that they reach the levels of the "low salinity" areas. Water (or leachate) run through the cores was collected and analyzed. Before you read further, understand that lab work is helpful when you want to control conditions and fast forward through a process that may be very slow to happen. That being said, we understand that lab experiments may not fully represent field conditions. A couple surprising results. First, water applied per wet-dry cycle (6 inches water applied per cycle) moved through the cores in less than 24 hours or 4,000 times greater than expected for high clay soils. Second, salt removal was relatively low even with the rapid water movement through the intact cores. Results suggest it would take 94 leaching events (560 inches of water) to remove enough salts from the "high salinity" area to make it similar to the "low salinity" area.
In a field situation, water may move rapidly through the cracks in a high clay, shrink-swell soil when the conditions are dry. There may not be much time for that water to come in contact with salts in the soil to dissolve and remove them from the soil profile. One possible solution to increase time the water is in contact with the salts to get more salt leaching - create smaller pore space through aggregation and leave root channels intact for more effective water movement through the soil by reducing tillage.
Tiled and un-tiled field comparison: In 2014, tile drainage was installed (40 foot spacing, 4 foot depth) on the northern 80 acres of the SHARE Farm, leaving the southern 80 acres un-tiled. We have been monitoring the depth to the water table throughout the year on both the tiled and non-tiled portions of the field. Through time, the depth from the surface to water table on the non-tiled part of the field has significantly decreased (meaning the water table is getting closer to the surface) while the water table depth has stayed the same in the tiled portion of the field. Makes sense, water table is a problem in this area and tile drainage is helping to keep the water table at 4 feet. Surface soil salinity levels (0-6 inch depth) are "trending" (not statistically significant) to decrease in the tiled part of the field and remaining unchanged in the non-tiled part of the field (Figure 2). But keep in mind, this is just a trend and not statistically significant (the "p values" are greater than 0.05 and that means there is too much variability to say confidently that there is a difference). Maybe more time will tell us something more definitive. The big picture, interception of groundwater by the tile drainage may be an important salinity management strategy that we observed when making field measurements.
Tile drainage shows to be an important tool, but the lab and field results indicate that tiliing should not be the only tool used to manage salinity in high clay soils of the Red River Valley. Other tools like crop rotation, cover crops and reducing tillage could be used in combination with tile drainage (I'll dig into these topics in the next couple months). However, knowing that interception of groundwater by tile may be more beneficial than relying on the leaching of salts from the surface in high clay soils is important to know when making a decision to use this approach.
Research Credits: Frank Casey, Nate Derby and Rebecca Doyle (NDSU Soil Science) led research presented in this Soil Health Minute.