Dissolved phosphorus joins nitrate as a concern with subsurface drainage
Thousands of acres of yield maps have shown Farm Journal Field Agronomist Ken Ferrie improving drainage is the first step a farmer should take, if possible, to increase crop yield. The volume of tiling, especially in the Corn Belt where soils lend themselves to subsurface drainage, shows producers understand that, too.
But early observations from a Farm Journal study suggest tile drainage might require farmers to sharpen their nutrient management strategy to safeguard water quality.
Although the tile drainage study has been in place for a long time, Ferrie has only been analyzing the impact of drainage on water quality for one season. “While our observations are preliminary, in the first season, we’ve learned some surprising things,” he says.
Noting tile drainage gets some negative press, Ferrie enumerates the benefits of tile not just to farmers but also the environment:
“Among factors allowing us to feed our growing world population, subsurface drainage ranks right up there with the Green Revolution,” Ferrie says. “It’s not uncommon to see crop yields double, or even triple, when tile is installed to drain areas that drown out nearly every year. It’s true those areas can produce a crop in an unusually dry season; but, most years, just two weeks of wet weather is enough to severely reduce yield.
“If you can double yield by draining wet areas, that means 100 acres of tiled land can produce as much food as 200 acres of undrained land,” Ferrie explains. “If we take an equivalent amount of marginal land out of production, that’s 100 fewer acres of fertilizer, herbicide and insecticide applied, some of which might be lost and wind up in water supplies. If we put the 100 acres of marginal land into timber or pasture, there will be less soil erosion. Or, it can be devoted to wildlife and pollinator habitat, perhaps through the Conservation Reserve Program.”
With all the good drainage does, it also brings pollution risks farmers must learn to manage. “One of the biggest aspects of stream pollution has always been surface runoff and soil erosion,” Ferrie explains. “Tile drainage helped reduce this problem by letting us take some marginal land out of production. Now we must be concerned about dissolved nutrients, mainly nitrate and phosphorus, coming out of the tile lines.”
Where water goes, nitrate follows, whether it’s from a tiled crop field, pasture or timber. But, as the Farm Journal study and others show, ortho, or dissolved, phosphate also is exiting fields in tile drainage water. It is present in minute amounts, which until recently were undetectable but still capable of causing water quality problems, Ferrie adds.
Ortho phosphate makes up only a small percentage of phosphorus in streams. Most of that phosphorus is particulate phosphorus, attached to soil particles or organic matter. But ortho phosphate is much more bio-reactive than particulate phosphorus. A mere 0.02 ppm ortho phosphate, in combination with nitrate, is enough to trigger algae growth. Algae growth becomes rapid at 0.1 ppm. (In comparison, it takes 10 ppm of nitrate to make drinking water hazardous to pregnant women and small children.)
Algae might impart an odor to drinking water and interfere with recreational use. When large algae blooms decompose, they can cause hypoxia, or oxygen depletion, in water bodies such as the Gulf of Mexico.
Information gleaned from the first year of the Farm Journal study suggests how farmers can minimize nitrate and phosphorus loss through tile water. Most of the practices involve the 4Rs, which you’ve probably already implemented: the right product, at the right rate, with the right timing and the right placement.
Here’s what we know and have learned about phosphorus through the Farm Journal study:
- The level of phosphorus in tile drainage water correlates to the time of year. In March and April, when soil microbes have not yet fully awakened, phosphorus levels are almost undetectable. In May and June, microbes cycle nutrients and the soil comes alive, which means there’s an uptick of phosphorus in tile water.
- High-pH soils tie up phosphorus. Water coming out of soils with a pH of 7.2 or higher contains almost no phosphorus. This indicates the pH level of the subsoil, or B horizon, is a factor in the risk of phosphorus loss.
- There’s a correlation between soil phosphorus values and phosphorus levels in tile drainage water. “This tells us that knowing our base fertility level can help us manage phosphorus in drainage water,” Ferrie says.
- There’s no correlation between the timing of phosphorus applications and the level of phosphorus in tile water. “This is different from what we see with nitrogen,” Ferrie says. “If we apply fertilizer containing some nitrate and get a heavy rain, we will see a spike in nitrate in the water coming out of our tile. But the amount of phosphorus in tile water is influenced more by the amount of phosphorus in the soil and biological activity.”
- Monitoring the volume of tile line discharge revealed how rapidly rainfall makes its way into tile lines. “Following a rain, tile flow spiked in the first 24 to 48 hours—faster than I anticipated,” Ferrie says. “So water exits the tile system into a ditch almost as quickly as it runs off the soil surface. I shouldn’t have been surprised because the purpose of a properly designed tile system is to let oxygen into the soil within 24 hours after a rain, so crop roots don’t suffer.
“This occurred in healthy soil with good water infiltration,” he adds. “This—along with the fact increased microbial activity makes more phosphorus available in the soil—suggests as we improve soil health, we might increase the risk of dissolved phosphorus entering tile drainage water.”
The flow rate came back down almost as quickly as it rose. “So the greatest risk of nutrient loss to tile water is in the first 24 to 48 hours after a rain,” Ferrie says.
- There’s a strong correlation between the amount of nitrate in soil and the amount that showed up in tile lines following a rain event. “Unlike phosphorus, the amount of nitrate in soil is often correlated to application timing,” Ferrie says. “If you apply all your nitrogen in the spring, there will be more nitrate in the soil and more risk of it exiting through tile water.”
Based on Farm Journal studies, Ferrie says if soil tests 60 ppm nitrate, you might lose half of it, or 30 ppm, during one rain event. But if soil tests only 20 ppm nitrate, you might lose only 5 ppm.
“Keeping soil nitrate at reasonable levels reduces the risk of loss through tile water,” Ferrie says. “We can do that, to some extent, by carefully timing our nitrogen applications.”
- Although bio-reactors installed at the end of a tile line can capture normal levels of tile flow, during peak flow much of the water bypasses them. “In our study, that’s the time when nitrate levels in tile water are highest,” Ferrie says.
- Cover crops might help reduce the amount of nutrients in discharge water, Ferrie notes. Cover crops are not included in this study, but shallow tile lines and gated structures were included as tools to manage water quality.
- When installing new tile systems, it might help to make the depth as shallow as practical, Ferrie says. “Our long-term drainage study showed the deeper we install our tile lines, the more water we remove from a field. The water table seeks the depth of the tile line, whether tile lines are spaced at 30', 60' or 120' intervals. If we remove less water from the field, we will remove fewer dissolved nutrients.”
- Gated structures help regulate the amount of water and nutrients leaving the field through tile lines. “Right after planting, we put in gates to hold the water table within 2' of the soil surface,” Ferrie says. “So very little drainage occurred from April through the first half of June. Only when July rains arrived did water flow over the gates.
“As the corn plants got taller and deeper-rooted, we had to remove some of the gates. We drastically reduced water flow, and nutrient loss, for about 50 days. Although we can’t say whether we improved water quality in the ditch, there was a substantial reduction in nutrients leaving the field.”
“We’re just beginning to learn about the role of tile drainage in the overall nutrient situation,” Ferrie concludes. “But we do know it is a factor affecting water quality. Using sound fertilizer practices is a small price to pay for all the benefits of drainage, not just to farmers but to hungry people all over the world.”
Can Tile Drainage Improve Water Quality?
Theoretically, if the discharge water from a farm’s tile system contains fewer nutrients than the water already flowing in a drainage ditch, the farmer is improving the quality of the water downstream. (As an old saying puts it, “The solution to pollution is dilution.”) But that might be difficult to achieve, says Farm Journal Field Agronomist Ken Ferrie.
“In our drainage study, early in the season, the discharge water was lower in nitrate than the water in the ditch,” Ferrie says. “But at the peak of nitrogen uptake by the crop, when the nitrate supply in the soil was highest, nitrate levels were higher in the tile water than in the ditch water, so we were adding nitrate.”
Throughout the growing period, the study found the farm’s tile water was considerably lower in total phosphorus than the water in the drainage ditch. But that was because the tile water contained less particulate phosphorus. Unfortunately, because the dissolved phosphorus in the drainage water is more bio-reactive, even a minute amount might trigger algae growth.