Increased biological activity might release water-soluble phosphorus into water supplies
Success in one area often creates challenges in another. Early results from studies conducted by Farm
Journal Field Agronomist Ken Ferrie suggest that could be the situation with no-till and cover crops.
The studies involve strip trials of no-till, strip-till, conventional tillage and cover crops on more than a dozen farms throughout Illinois over several years. After as little as one year, the cover crops are producing measurable increases in microbial activity, organic carbon and water infiltration. All are indicators of improving soil health.
But those same studies suggest as farmers improve health, they might have to sharpen their fertility management to minimize nutrient pollution of water supplies.
“My concerns are nitrate and water-soluble phosphorus,” Ferrie explains. (Water-soluble phosphorus is also called ortho-phosphate.) “Farmers are used to managing nitrate in soil, but most probably haven’t given much thought to water-soluble phosphorus.”
Water-soluble phosphorus, as the name implies, can be leached through soil along with water (just like nitrate). That presents a new challenge compared with the more familiar form of particulate phosphorus that reaches water through runoff and soil erosion, which farmers are used to managing.
In soil samples collected in Ferrie’s studies, water-soluble phosphorus values ranged from 0.01 ppm to 4.0 ppm. But it’s a potent nutrient—95% bio-active in the environment versus 30% bio-active for the particulate form.
In water, only 0.02 ppm water-soluble phosphate, paired with nitrate, is enough to trigger algae growth in lakes and rivers. “Some algae is a necessity because it is the foundation of the food chain for many aquatic organisms,” Ferrie points out. “But too much of it can create problems.”
Algae growth becomes rapid at 0.1 ppm water-soluble phosphate. Algae blooms are an issue in water bodies such as the Chesapeake Bay and Gulf of Mexico. When the algae dies and decomposes, it leaves the water deficient in oxygen, a condition called hypoxia.
In contrast to water-soluble phosphorus, nitrate doesn’t become a water quality issue until it reaches 10 ppm (when it becomes a concern for pregnant women and small children).
The concern with water-soluble phosphorus is healthier soil, which has more microbial activity, produces more of it. “Our studies show water-soluble phosphorus can be used as an indicator of the amount of microbial activity in the soil,” Ferrie says.
“Microorganisms release water-soluble phosphorus—so the more water-soluble phosphorus, the more microbial activity in the soil. Microbial activity is good. Water-soluble phosphorus, which is used by plants and microbes as an energy source, is also good, but it can be leached into water supplies,” he adds.
Water-soluble phosphorus is not measured by the standard soil tests used for nutrient planning. Those tests, such as the Bray and Mehlich methods, use various types and strengths of acids to release phosphorus tied up in the soil. They predict how much phosphorus is likely to become available to plants in the next several years.
Water-soluble phosphorus is contained in the soil solution. To test for its presence, lab technicians simply run water through a soil sample and flush out the phosphorus.
The amount of water-soluble phosphorus in soil varies from day to day, and even from hour to hour because of changing environmental conditions. When soil is warm and moist, microorganisms are more active, releasing more water-soluble phosphorus into the soil solution.
“So the water-soluble phosphorus test produces a snapshot in time,” Ferrie says. “Because it is affected by various factors, such as soil moisture, temperature, pH and time of sampling, it can be difficult to use the result to make long-term predictions.
“After water-soluble phosphorus is created by microbes, it is consumed by microbes for energy, taken up by plants for food or tied up in the soil,” Ferrie says. “Like nitrogen, phosphorus is constantly cycling from one form to another in the soil. Weekly water-soluble phosphorus values give us an indication of what’s happening with soil microbes. As the soil warms, water-soluble phosphorus values climb.”
As with nitrogen, soil microorganisms not only mineralize, or release, phosphorus, but they immobilize it, making it unavailable. “Both processes go on at the same time,” Ferrie says. “Sometimes you have more of one or the other, creating net mineralization or net immobilization. The reason we get a stronger response to starter fertilizer in continuous corn is because we are helping the corn get through a net immobilization phase of phosphorus, which we call the carbon penalty.”
Once you know how microbes cycle phosphorus, you can see how cover crops impact the level of water-soluble phosphorus. “The water-soluble phosphorus values tell us cover crops lead to more microbial activity,” Ferrie says. “In the fall, if we compare a live cover crop strip to a check strip, we find lower water-soluble phosphorus values in the cover cropped soil. That’s because the cover crop is consuming water-soluble phosphorus in the soil, pulling it into the plants.”
In the trials, Ferrie found after a cover crop winterkills or is terminated in early spring, there might not be much difference in water-soluble phosphorus values between the cover crop and the check, depending on the cover. “As the soil warms up, and microorganisms start decomposing the cover crop, we saw water-soluble phosphorus values climb, compared with the checks,” he notes.
Depending on when the cover crop was killed, Ferrie also documented the effect of the carbon/nitrogen and carbon/phosphorus ratios of the cover crop. For instance, with oats and radishes or turnips and radishes, which winterkill, water-soluble phosphorus cycles faster in the soil. The earlier the cover crop dies or is killed, the sooner water-soluble phosphorus values started to climb, as microbial activity reached the point of net mineralization.
In cover crops such as annual ryegrass, which continue to grow in the spring, Ferrie’s tests reveal lower water-soluble phosphorus values because the cover crop is still taking up water-soluble phosphorus.
“After we kill the cover crop, there will be a period of net immobilization as the cover crop decomposes,” Ferrie explains. “After 30 to 50 days, net mineralization occurs, and the water-soluble phosphorus value climbs. We can actually determine how fast mineralization occurs with various cover crops and plan our nutrient applications accordingly.”
Here’s why Ferris is concerned about water-soluble phosphorus: Testing soil at depths of zero to 6", 6" to 12" and 12" to 24" revealed water-soluble phosphorus is carried deeper into the soil profile in no-till compared with tilled soil because no-till soil has a higher infiltration rate.
“Using weekly water-soluble phosphorus tests, we could track the effect of every rain event, as rainfall moved water-soluble phosphorus downward through the soil,” Ferrie says.
The strip trials showed more aggressively growing cover crops resulted in more water-soluble phosphorus deeper in the profile. “With covers that winterkilled, nutrient mineralization showed up faster than with cover crops killed in the spring,” Ferrie says.
In one trial, the measured infiltration rates for strip-till with and without cover crops were similar. There, cover crops with more above-ground growth (and correspondingly more root mass) resulted in higher water-soluble phosphorus values deeper in the soil than strip-till without a cover crop.
“Because the cover crop strips and the no-till strips had the same infiltration rates, we surmise the higher water-soluble phosphorus values deep in the soil resulted from deeper roots that, when decomposed by microbes, released water-soluble phosphorus,” Ferrie says.
The fact cover crops can produce a deeper feeding area for crop roots is exciting for crop production. “But from a water quality standpoint, it raises concern,” he says. “When we increase the amount of water-soluble phosphorus at the 2' depth, it just needs to move down one more foot to reach drainage tile.
“As cover crops biologically improve soil, the soil produces more nutrients, so we may be able to reduce nutrient applications,” Ferrie summarizes. “We also get better infiltration. But now we have to think about keeping the water-soluble phosphorus out of tile lines.”
Ferrie and his staff recently began studying the quality of tile drainage water. “The level of nitrate in drainage water is strongly correlated to the level of nitrate in soil when a rain event occurs,” he says. “The amount of water-soluble phosphorus in tile water also appears to be somewhat tied to the level in the soil.”
In the spring, when soil nitrate levels tend to correlate with nitrogen applications, the amount of water-soluble phosphorus tends to be low to nonexistent. “But by midsummer, after the carbon penalty phase, when there is a lot of microbial activity going on, the amount of water-soluble phosphorus in the tile water appears to mimic the soil test value,” Ferrie says. “The good part of this is the cash crop is taking up phosphorus at that point in the growing season.”
At the end of the growing season, however, the cash crop is no longer taking up nutrients but biologically active soil is still producing them. At that point, there’s little control over the nutrients left in the soil and no easy way to make sure water-soluble phosphorus doesn’t move into the tile water and get carried out of the field into water supplies.
Perhaps the solution is to become more sophisticated with cover crops. “We may need to find ways to plant cover crops that can take up those leftover nutrients late in the summer,” Ferrie says. “We need to learn to establish cover crops as quickly as possible and grow them every year.”
Ferrie is confident farmers and scientists will solve this environmental challenge—while increasing yield to feed a growing world population—just as they have solved other challenges. “But right now, I’m still scratching my head,” he says. “We need a lot more research.”
Watch for more articles on how to mind water quality and keep nutrients in your fields where they belong at www.FarmJournal.com/water_quality.
Tools You Can Use Now
While we search for more answers to potential issues with water-soluble phosphorus, here are some steps that can help:
- “In the middle of the growing season, when there is a lot of microbial activity, there is a strong correlation between the level of phosphorus in soil, measured by standard soil tests, and the amount of water-soluble phosphorus,” says Farm Journal Field Agronomist Ken Ferrie. “As a result, try to keep soil phosphorus levels at moderate to optimum levels. Pay close attention to your soil test values, and implement all of the 4Rs—the right product at the rate, the right time and the right place.”
- Keep your soil pH value in the optimum range. Phosphorus is more easily leached out of acid soil.
- Where they fit, gated tile systems can be used to hold back drainage water until a cover crop is established.