December 27, 2021 at 3:36 p.m.

Balancing phosphorus trade-offs

Seasonal shifts, farming practices impact nutrient loss
Eric Cooley, Discovery Farms Director
Eric Cooley, Discovery Farms Director

By Stacey [email protected] | Comments: 0 | Leave a comment

WISCONSIN DELLS, Wis. – Depending on the time of year and farming practices used, phosphorus loss can take different forms and intensities. What is the best method for retaining this important nutrient on the landscape?
This topic kicked off the Discovery Farms 10th annual conference Dec. 15 in Wisconsin Dells during a presentation by Eric Cooley, Discovery Farms director, entitled “Balancing phosphorus trade-offs.”
Focused on the relationship between agriculture and water quality, Discovery Farms is a farmer-led research and outreach program of the University of Wisconsin-Madison Division of Extension. Conducting research on privately-owned farms throughout Wisconsin, Discovery Farms works with the U.S. Geological Survey to gather credible and unbiased water quality information from monitored sites.
“There are two forms of phosphorus – dissolved fraction and particulate fraction,” Cooley said. “A balancing act occurs between the loss of each, but Mother Nature likes to throw curveballs at us, making it far more complex than just a nice teeter tottering effect. The weather can really shift things back and forth with the balance of phosphorus and particulate or dissolved fractions of that phosphorus.”
Particulate loss is more prominent in tillage systems, in the incorporation of manure and fertilizer, and in bare soil; whereas, dissolved phosphorus loss occurs more readily in no-till systems, on fields where manure is surface applied and atop a crop or residue cover.
“The loss of phosphorus shifts up or down based on management systems and other factors,” Cooley said.
The loss of particulate phosphorus is driven by non-frozen soil runoff that typically occurs less than two weeks after manure or commercial fertilizer application.
“Particulate loss takes place when you see a lot of soil moving around,” Cooley said. “But soil conservation practices can target the phosphorus pathway and reduce this loss.”
The loss of dissolved phosphorus is seen when runoff occurs on frozen soil.
“Winter application of manure, especially later in the year, can drive dissolved phosphorus losses, but testing soil for phosphorus levels – especially the upper inch – can really have some significant differences in dissolved fractions we see coming off the land,” Cooley said. “For all the different farms we monitor, half of the water comes off when the soil is frozen and half comes off when it’s not frozen.”
March produces the most surface runoff, averaging almost an inch, while October is the month with the least amount of runoff at .02 inch.
 “We almost always had runoff at every Discovery Farms site during the month of March,” Cooley said. “This is when most of the water is coming off the land from snowmelt and subsequent rain, creating high-moisture content soils. If we know when runoff will occur, we can try to mitigate its effects during the times we see water moving.”
A tool that can help with decision making is the Runoff Risk Advisory Forecast from the Wisconsin Manure Management Advisory System. This map illustrates risk severity for runoff throughout the state.
“They continue to make this tool better so it produces less false positives and fewer false negatives,” Cooley said.
Soil loss and particulate phosphorus loss are directly correlated; therefore, the ability to control runoff saves phosphorus from leaving the field as well.
“We know that surface runoff will happen, but you can control what is in it,” Cooley said. “Reducing the risk of phosphorus loss requires a balance of practices based on the system’s biggest risk.”
Total phosphorus loss throughout the year mirrors runoff with March being the No.1 contender at 21%. However, the months of May and June see a 16% and 21% loss, respectively. Dissolved and particulate losses are split evenly at half and half on an annual basis for all farm data combined.
“How we manage farms to limit the loss during these two times is very different,” Cooley said. “There’s higher potential for loss at one of these two times based on your management practices. Some farming systems have benefits in wintertime; others have more benefits during spring planting time.”
Runoff during the snowmelt period is mostly dissolved phosphorus. During the spring, it is mostly particulate phosphorus. January through March, when soils are frozen, account for 40% of the year’s phosphorus losses with 70% of that being in dissolved form. May through July sees 45% of annual phosphorus losses, and 70% of those are in particulate form.
“When soils are frozen, we don’t see a lot of soil loss, but we do see a lot of water interact with the surface of soil and phosphorus loss is in dissolved form,” Cooley said. “Frost kill can result in significant phosphorus loss as well.”
The critical period for soil loss is May and June, which explains that second peak of phosphorus loss in the year.
“Controlling soil loss can control particulate phosphorus loss; therefore, soil protection becomes paramount to reducing the amount of both soil and phosphorus loss,” Cooley said.  
The level of soil protection is site specific but may include residue management, cover crops that involve planting green versus winter kill, the use of perennial crops, and no till or reduced till. Tools for nutrient retention include cover crops, reduced tillage, soil testing, nutrient incorporation, residue cover and better timing of application.
When looking at data from Wisconsin and Minnesota regarding total annual soil loss in tillage versus no-till systems, there was four times more soil loss in tillage systems compared to no till. Tillage systems resulted in a soil loss of 193 pounds per acre, while no-till systems incurred 46 pounds of soil loss per acre.
“No till definitely has the advantage when it comes to soil loss,” Cooley said.
When it comes to phosphorus, however, total loss between the two systems is much closer with no till seeing larger numbers than tillage at 1.09 pounds per acre versus 0.79 pounds per acre.
For dissolved phosphorus, there was far more loss in no-till sites at 0.80 pounds per acre compared to tilled land at 0.30 pounds of loss per acre. Cooley has seen instances where 90% of phosphorus loss was in the dissolved form in no-till systems.
“How do we get the best of both worlds with reducing soil disruption yet getting those nutrients, specifically phosphorus, below the soil surface?” Cooley said. “We need to try and find the happy mediums, the new technologies, and a new way of getting phosphorus down below the soil surface to prevent runoff yet not disrupt soil and cause soil loss. Nutrients should be placed below the soil surface but not with so much disturbance that soil loss becomes an issue.”
Delivering nutrients below the surface reduces dissolved phosphorus loss, especially in winter runoff. In one study, manure incorporation decreased annual dissolved phosphorus loss by 50% or more compared to surface applied manure in no-till fields and pasture. But in years with high soil loss, manure incorporation increased annual soil and total phosphorus loss by 50% or more compared to surface applied manure in no-till fields and pasture.
“This shows there are tradeoffs both seasonally and with the different farming systems we have out there,” Cooley said. “There isn’t one perfect farming system. They all have tradeoffs.”


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