When Laura Paletta, outreach specialist with the University of Wisconsin, talks about manure, she emphasizes the importance of knowing when nutrient losses occur. This knowledge helps farm managers develop best management practices for the operation.

March is the month most runoff occurs, primarily due to snow melt and soil thawing. Soil loss occurs primarily in May, June and July because tillage is taking place and soil isn’t fully covered.

Paletta said most dissolved phosphorus (P) is lost in January, February and March. “Dissolved phosphorus is the portion completely dissolved in solution, so when there’s a lot of runoff, the dissolved phosphorus is also lost,” she said. “Dissolved phosphorus is linked to eutrophication and algal blooms.”

Particulate P is the portion of P bound to soil particles, so when there’s increased soil loss, there’s also increased particulate P loss. Most particulate P loss occurs in May, June and July.

“With particulate soil loss, there’s an increase in sedimentation in water bodies that significantly impacts aquatic wildlife,” said Paletta. “We’re out there tilling and tractors are on and off the field kicking up soil, which is why we see an increase in soil and particulate phosphorus loss.”

Typical manure spreading windows are in spring, before crops are planted, and in autumn after harvest. Springtime brings more P losses as manure nutrients are being added, so limiting loss is key.

“This time of year, it’s all about soil coverage,” said Paletta. “If soil coverage is optimum, it can help limit losses.”

Overwintering cover crops will help limit losses and provide soil coverage in April and May. Reduced tillage helps keep residue on soil, and planting into living matter helps maintain coverage.

The optimum window for autumn manure application is after the soil temperature drops but before soil freezes. “We spread when soil temperature drops because that’s when microbial activity is also reduced,” said Paletta. “When microbial activity is reduced, nitrifying bacteria also decreases, and that can help limit nitrate losses through winter and into spring. Spread before the soil freezes because once the soil is frozen, the soil is like a concrete layer and infiltration is greatly reduced.”

Winter manure spreading is dependent on state regulations. Paletta recommended not spreading in winter, but if necessary and allowable, spread on internally drained fields with little or no slope that are far from surface water. Fields with cover crops are ideal for winter spreading.

Summer spreading is another option. “There’s research at Ohio State where researchers found spreading in summer could potentially help increase crop yields,” said Paletta. “This opens more time to spread. Spreading with a dragline when corn is in V3 to V4 stage increases yield because the growing crop takes up nutrients.”

Always be aware of large runoff events and timing of manure application. When soil loss increases, total P loss is higher. Runoff issues can be avoided by keeping soil covered when possible and being aware of field conditions.

Low disturbance manure injection is an option that works for some farms. Paletta explained a project in which a no-till field and a pasture both had surface manure application, and a third field was chisel plowed. When manure was incorporated, annual dissolved losses decreased by 50% or more. Phosphorus was moved deeper into the soil profile, helping to decrease dissolved P losses.

Paletta said chisel plowing breaks up the soil surface, increasing total P losses. “There’s middle ground to avoid having dissolved phosphorus losses,” she said. “That’s where low disturbance manure injection comes in. Nutrients are under the surface to reduce phosphorus stratification along with minimal disruption to help reduce annual soil and total phosphorus losses.”

She recommends choosing multiple practices such as grassed buffers and low disturbance injection to reduce losses throughout the year.

Dr. Francisco Arriaga, associate professor and Extension specialist in the Department of Soil Science at the University of Wisconsin-Madison, explained what happens in soil during winter.

“When soil freezes, ice gets in the pores and blocks them, reducing the capacity of soil infiltration,” he said. “That’s the big problem we have in winter. The mechanics and physical process are different from the rest of the year when soil is not frozen and there’s no ice in pore spaces.”

Arriaga explained a research project to test the effects of liquid manure application in winter. “Liquid dairy manure application increases the amount of energy the snowpack can take up from the sun,” he said. “Solid manure acts as more of an insulator and slows down the melt rate, while liquid manure increases melt rate.” Manure also changes the color of the snow, which changes energy absorption.

There are considerably benefits when manure is applied to tilled ground. “The chisel creates depressions that provide more time for water to infiltrate,” said Arriaga. “Water is stored and has a little bit greater contact with the soil. As it melts, it can go into the soil versus a flat surface where it just runs off.”

Runoff losses are affected by soil surface roughness. Late manure applications result in greater P losses.

Arriaga said overall, losses are greater on no-till ground than on chiseled ground. No-till produced 48% greater runoff compared to chisel, and no-till runoff is dominant during the non-growing season when soil is frozen. Chisel runoff was dominant during the growing season.

“One thing to note is that losses during the time soil was frozen dominated the entire year,” said Arriaga. “When we look at the entire year, the no-till still had greater losses for the year because of the importance of the frozen period and things starting to thaw out. Weather and field management affect the rate of snow melt.”

The reduced losses in chisel tillage are mainly associated with surface roughness and depressional storage capacity during frozen conditions – water has more time to infiltrate.

Arriaga said tillage management decisions should be made based on annual effects and site-specific runoff characteristics. Due to variation from year to year, what worked this year might not work next year.

“Manure management doesn’t have to be complicated,” said Arriaga. “Apply manure to meet phosphorus needs. Soil test phosphorus (STP) is important; frequent soil sampling is important. Inject manure whenever possible at reasonable rates. Apply manure when soil temperatures are below 50º helps save nitrogen. Avoid applying manure to frozen soil and use necessary setbacks from streams and other landscape features. Avoid application to fields with slopes greater than 6% when the ground is frozen, and keep in mind that conservation practices such as buffer strips and setbacks help keep nutrients in the field.”

by Sally Colby