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To till or not to till? Phosphorus trade-offs in manure application methods

Progressive Dairy Editor Karen Lee Published on 01 July 2019
manure application

Farmers have been adopting no-till cropping methods to reduce sediment loss. However, tillage has been touted as a way to incorporate manure into soils to minimize nutrient loss.

“There are significant trade-offs at work in till versus no-till,” Clinton Church said at the Midwest Manure Summit, Feb. 27, in Green Bay, Wisconsin.

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A research chemist for the USDA Agricultural Research Service, stationed in University Park, Pennsylvania, Church pointed out the pros and cons to both methods.

Tillage

Pros:

  • Reduces nitrogen volatilization loss to the air
  • Reduces soluble phosphorus loss because the manure is off the surface where the rain can’t wash it away
  • Reduces odor
  • Reduces leaching

Cons:

  • Increases erosion soil loss, including the erosion of phosphorus as those particles are attached to the soil

No-till

Pros: 

  • Reduces erosion of sediment
  • Reduces erosion phosphorus loss

Cons: 

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  • Increases nitrogen volatilization to the air when manure sits on the surface 
  • Increases soluble phosphorus loss
  • Increases odor 
  • Increases leaching

“In terms of phosphorus loss, no-till is a net benefit,” Church said. “However, the trade-off is it increases problems for the application of manure.” 

Church and his colleagues, Peter Kleinman, Curtis Dell and Ray Bryant, have researched various manure application methods as a means to get manure off the surface without resorting to a full-tillage scenario.

“My colleagues have put a lot of effort into testing different kinds of liquid manure injectors that get manure below the surface and are still considered no-till even though it is minimal disturbance and not zero disturbance,” he said.

In one trial conducted in Maryland and Pennsylvania, they compared broadcast application, broadcast with immediate tillage, aeration, shallow disk injector, shallow disk injector with anti-leaching sweeps and high-pressure injection. Manure was applied at 6,000 gallons an acre.

With the broadcast application, the manure remained on the surface of the soil.

The aeration unit punched holes in the ground and then applied the manure. Some of it ends up on the surface and some falls in the holes.

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The shallow disk injector has a cutting wheel to slice the ground open, a pipe deposits manure in the trench, and closing wheels squeeze the soil shut. The manure is in these slots, and very little is on the surface.

A newer disk injector was outfitted with wings to reduce soil leaching. The wings travel below where the manure is injected to break up and close off macropores that have been found to drain to tile lines and groundwater supplies.

The high-pressure injection system blasts manure down a hole and spreads it out about 3 inches below the surface.

Compared to the control, which was no manure injected at all, the chisel plow (broadcast with tillage) left very little residue cover on the surface. The high-pressure and disk injectors were comparable to the control for residue cover, and the aeration and surface application had more residue on the surface after the application than what there was before.

As far as ammonia loss, the surface application had the most lost to the air through volatilization. All of the other applications had less ammonia loss, with chisel plow and disk injection as the lowest losses.

“When you look at ammonia, the more manure that’s on the surface, the more ammonia gets lost to the atmosphere,” Church said.

Odor results were similar, but not identical to ammonia results. “Obviously, with surface application, you smell it a lot. When you get it below the surface, you don’t smell it so much,” he said.

For dissolved phosphorus, the more manure left on the surface, the more that can end up in runoff.

In order to analyze an economic benefit to these systems, the researchers conferred with their colleague Al Rotz, who developed the Integrated Farm System Model. This model tries to take into account everything that happens on the farm. They found the total cost for all application methods to be similar, however, there were differences in net return.

“A lot of those differences were in fuel,” Church said, noting the more times you have to drive the field, the more it costs you.

“Unfortunately, the trade-off of decreased runoff with manure injection has also been shown to increase leaching,” Church said.

Injecting manure into soils filled with macropores enhances the amount of phosphorus lost to leaching.

The disk injection with anti-leaching sweeps was found to have considerably less phosphorus lost in leachate, but there wasn’t much difference in nitrogen lost through leachate.

“There are significant trade-offs with all of the manure application methods,” Church concluded.

All forms of application have pros and cons, but no matter what method is used, too much phosphorus will build up in soil over time.  end mark

PHOTO: Disk injection was one of the methods researchers tested for nutrient losses in a minimal tillage scenario. Photo by Karen Lee.

Karen Lee
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