How long can manure provide value to farmers’ fields after application?
The short answer is that it depends on what you mean by ‘manure,’ and how and when you’re applying it, according to a 10-year study from the University of Manitoba’s National Centre for Livestock & Environment (NCLE).
NCLE has been evaluating the short- and long-term fertility value of manure, comparing synthetic fertilizers and manures on a perennial and annual crop rotation.
For the first phase of the study, from 2007 to 2015, researchers applied different sources of manure annually — liquid and solid pig manure and solid dairy manure — as well as synthetic fertilizer, which they applied yearly or intermittently.
The yearly applications were designed to meet crop nitrogen (N) requirements. The intermittent applications were to meet crop N requirements in the first one or two years and then a synthetic fertilizer was used from that point on, to try and use up some of the excess phosphorous (P) applied. All manure was fall applied, except for one spring application.
In the second phase of the study from 2015 on, the researchers stopped applying any form of nutrient in some of the plots that had received yearly applications of manure or fertilizer to see how much N was coming out of those historic treatments in subsequent years, and also the P drawdown rates.
Don Flaten, a University of Manitoba soil scientist, presented the data at the recent Manitoba Agronomists Conference in Winnipeg, where he dove down into the data.
“We tried to estimate the available N in the manure that we were applying according to the traditional formula in Manitoba,” Flaten said.
That formula takes into consideration the expected N losses due to volatilization and also winter losses from applying in the fall. What they found is that what form the manure came in had a dramatic effect on how it worked in the field.
Not all manures are equal
Different manure sources contain different types of N. Liquid manure is largely ammonium N and solid manure is primarily organic N. Liquid manure has a higher N-to-P ratio than solid manure, especially solid pig manure.
“You can anticipate if you’re going to meet crop N requirements, you’re going to be putting on a lot of P with solid pig manure,” said Flaten.
In the annual crop system, researchers indexed crop yields according to synthetic fertilizer response and although the liquid pig manure treatment took awhile to get going, within a couple of years, it was matching the yield response to the N that the synthetic fertilizer was providing. The solid manures were providing little or no yield increase because the carbon in the straw bedding in both the solid dairy and pig manure slowed down the release of N.
In Phase 2, when the team began adding N fertilizer to solid manure applications to accelerate P drawdown, crop yields increased.
“The combination of that historic solid manure with some immediately available synthetic N fertilizer, was excellent for the annual cropping system,” Flaten said.
In the perennial cropping system, liquid pig manure increased dry matter yield compared to the synthetic fertilizer and exceeded expectations in terms of keeping down N losses.
“We had fewer losses than we expected,” Flaten said.
The solid manures performed more poorly in the perennial rotation than the annual crop rotation.
“It isn’t until we get annual crops in there and start tilling up that land, mixing up the solid manure and exposing it to microbial processes that we see the solid manure is starting to take off and release N but it’s taken us years to get that far,” Flaten said. “When we look at the solid manures that are converted over to a synthetic fertilizer system in the second phase to try and draw down their P soil test levels, the treatments with the solid manures take off; they’re very responsive plots, but they just weren’t getting the N that they needed.”
Solid pig manure treatments had the largest increases in soil test P during the first phase of the study.
“Our intermittent manure applications didn’t change the soil test P at all,” Flaten said.
The first threshold for P in Manitoba’s regulatory system is 60 ppm P, which is four times the average soil test P in the province.
“We exceeded that threshold with the yearly applications of solid dairy and liquid pig manure after like five or six years (in the annual crop rotations). But if you really want to jack up your soil test P level, put on the solid pig manure at an N-base rate and we could get that soil test P level all the way up to nearly 180 ppm within eight years. That’s because it’s got such a low N-to-P ratio, so you’ve got to put a lot on with the hope that it’s going to supply N, which it doesn’t really do.”
In the perennial crop system, yearly application of solid dairy and liquid pig manure exceeded 60 ppm after six or seven years, similar to the annual crop system.
“But if we went on with our solid pig manure with that super-low N-to-P ratio, we could exceed 180 ppm,” Flaten said. “So once again there were super increases in soil test P from the solid manures in particular. Intermittent applications are not a problem, but do it year after year and you can just see your soil P testing going very high.”
Phase 2 looked at the residual benefit of applying manure in terms of N release after eight to 10 years.
The plots that had repeated daily applications of manure built reserves of organic N, and it (eventually) came out, and paid dividends going forward in the future, but the amount and timing of that nitrogen release varied. It depended on the cropping system.
“There were big differences between annual and perennial systems, with much faster release of that organic N in the tilled annual crop system compared to the untilled perennial grass forage,” Flaten said. “We ended up with a lot more mineralized N coming out of the annual crop system where the historic applications had occurred.”
In 2016 and 2017, the researchers stopped applying any manure or synthetic fertilizer in most of the plots and measured the effect on yields compared to a control that had never received any manure or fertilizer since 2007.
The long-term control plots yielded an average of 15 bushels of wheat and 15 bushels of canola. With continuous synthetic fertilizer, plots yielded 57 bushels of wheat and 60 bushels of canola. The year after discontinuing synthetic fertilizer, the yields were 44 bushels of wheat and 36 bushels of canola.
“We could recover 60 per cent of the synthetic fertilizer in the first eight years, and in years nine and 10, we’re still getting benefits from that synthetic fertilizer,” Flaten said. “Forty per cent of that synthetic fertilizer wasn’t lost; a large proportion was immobilized into the organic matter and it’s continuing to pay dividends.”
Those dividends, though, aren’t typically as large as the dividends that come from historic applications of manures. All the treatments – liquid pig manure, solid pig manure and solid dairy manure, yielded as well as the continuous synthetic fertilizer treatment.
“The historic investment in the manure and fertilizer is paying dividends later on in terms of N mineralization in the annual cropping systems,” Flaten said.
The two-year cumulative yield, in the annual cropping system, was three to four times more in the discontinued treatments as the long-term control. “It’s important to consider the long-term benefits of a well-managed crop nutrition program,” Flaten said.
In the untilled, perennial crop system the N mineralization and yield numbers are lower than in the annual system, but the same principles apply.
“Tillage accelerates mineralization whether it’s the soil’s ancient organic matter or it’s residual benefits of organic N contributed from synthetic fertilization or livestock manure applications,” Flaten said. “Mineralization is dramatically influenced by the cropping and tillage system.”
What about the P?
The annual crop treatments that had the largest increases in soil test P in Phase 1 of the study had the largest decreases in Phase 2.
“The faster the soil test P went up, the faster it came down,” Flaten said.
The solid pig manure treatment went up to 180 ppm at one stage during Phase 1, was 176 ppm at the start of Phase 2 and two years later it was 119 ppm, a loss of 57 ppm of P.
“If we accounted for that only by crop uptake, we’d have to have grown about 2,500 bushels of canola over that two-year period per acre,” Flaten said. “A lot of that drop in soil test P is due to the stabilization of the P. It’s changing chemically from a more available form to a less available form.”
The same thing happened in the perennial cropping system, with around a 60-ppm decrease with the solid pig manure treatment over the same time period.
Adding urea fertilizer did not accelerate P drawdown.
“Because we had such good yields from the manure treatments without any fertilizer, we didn’t need N fertilizer to accelerate P removal because we had all those treatments giving so much mineralization of the historic applications,” Flaten said.
In summary, N release from liquid manure’s short-term N availability generally matches the estimates of the traditional formula. In some cases, it exceeded expectations and N losses weren’t as large as predicted. Solid manure’s short-term N availability is much less than predicted from the Manitoba traditional formula.
“Our soil test-based recommendations for fertilizer manure application pose little risk of excess residual nitrate,” Flaten said. “In cases where the liquid pig manure provided more N than expected, we ended up with more N left over in the fall and we downgraded the rate that we applied the next year. An annual fall nitrate testing can basically deal with all these issues.”
P released from manure is rapid. In early-application manure, the N-based rate raises soil test P very quickly. Intermittent applications of manure reduce or eliminate the risk of excess soil P.
The effect of manure on changes in soil test P varies with the year and manure source. It is quite variable and not very predictable.
N mineralization is high after discontinuing yearly applications of manure especially in annual cropping systems where tillage accelerated the mineralization process, and the crops didn’t need any additional synthetic fertilizer to match the yields of the continuous synthetic fertilizer treatment.
Drawdown of soil test P was very rapid after discontinuing manure treatments, probably due to stabilization of P in these high P treatments.
Nitrogen fertilizer did not help draw down the P faster because there was so much mineralization coming from historic manure applications.