Getting your nitrogen fix from manure a slow and steady process

New research says standard formulas overstate 
the amount of nitrogen released

Large truck in a field spreading manure

Standard calculation formulas overestimate the amount of nitrogen that is available to crops the year after solid manure is applied to the field, researchers with the National Centre for Livestock and the Environment (NCLE) say.

Don Flaten and Wole Akinremi say the formula that is used in Manitoba is based on the assumption that 25 per cent of the organic nitrogen in manure will be available for crop use in the first growing season after the manure is applied.

“Not only are we seeing lower availability, but it is also generally slower, with small amounts of organic nitrogen from repeated applications gradually becoming available over time,” said Flaten in an NCLE release.

“If conditions are right, there is potential for these regularly manured soils to provide a substantial amount of N for crop use. For the perennial rotation at our site, total nitrogen uptake under annual N-based solid manure applications at year six is now comparable to synthetic fertilizer.”

But he warned that doesn’t always happen.

“Nitrogen release from solid manure is a biological process and it can be highly variable and unpredictable from one site to another or from one year to the next.”

In a separate study where nutrient release was monitored for three years following beef cattle manure application to cropland, manure nitrogen availability in year two was overall superior to urea fertilizer (46-0-0) when applied to perennial crops at one site. However, at another site, there was no apparent benefit from the manure in the second or third years following application to annual cropland.

Adding to this field variability is the variability of manure itself. For example, use of extra straw in particularly cold or wet winters increases the carbon content of manure. When there is a high amount of carbon relative to nitrogen, microbial breakdown of the manure may tie up nitrogen that would otherwise be available to crops. This can delay the release of nitrogen from months to years.

Akinremi said there is a bright side to this slower release. “Solid manures containing a large amount of straw may reduce the risk of nitrate leaching, particularly in a perennial system where the established root system can intercept nitrates before they move below the root zone. We observed this in our perennial plots where nitrate leaching with manure was small and no different than from plots that were not fertilized.”

Akinremi and his team are focusing on how to better predict the release rate of nitrogen from different solid manures. “By understanding how these biological processes respond to different soil and weather conditions, we should be able to better estimate nitrogen availability,” says Akinremi.

Their main finding so far is that the breakdown of organic nitrogen into plant-available forms was much slower for the clay soil than for the loam-textured soil used in the study. Nitrogen release rates were less than half of the 25 per cent estimated release rate used in Manitoba’s standard formula.

Based on these findings, Akinremi tested a modified formula for calculating N-based manure application rates using a 12 per cent estimated release rate of organic nitrogen. His team assessed nitrogen availability and crop response for dairy manures applied according to the new formula compared to the standard calculation.

While overall they observed trends in higher grain yield, nitrogen uptake and nitrogen use efficiency with the revised formula, they advise to proceed with caution. With only one year of field data in a year that had greater-than-normal yields, the results are preliminary.

Negative consequences

Also important to keep in mind is that high rates or repeated applications of solid manure to the same field can have negative consequences. Compared with nitrogen, solid manures are relatively rich in plant-available phosphorus and potassium. High levels of soil test phosphorus pose an increased risk to surface water quality while elevated potassium levels may pose a risk to cattle health and nutrition.

After six years of annual N-based applications of solid manure at both long-term sites, soil test phosphorus in the upper layer of soil has risen to levels that would trigger a shift from N-based to P-based manure management practices. This increase did not occur with intermittent applications of manure based on crop removal of phosphorus.

At the NCLE site soil test potassium increased over time in proportion to the amount of manure applied. While this increase could be beneficial for potassium-deficient soils, it could induce nutritional problems in cattle if forage is grown on soils with excessive amounts of potassium.

From an environmental perspective, Akinremi’s study is designed to assess the risk for nutrient leaching below the root zone in a sandy loam soil. Flaten’s study on heavy clay soil at the NCLE is mostly focused on nutrient accumulations in soil, rather than losses.

Their research receives financial support from Manitoba Beef Producers, Dairy Farmers of Manitoba, Manitoba Pork Council, the Canada-Manitoba Agri-Food Development Initiative (ARDI), the Manitoba Rural Adaptation Council (MRAC), and the Canada Foundation for Innovation (CFI).

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