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Managing phosphorus-based manure inputs

Managing manure requires  an agronomic as well as an environmental perspective.

November 22, 2007
By Bruce Barker


Farmers in Manitoba do not have to be reminded of the environmental concerns about manure and fertilizer applications. Huge algae blooms that covered almost all of Lake Winnipeg generated headlines and public concerns over the main culprit, excess phosphorus (P) in the watershed. Where the P came from was not the issue for sun-worshipping urbanites on the beaches of Lake Winnipeg. Rather, they saw a problem and wanted it fixed.
“Fifty three percent of phosphorus in Lake Winnipeg comes from jurisdictions outside Manitoba, such as Saskatchewan and the US,” says Don Flaten, a professor in the Department of Soil Science at the University of Manitoba. “When you look at the sources of phosphorus in the watersheds over the long-term, approximately one-third comes from cities and towns, one-third from agricultural sources and one-third from natural sources.”

Flaten explains that research has shown that the concentration of P in most Manitoba rivers and streams is increasing, and the problem for Lake Winnipeg is compounded by lower flow rates in the Saskatchewan River system. He says there is a substantial increase in the P concentrations in the Red River and Assiniboine Rivers, which account for more than 60 percent of the increasing P levels found in Lake Winnipeg.

With the goal of reducing P concentrations in watersheds, two new sets of regulations are in the works to regulate the application of nutrients onto agricultural land in Manitoba. One set of proposed regulations is meant to restrict the application of manure phosphorus from livestock operations in Manitoba, through amendments to the Manitoba Conservation’s Livestock Manure and Mortalities Management Regulation (LMMMR). The other set is much more comprehensive, designed to prevent over-application of N and P from all sources of nutrients (livestock manure, commercial fertilizers and municipal bio-solids) on all agricultural land in Manitoba, through the establishment of Water Quality Management Zones (WQMZ) under Manitoba Water Stewardship’s Water Protection Act (WSWPA).


The basic approach that the regulations take is that for long-term sustainability, nutrient application from all sources should equal nutrient removal by the crop, given that the soil is in a relatively ‘balanced’ state. Generally, crop removal and replacement of P over the last 40 years has reflected that goal, until the mid 1990s when the amount of P applied has been greater than crop removal in some areas of Manitoba, mostly because of manure application.

What are the key changes?
While the LMMMR deals with manure-P application, the WSWPA looks at the sustainable application of all nutrients. In order to prevent over-application of nutrients, the regulations focus on restricting nutrient management practices according to soil test concentrations, along with a rating of the land’s crop productivity such as nutrient removal capacity, and susceptibility of transport of excess nutrients to surface or groundwater. Flaten likens the degree of restriction imposed by the regulation to that imposed by a traffic light, where green indicates minor restrictions, yellow indicates significant restrictions and red signals no application allowed.

WQMZ subsequently lay out the amount of nutrients that can be applied to that soil, based on the crop grown and the land’s productivity. The zones are based on the Canadian Land Inventory Agricultural Capability Classes.

Examples of the proposed limits for nutrient application are shown in Table 2 for N application in the water quality management zones, and P restrictions in Table 3 (as of November 2006, these restrictions already apply to manure P).

Flaten says there are two possible exemptions to these standard tables of limits for farmers. An exemption is currently possible if the farm has a registered manure management plan that addresses all sources of nutrients applied to the land, including livestock manure, synthetic fertilizers and municipal wastewater sludge, and demonstrates that N and P are not
being applied in excess of crop needs. Under a new proposal, an approved nutrient management plan for other sources of nutrients would also offer an exemption.

For farmers in the flood plains of the Red and other rivers, winter application of manure is prohibited. Also, if manure is applied in these areas during the fall, manure must be incorporated within 48 hours of surface manure applications, or applied with injection on tilled soils. Zero-till and perennial forages are exempt.

More land required for manure application
The challenge for manure applications is that nutrients in manure do not match crop requirements. Generally, crops use at least four times as much N as P, while the N and P concentration in manure has three times as much N as P, or less. As a result, if the manure is applied to meet crop N needs, P is applied at greater levels than the crop requires, resulting in an accumulation of P in the soil. This extra P ends up overloading the soil’s capacity to retain P, resulting in off-site movement of P into the watershed.

“Some farmers will be challenged to move from N-based to P-based applications. For cropland, you would need about two to four times more land to meet the P-based requirement. For grazing land, P-based manure applications would require 20 to 40 times more land, due to the low net rates of removal by cattle,” explains Flaten.

Flaten says that unless extraordinary measures are taken to increase N:P ratios in manure, most fields that receive annual applications of manure at a N-based rate will eventually reach the soil test P thresholds in the proposed regulations. When a field reaches the 60ppm or 120ppm threshold, the P-based rates of manure that can be applied will probably be much lower than what is normally applied according to a crop’s N requirements.

While the regulations will be an additional challenge for farmers, a positive side effect is that the livestock industry will develop more effective manure application and management methods. For example, better application methods will need to be developed to reduce N losses, which effectively results in the raising of the N:P ratio. With today’s high synthetic fertilizer prices, there will be a greater opportunity for livestock producers to apply manure on land owned by crop producers.

However, meeting those challenges will not be easy, as a wide range of issues still need to be addressed. Flaten says that some Beneficial Management Practices (BMP) imported from other geographies did not work in Manitoba. Most of the P lost from prairie soils is in the soluble form, like phosphorus ‘tea’. As a result, the concentration of P in Manitoba rivers is associated mainly with soil test P concentrations, not the rate of soil erosion as has been
found in the eastern US and Canadian jurisdictions where erosion of soil particles is the most important contributor of excess P found in watersheds. Additionally, research in Manitoba could not prove that no-till is better than conventional tillage in holding P back from watersheds, although more research is required in that area, and vegetated buffer strips have been found to reduce P contributions by only four percent.

Flaten says the most important and reliable BMP, which prairie farmers should focus on, is to balance the removal of P with the application of P because scientists have not identified how to intercept excess P to prevent it from moving into the watershed. This BMP, though, brings an entire new set of challenges. Soil sampling results are variable by location, depth and time of year. The Olsen-P analytical method is proving to be the most reliable test for estimating P loss in runoff for Manitoba soils. The Olsen test is also the test used to define Manitoba’s P regulations, but some laboratories use other methodologies. Accurate manure
analysis is also a challenge, as is manure use efficiency.

For farmers in Alberta and Saskatchewan where regulations have not focussed on P-based manure applications, likely the best advice, from an agronomic and environmental perspective, is to also try to balance crop needs with nutrient application rates. While there is more flexibility in P application in these provinces, from an environmental perspective, following a needs based approach is preferable. This strategy will help ensure the long-term viability of the farm if or when P regulations are eventually introduced.

“Regulations are coming to Manitoba for manure and fertilizer. The best advice is to use common sense for applying P because at this point, no other effective BMPs for our situation are yet proven,” says Flaten. “P balance should be the priority for producers now, but that won’t be easy for everyone, especially if there is little land available for manure application near the farm.”