Recipe for Keeping Clean Water When Applying Liquid Manure?

At forty-two cents per pound of actual N, nitrogen from manure is looking pretty good. Liquid manure is especially valuable from a nitrogen perspective. However, with the application of liquid manure also comes the increased risk of “preferential flow”. Preferential flow is commonly referred to as macro-pore movement.

Preferential flow is the direct movement of liquid materials through large cracks or pores (earthworm or old root channels) in the soil. In Ontario, movement is most often to tile drains. From 1988 to 2001, 44% of the MOE reported spills impacted field tile. However, preferential flow can also occur to groundwater - either through natural cracks in the bedrock in shallow soils, or through “shallow to the water table” soils. The end result with preferential flow is contaminated water. Contaminants could include nitrates, ammonia and pathogens.

Preventing Preferential Flow

The conditions that favour preferential flow include:

• tiles that are flowing, but soils are not saturated above the tiles
• application of high volumes of liquids, and
• application to soils with excellent tilth, and many macropores.

There are some guidelines that will help prevent preferential flow occurring at the time of application.

Options for Application on Tile-Drained Soils

• Pre-tillage, done up to a week prior to application, is the best option because it will break the macropores in the soil and increase soil surface area for infiltration.
• An application rate of less than 3,600 gal/ac (40 m³/ha) will be low enough to reduce the risk of movement to tile drains, even if pre-tillage has not occurred. This option is often the preference for fields where no-till is practiced. Two applications up to this rate could be applied several days apart if one low application doesn’t provide all the required crop nutrients.
• Tile drains are monitored and appropriate action is taken if contamination occurs. Appropriate action includes stopping application, blocking outlets (for at least 72 hours) and removing contaminated water (e.g. by using a vacuum tanker)
• A suggested monitoring schedule that observes colour (compared to pre-application) of the water discharging from the tile drainage system:
  • Once 10-20 minutes following the start of manure application, and
  • Once each hour when application rate is greater than 90 m³ or 20,000 gallons per hour, or once for each 20,000 gallons applied up to one hour after completing application.
  • Stop application immediately if discolouration is observed and implement the contingency plan.
• An alternative to manual monitoring could include continuous monitoring using an automated system capable of detecting contamination in tile flow, and of signaling the operator when contaminates are detected
• Apply liquid manure/organic material over a “representative” tile and observe the tile outlet for manure.
• Treat tile effluent to remove contaminant (e.g. biofilter, dispersion sandwich, retention/irrigation pond)

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Options for Application on Shallow to Bedrock Soils (without tile drainage)

• No manure should be applied on or within 10 ft (3 m) of exposed bedrock. The risk of contamination in these areas is the highest.
• On soils with less than 6 inches (15 cm) of soil over bedrock, liquid manure application is not recommended. Solid manure should be applied at less than 10 ton/ac (22 t/ha) between June and September.
• On soils with 6 - 12 inches (15 to 30 cm) of soil over bedrock, liquid manure should be applied to pre-tilled soils or at low rates (<3,600 gal/ac) between June and September. Solid manure should be applied to pre-tilled soils or at rates under 20 ton/ac (45 t/ha).
• On soils with 1 - 3 feet (30 – 90 cm) of soil over bedrock, liquid manure should be applied to pre-tilled soils or at low rates (<3,600 gal/ac).

Options for Application on Shallow to Groundwater Soils (without tile drainage)

The most difficult aspect of liquid manure application to soils with a high water table is knowing the depth of the groundwater at the time of application. Water table depths can fluctuate significantly, depending on the season. In Ontario, the water table is usually highest in the spring or fall. Depth to water table can be assessed by:

• digging a hole in June or September and observing the depth to free water in the hole
• using soil colour features (rust spots and blue-grey colours in soil layers) and the soil drainage method to assess drainage class
• referring to a local soil map to assess drainage class (e.g. imperfect drainage would tend to indicate groundwater between 60-90 cm while poor drainage could indicate groundwater between 30-60 cm.)

Options to Prevent Preferential Flow to Groundwater Include:

• No manure application should occur on soils less than 30 cm from groundwater. Generally, manure application in these conditions would leave ruts in the field.
• On sandy soils 1-2 feet (30-60 cm) from the water table, liquid manure should not be applied. Solid manure should be applied to pre-tilled soil, or at less than 20 ton/ac.
• On all other soils shallow to groundwater, liquid manure should be applied to pre-tilled soils and/or at reduced rates. Heavy clay soils have no suggested restrictions when groundwater is greater than 30 cm from the surface.

Avoid the application of liquid manure onto wet soils or during wet weather. For example, avoid application when water is flowing from tiles and when heavy rains are forecast within 12-24 hours of spreading. Where possible, incorporate manure as quickly as possible when applying under the threat of thunderstorms. Contaminants from solid manure can reach tiles or groundwater when rainfall occurs shortly after application.

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