This article was originally published in the November 2018 issue of the IMMAG News Update and has been reproduced here with permission.

As we deal with frozen soils, switching to surface manure application, and the challenges of dealing with manure application during colder temperatures we need to be mindful of our application practices. While these application conditions don’t necessarily increase nutrient loss, they do increase the risk of potential loss.

Nutrient movement is always driven by water movement - so what makes winter application riskier? Winter often has a mix of freezing and thawing, and these freeze-thaw cycles affect soil structure, infiltration, and water movement. Often what happens is freezing and thawing break up surface soil aggregates and cause a crust to develop. This makes it more difficult for water to move through the soil and makes it harder to resist erosion, but it’s really not as cut and dried. In 1955 they identified four types of frozen soil structures: concrete, honeycomb, stalactite, and granular. Of these structures, it’s the concrete structure that really slows water infiltration. Unfortunately, this is also one of the more common structures to develop, especially if soils are wetter when they freeze, which is what we normally see after a few freeze thaw cycles that have left soils close to saturation. This is one of the reasons if winter manure application is necessary, earlier application (in the winter) is safer, because typically they tend to be a bit drier than they would be later in the winter.

One thing all the studies of manure application to frozen ground have in common is variability. Every situation is different - the weather, the soil, the manure characteristics all play a role. However, what we do know is the response is typically driven by the hydraulic conditions. If rapid snowmelt or rainfall is imminent, don’t apply.

Best management practices for winter manure application include applying to level ground and where soil erosion is controlled. If you do need to apply, timing and weather conditions are two of the most important factors affecting the amount of manure nutrient we lose. Nutrient loss requires something to move the manure nutrients from the field to a water body; this is usually either snowmelt or a rainfall event onto the frozen soil. If these events are small, nutrient losses tend to be low; if it is a larger runoff event then nutrient losses are higher. In general, the more time that passes between the manure application and the first runoff event, the less risk of environmental impact from nutrient transport. This means watching the weather forecast and avoiding manure application for a few days before anticipated snowmelts or rainfalls can make a big difference in limiting manure nutrient loss.

Additional recommendations include incorporating the manure when you can, avoiding areas of concentrated flow such as waterways, ditches, or similar areas, using setbacks from sensitive areas like stream banks, sinkholes, and similar, and if possible avoiding application near areas that drain to surface tile inlets. If these areas can’t be avoided, add protection around drainage tile intakes to prevent entry by manure or runoff water.

In summary, a few things you can do are:

• Take into account soil and weather conditions and make the best decision based on the conditions you face
• Avoid applying before a snowmelt or rainfall event
• Apply to areas of level ground or soil erosion is controlled to help prevent nutrient transport
• Follow appropriate setback distance requirements
• Sleeve surface tile inlets to prevent runoff from entering tiles