Certainly one discussion point when we talk about using manure as fertilizer is that we want to try to match manure nutrient availability to crop nutrient need and demand. At first glance this might seem like well nothing is growing in the winter, why would we put manure on then? Although some reasons may exist, such as well its better than having a storage overflow, or it reduces some compaction risk especially if we’ve had a wet fall or may have a wet spring, but I think we can all recognize that it some ways the risk of nutrient loss may be higher. However, that means we need to really know and understand what causes the risk on how to best minimize it if winter manure application is necessary. So what science is out there?
Let’s start with this, if it is risky why might we still practice winter manure application? In my mind there are probably three driving factors for this: 1. Reduced manure storage structure size/management of the storage structure, 2. Time available for manure application, and 3. Compaction risk. Constructing a storage is an expense, it may offer vale in that it improves our manure application timing, but accounting for this value can be difficult. Spring and fall are busy times on the farm and in sometimes we find ourselves in a situation with more hours than work and as winter generally has less demands on our time, it becomes a manure application window. Finally, frozen soils offer more support for heavy equipment and thus reduce compaction risk.
The downside of winter manure is risk of nutrient transport. Nutrient movement is always driven by water movement – so what makes winter application riskier? Well, given the last week winter might seem cold, but generally winter is a mix of freezing and thawing, and these freeze-thaw cycles affect soil structure, infiltration, and water movement. Often what you see happen is freezing and thawing break up surface soil aggregates increasing crusting (or even getting the top layer of soil filled with water and freezing solid). This make is 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 as that, way back 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. This is one of the reasons that if winter manure application is necessary, earlier application (in the winter) is safer, because after several freeze-thaw cycles soils tend to be wetter from the snow water they have infiltrated.
One thing that 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 that the response is typically driven by the hydraulic conditions. If rapid snowmelt or rainfall is imminent, don’t apply. Runoff will move manure nutrients. Anticipate problems early in the winter and get manure on then, especially before snow cover develops or before freeze-thaw cycles cause the soil to get wetter and refreeze. If applications must be made later in the winter, choose fields that get the manure closer to the soil surface and look for weather conditions when rain and melt is not imminent.