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.
