With
the very idea of the topic, questions begin to emerge: what, why, and how to use the technology. We
will try to walk through a few of these questions to address what we do and
don’t know. With today’s technology, including things like GPS location
guidance, flow controllers, or weigh scales on manure spreaders it is possible
to make maps of how many gallons per acre are applied.
In
terms of solid manure, where decisions are often made based on phosphorus
management, grid sampling can be used to determine current soil phosphorus
levels. A map is generated of how much phosphorus we want to add to hit a
certain level and this prescription map used to determine manure application
rates on the go. Some current equipment even has the capability of using these
prescription maps on-the-go to change the rate as you move through the field. This
is effectively how variable-rate of application other commercial fertilizers
has been done for a while, but there are some additional challenges with
manures.
However,
the question when using manure as the fertilizer source substantially increases
these questions. Things like how accurately do we know the manure nutrient
content, how variable is the nutrient content during application, how
accurately can you hit rate, how uniform is the application, how good is the
application method, and we are left with questions about if we can control
these variables accurately enough to make variable rate application pay . If we
try to extend this to nitrogen, it can get even more complicated as we now need
to consider additional factors such as the quality of injection/incorporation
throughout the field and its impact on ammonia volatilization and the variation
in nitrogen mineralization and variability. This is to say, getting a firm
grasp on these details would be the first step towards working towards a variable-rate
manure application.
In
terms of variable rate nitrogen application with manure, the first step would
be determining what we parameter we want to vary nitrogen application rate
based on. Some ideas that have been
proposed, include previous year’s yield maps, soil type, or soil organic carbon
levels. Two weeks from now we will take a closer look at each of these
potential methods, why they may be considered, and science available behind how
well it works.
