When it comes to selecting the right manure injection tool
for the job there are many variables; the application rate, the amount of power
to pull it, the soil type and conditions, the desired amount of residue cover
left, or even the speed we can pull it through the field. All these constraints
are important to consider, but the one we are going to discuss today is how
much space do we need to create in the soil to have room to get the manure in.
It is intuitive that injection tools that create a larger cavity
below the ground for the manure are capable of achieving good injection at
higher application rates if the soil conditions are right, but they also
require more power to pull, so trade-offs are required. One newer example
injector is shown below. This one uses a fluted coulter to open an injection
cavity.
Figure 1. A fluted
disc manure injector followed by two concave discs. The fluted coulter opens
the injection trench while the discs close and cover the injection furrow.
When it comes to injection, we want no overflow of manure
out of the injection cavity. Two things are to achieve this, the first is that
we must not have overflow manure. Overflow manure is when our injection furrow isn’t
big enough to hold all the manure and as a result, it bubbles back to the
surface. To avoid this the tool capacity has to be greater than the application
rate (we’ll discuss in more detail below). The second thing we have to avoid is
in-furrow manure; this manure stays in the injection furrow like we want, but
we fail to cover up the furrow after putting the manure in it. Avoiding these
two conditions limits the manure from air exposure, keeping odor and ammonia
volatilization low. The example injector shown in figure 1 demonstrates both of
these operations. In this case, the fluted coulter cuts the injection cavity. To
be successful this cavity must be big enough to hold the manure we are putting
down. The two concave trailing discs then cover the applied manure so we can’t
see the furrow. To be successful both parts must be set correctly for the soil
conditions and manure application rate we are trying to achieve. Below (in
figure 2) you can see two examples of manure injection, the one on the left
where the manure is covered, and the one on the right where we coverage of the
injection furrow wasn’t achieved.
Figure 2. Good injections as compared
to in-furrow manure injection.
So how can we determine how much
injection capacity is needed for our manure? Well, it’s based primarily on two
factors, the application rate you are trying to achieve and your tool spacing.
Higher application rates require more capacity, while narrower spacing reduces
required capacity (because each knife has to put down less manure per acre).
Next, we will take a look at the requirement for two reasonable manure application
rates, a swine finishing manure applied at 3,000 gallons an acre and a dairy
manure applied at 12,000 gallons an acre. In both cases, we will assume the manure
injector are on 30-inch centers.
The first thing we need to do is calculate the amount of manure each injector will receive. In the 3,000 gallon per acre
case this is calculated by multiplying the application rate (3000 gallons per
acre) by the tool spacing (2.5 feet), dividing by 43,560 to convert from acres
to feet, multiplying by 0.134 to convert from gallons to cubic feet, and then
multiplying by 144 to get the injection cavity cross-sectional area in feet.
For the swine manure, we need about 3.3 square inches, as the dairy manure application
rate was 4 times as much, four times this much area, almost 13.25 square inches,
is needed.
What does this mean in practice? Let’s assume we are using the
fluted disc (or similar to that shown in figure 1). Based on our soil
conditions (current soil moisture, soil structure, residue cover, and the down
pressure on our toolbar) it is cutting a cavity 4 inches deep by 2.5 inches
wide, is our tool capacity sufficient for these application rates? The tool
capacity is equal to the cross sectional area we are cutting so in this case it
would be 4 inches times 2.5 inches, or approximately 10 square inches, which is
enough for the deep pit swine manure example (3.3 square inches required), but
not enough for the dairy manure example (13.25 square inches required).
So what options are there to increase capacity? A few things
could be done: (1) We could reduce the manure application rate to be in line
with what the equipment can handle (to achieve the desired application rate we
would need to apply twice), (2) we could reduce the tool spacing as this
reduces the amount of manure each tool needs to inject, (3) we could try
running the tool deeper to get a larger cavity, or (4) we could use a tool
with a larger area, potentially a knife or sweep.
