Tuesday, March 3, 2015
Does better nutrient conservation pay? The story of injection and incorporation
Let’s take the time to talk about another practice – injection and incorporation. So the approach of this little write up will be to explore how the costs of manure management change with the surface application as opposed to injection/incorporation and how the benefit we get from our manure changes.
Let’s start with swine manure. In working these calculations, I’m going to be working with what I consider to be an average swine manure from a finishing operation with a deep pit, something like a 60-30-25 (N-P2O5-K2O) pounds per 1000 gallons.
In this case we are working with a liquid manure and it makes some sense to perform some sort of injection/immediate incorporation rather than just surface applying. One of the reasons to do this is because it places the phosphorus below the ground surface, which helps protect it from surface runoff and keeps it there when we need it. However, the primary reason for injection/immediate incorporation is for nitrogen management. Getting the manure in the ground, mixed with soil, and covered reduces the amount of nitrogen we lose through volatilization.
So let’s work some examples to see how these practices compare. In the case of direct injection of the manure we do a good job of limiting ammonia volatilization, reducing it so it is typically between 0-2% of the total amount of nitrogen we applied. If we had just surface applied this liquid manure we’d expect that between 10-25% of our nitrogen would be lost. So what value does this have at our farm?
Alright, so let’s assume we are working in a corn-soybean rotation and we plan to apply our manure at the maximum return to nitrogen suggested rate. In Iowa this would mean our ideal rate (based on current corn and nitrogen prices) would be 134 lbs N per acre that we want to supply to our crop. In the case of the swine manure we proposed that would mean we’d want to apply about
In the injection system to do this we’d need to apply about 2400 gallons per acre, because we are getting about 56 lbs of available N per 1000 gallons of manure. To calculate this I adjusted the total amount of N in the manure times and availability factor and a loss factor. I used 95% available in this case, based on ISU PM 1003 which suggests swine manure will be 90-100% available, and correct for nitrogen losses during application by multiplying by 0.99, which assumes 1% of the applied N will be lost to volatilization.
134 lbs N/acre / (0.99 * .95 * 60 lbs N/1000 gallons) = 2400 gallons per acre
If we had surface applied without incorporation how would have this changed? Well, we’d still have the same manure and I’d assume the same N availability, but my correction for ammonia volatilization would be different. In this case, I’d assume that between 10 and 25% of the applied N would be lost. We’d have to apply closer to 2900 gallons per acre.
134 lbs N/acre / (0.75 * .95 * 60 lbs N/1000 gallons) = 3150 gallons per acre
So what does this mean? We’ll I had to apply an extra 750 gallons of manure per acre to get the same crop production, reducing the number of acres we cover with our manure. This means we are applying, and wasting, about $18.50 worth of extra nitrogen value.
The second part of this question is how did my costs change? To estimate this I need to do some quick estimating about how this would affect my manure application expenses. It’s not perfect, and every situation is different, but to get a rough cause of the cost I use the figure below.
So, how do you use this graph? Well the blue diamond graph represents injection while the red squares represent broadcast (we could talk about why the graphs have different shapes, but not today). So in the case of injection we said we wanted 2400 gallons per acre, and using the top line I’d estimate an application cost of about $0.02 per gallon, which would give an application cost of about $48.50 per acre. In the case of surface application, we wanted about3150, which has an application cost of around $0.01 per gallon, or about $30 per acre. So, that means injecting cost us about $18 more an acre, while we only saved an extra $18.50 worth of nitrogen value. In this case the value of the conserved nitrogen covered our additional application expenses.
So, what about dairy manure? In this case I estimate dairy slurry will have about 25 lbs N/1000 gallons, which will be about 35% available.
Using the same assumptions as before I’d calculate a manure application rate of 15,000 gallons per area to supply 134 lbs N to our corn crop, while for surface application it would be 20,000 gallons per acre. In this case, I’d estimate an application cost of about $0.011 for injection (total cost of $169 and acre where as in the case of surface application the cost was about $0.0078 per gallon (total cost of $159 an acre).
The extra 5,000 gallons would contain about $19 worth of fertilizer value, once again making injection a great choice to increase the value of your manure and to help protect the environment.