Energy Embedded in Fertilizer and Impacts on Manure Carbon Management

 

Last month we looked at how a few different manure management decisions impacted carbon cycling in the manure. This time, we are going to look at a different aspect of carbon, that being energy. While we often think about the economics of fertilizer, with the greater emphasis on climate change, greenhouse gases, and sustainable energy, it is important to look at how different manure management decisions impact energy from a fertility perspective.

 

Obtaining fertilizers takes energy. In the case of phosphorus and potassium, this energy is related to the difficulty of mining and processing the fertilizers (for P₂O₅, this is 5600 Btu per pound, and for K₂O, it is 4700 Btu per pound). For nitrogen, it is the energy required to synthesize nitrogen from diatomic nitrogen in the air (18,100 Btu per pound of N). Considering what this means in terms of carbon emissions, it implies every pound of P₂O₅ is worth 0.65 lb CO₂, a pound of K₂O is worth 0.55 lb CO₂, and a pound of N is worth 2.1 lb CO₂.

 

Let’s look at what this implies for beef finishing farms employing different manure management.

 

Table 1: Summarizes the N, P₂O₅, and K₂O contents of typical manure from different systems and what this means for differences for CO₂ use.

	N	P2O5	K2O	CO2	CO2/savings
Manure System	lb	lb	lb	lb	Savings per Head
Bedded Pack	44.5	25.9	26.4	125	34
Deep Pit	42.7	26.8	37.3	128	37
Open Lot	30.0	21.8	24.5	91	-

 

If we compare these options by holding onto the manure’s fertilizer value, we save approximately 34-37 lb of CO₂ per animal in extra nutrients that have been saved in manure. While this is important and useful, one thing to keep in mind is that the carbon cycle itself accounts for around 500-1000 lbs of CO₂ per pig space per year, which is a fair amount more than the energy in the fertilizer value of the manure. However, some of that CO₂ was recently recycled from the atmosphere to plant material and then through the animal. In terms of energy not used to make or obtain new fertilizer, carbon that has not been emitted represents a true change in the carbon cycle.