Tuesday, January 25, 2022

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.