This post is a part of the series An Acre of Sunshine.
Going along with the main theme of this series, the following post gives some quick estimates about the energy yields that different crops can produce, starting with a more in-depth discussion of corn.
Estimate #1 for Corn - From first principles
Cereal crops are plants that are
grown for their starchy seeds, including corn, wheat, barley, oats, and
others. When it comes right down to it, these are some of the most important plants for feeding
the world. Corn makes a good example, and is one of the most productive
crops per acre, period. The following estimates are for field corn,
which is quite different than the sweet
corn that you have eaten at dinner. It is much richer in oils, yields
much more energy per acre, and is primarily used for animal feed,
ethanol fuel production, as well as thousands of other uses in processed
foods and chemical products.
looking at photosynthesis, we found that our farm has about
36000 kWh/acre/year of basic photosynthetic energy production. Out of
that amount, a plant needs to grow, metabolize, fight off predators, as
well as to create that portion of the plant that is useful to us. In
this case, what we actually want is the kernels of corn on each ear. Research on this subject shows that approximately 50% of a mature
corn plant's energy is found in the kernels on the ears of corn, while the other 50% is in
the stalk, leaves, and root system. This is actually a tremendous
proportion of the energy of a corn plant that is found in the kernels. It is pretty incredible that these plants are able to funnel fully half of their energy into their seeds
and that such a surprisingly small proportion is needed to grow the rest of
other thing to account for is what proportion of the energy that a
plant captures is put towards growth, and what proportion to maintain
the health of the plant as it lives day to day, known as respiration. One source
estimates respiration on a global scale at 20%, so as I didn't quickly find an actual figure for corn, we shall use that number. With this calculation, we get:
kWh/acre/year * .5 (proportion of stored energy in seeds) * .8 (losses
for respiration) = 14,315 kWh/acre/year of harvested corn kernels.
Estimate #2 for Corn - Real world yields
As I was not able to easily find Quebec data, I will instead use Ontario estimates
of corn production to make an estimate. These recent data state that corn yields are typically around
150 to 170 bushels per acre per year of field corn (a bushel of corn is 56 pounds). As our farmland is of a much lower quality than the average farmed acre in Ontario, it could produce perhaps only about 2/3 of the average production. This means that one of our acres could produce:
150 bushels/acre/year * 2/3 (reduction for poor quality land) * 56 pounds/bushel * 1550 Calories/pound * (1 kWh/860 Calories) = 10093 kWh/acre/year
In my last post, I showed a graph that included Calorie yields for many staple crops, and those are easy to convert to our usual unit of kWh. I also found plenty of sources (e.g., here and here) that listed the productivity of crops of all kinds, which often end up being much lower total energy because of how few calories many vegetables have (having high water content, high fiber, low fat). I've put a few of those estimates in the following table.
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