Friday, 31 March 2017

Food from the land - Annual crops

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.

In 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 the plant.

The 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:

35788 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

Other crops
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.

Previous Page: Food from the land - Growing domesticated crops 
Next Page: Energy from the land - Photovoltaic solar panels

No comments:

Post a Comment