Thursday, July 4, 2013

Energy to Produce Vegetables

 I just read the article  "The Permaculture Solution – an Interview with Warren Brush". It said "Estimates are that the modern agriculture system uses ten calories of energy to produce one calorie of food."

Fearing that I too may be growing in a style of negative net energy. I immediately looked at the energy costs involved with my soil-less gardens and I published an article.
Chris Carr was kind enough to remind me that nutritional calories (KCal) are equal to 1000 chemistry calories (cal).    A chemist would probably use Joules and Kilo-Joules (KJ) rather than KCal.   But the we think of food in terms of KCalories rather than Joules, so I'll keep it in KCal also known as large or nutrition calories as much as I can.  There were other errors in my first draft,  so here's the rewrite.

I get that using the rule of thumb that a soil garden should get about 10 gallons of water /day /100 sq. ft. is not exact, and everything from soil condition, weather, crop, and stage of growth will affect these numbers, but it's the average I want to start with.
Lets say you irrigate 100 square feet of garden with 10 gallons per day, and you are pumping water from let's say 100 feet below the surface. 

To calculate the power used to pump 10 gallons per minute 100'

Pwhp = q h sg / 3960
Pwhp = water horsepower (hp)
q = flow (gal/min)
    = (10)
h = head (ft)
              = (100)
sg = specific gravity = (1)
Pwhp = 10x100/3960 = .2525whp = 0.04497210699688 KCal/sec
This could also be expressed as 188.2892175745  J/sec which is the definition of a Watt.

If you are following along and checking my math you can use the 'Power Unit Conversion' to find the Power.
Energy is what is delivered and Power is the rate at which it is delivered.
In the above example we deliver 188 Watts for 1 minute or 3.138 Watt-Hours, but I'm going to stick with KCal units.

We pump for 1 minute so
0.04497210699688 KCal/s  x  60sec = 2.69 KCal / day to supply 1" of water to soil

Below is a list of common vegetables which I got from.

Tomato can be expected to yield as much as 37500 lb per acre or 37500/43560x100 = 86 lbs per 100sf
80 KCal per lb x 86 lb = 6880 KCal
Lettuce can be expected to yield as much as 27000 lb per acre or 27000/43560x100 = 61 lbs per 100sf
67.37 KCal per lb x 61 lb = 4176 KCal

The yield statistics below come from and

Using the numbers from vegguide I have determined that on average lettuce will take about 50 days to mature and tomatoes will take about 80.


Here is the energy required to pump the water for these two crops till harvested.
Tomatoes - 2.69 KCal x 80 days = 215.2KCal
Lettuce     - 2.69 KCal x 50 days = 134.5KCal

Tomatoes - It takes 215.2 KCal to produce 6880 KCal of tomatoes.  About 31.9 to 1
Lettuce     - It takes 134.5 KCal to produce 4176 KCal of lettuce.      About 31.0 to 1 .


I'm going to use a pump size that is far below what you are likely use to make the best case for soil-less gardening.
If you use a 20 watt pump 5 hours a day.
That's 20 watt-hours x 5 hours/day = 100 Watt-Hours / day 
Each W-Hr is equal to 0.859845227859  KCal
 0.859845227859  KCal  x 100 Watt-Hours / day = 86.0 KCal to pump water each day.

Tomatoes - 86.0 KCal/day x 80 days = 6880 KCal
Lettuce     - 86.0 KCal/day x 50 days = 4300 KCal
Tomatoes - 6880 KCal to produce 6880 KCal of tomatoes.  About 1 to 1 .   
Lettuce     - 4300 KCal to produce  4176 KCal of lettuce.       About 1.03 to 1 .

Compared to soil gardening the power efficiency of soil-less gardening is staggeringly bad, but water is also a commodity worth preserving.  Water use gets pretty complicated if you consider that some of the water applied to soil returns to the aquifer, but aquaponics and bioponics are definitely more efficient with water, and far more wasteful of power, which in some cases will be fossil fuel  

My comparison only scratches the surface.  Our home gardens may not be much better than the modern agriculture system which Warren Brush said uses 10 to 1 times more calories than it produces.  The cost of mining and producing nutrients for our gardens must also be included as well as the gas for those trips to the store for our gardening supplies and fish food.  Keep in mind petroleum is the main source of fertilizers, and even a source of power for production of organic products. The list goes on, and I'm sure I would not be able to think of all of the items on that list.

There are ways to improve each type of garden.  Solar power for water pumps, air pumps, and heaters.  Wicking beds and other Permaculture methods for soil based gardening as Warren has suggested,  Kratky Method for hydroponic gardens, but hydroponics use synthetic fertilizers.  In most cases I believe much more power is required in an aquaponic garden compared to a bioponic garden, but the trade off is protein from fish...  Finding the exact number seems out of reach - we can only try to make better decisions,
We are quickly out growing our planet, and its resources. I hope this has been  food for thought.   I hope you will share yours.

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