Ten Calories Of Energy To Produce One Calorie Of Food.

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 negative net energy food. I immediately looked at the energy costs involved with my soil-less gardens. My hydroponic pump uses 160 watts-hours per day or 137.6 KCal or about 576 KJ/day.   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 calories as much as I can.  There were other errors in my first draft,  so here's the rewrite.  Many of my calculations will be done using the 'Energy and Work Unit Conversion' at unit-conversion.info/energy.html. 

I did some research and found a daily harvest of 8lbs of lettuce and 100lbs of tomatoes to be respectable averages for a 100sf garden. So I will be using those figures throughout.
Just for grins 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'

P_whp = q h sg / 3960

where

P_whp = water horsepower (hp)
q = flow (gal/min)    = (10)
h = head (ft)              = (100)
sg = specific gravity = (1)

P_whp = 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.
Power and Energy are different.  Energy is what is delivered and Power is the rate at which it is delivered.
In the above example we deliver 188 Watts for 10 minutes or 31.38 Watt-Hours



We pump for 10 minute so
0.04497210699688 KCal/s  x  600sec = 26.9 KCal
So how many lbs of vegetables would you need to produce 26.9 KCal
Lettuce is 67.37 KCal per lb so you would need 26.9/67.37 = 0.40 lbs of lettuce per day from your 100 sq ft garden. If you harvest 8lbs / day the net gain is 19.97 times
Tomato is 80 KCal per lb so you would need 26.9/80 = 0.34 lbs of tomatoes per day from your 100 sq ft garden  If you harvest 100lbs / day the net gain is 296 times

Below is a list of common vegetables which I got from http://www.freedieting.com/tools/calories_in_vegetables.htm


Let's put this in perspective of a soil-less aquaponic or hydroponic system:
Let's say you use a 20 watt pump 5 hours a day.
    That's 20 watt-hours x 5 hours/day = 100 Watt-Hours / day  (W-Hr/day)
    Each W-Hr is equal to 0.859845227859  KCal so you spend about 86.0 KCal to pump water each day.

If you grow 8 lbs of lettuce per day on your 100 sq ft aquaponics garden,
8lb/day x 67368 cal/lb = 538944 cal/day or  ,  538.944Kcal/day

You would spend 86.0 KCal to pump water to produce 538.944 KCal of lettuce.
A return of over 6.27 to 1.  Not quite 19.97 times like soil gardening but pretty good!



How about tomatoes...
Lets say you grow 100 lbs of tomatoes per day in your 100 sq ft garden.   
Let also say you use the same amount of electricity - (86.0 KCal/day).
100 lb/day x 80 KCal/lb = 8000 calories/day
So you would spend 86.0 KCal/day to produce 8000 KCal of tomatoes.
A return of over 92 to 1.  Soil gardening was 296 but this is not bad!

Compared to soil gardening the power efficiency is not as good, but water is also a commodity worth preserving and my guess is any of the soil-less methods will beat soil based gardening several times over.   It gets pretty complicated if you consider that some of the water applied to soil returns to the aquifer but ponics are definitely more efficient with water.  

Back to my hydroponic system which consumes 137.6 KCal per day.  It would need to produce 1.72 lbs of tomatoes per day.  But this is just the power used to move water.  Warren Brush was including the power used to make fertilizers, mine, and apply nutrients, and maintain the crop from start to finish.  That's beyond the data available to me, but at least it still looks like a positive net gain whether you are gardening with soil or without and probably far better than the modern agriculture system using ten calories of energy to produce one calorie of food.

Aquaponics has a side benefit of fish protein which I have not included, but the energy used to make fish food should be considered.  Bioponics on the other hand is free of any further input save iron and magnesium which is also required in aquaponics.


I'll admit this article was difficult for me,  I feel like I have dyslexia where it comes to keeping units straight.  If you find any errors I will correct them, but to my best knowledge I have presented this correctly.