Saturday, December 29, 2007

Liquid gold calculations revisited

David William House replied to my Nitrogen Fixers Vs Liquid Gold post with some alternate numbers for nitrogen content and daily human production of urine and feces:

"One adult on an ordinary diet will produce from 100 to 250 grams of feces per day. On a vegetable diet, an adult will produce from 300 to 400 grams per day. (Respectively, 0.22 to 0.55, and 0.66 to 0.88 pounds per day.) Feces are usually neutral to slightly alkaline in pH, 24% to 27% TS [total solids] (dry weight), with a C/N [carbon-nitrogen ratio] of 6 to 10, nitrogen 4% to 6% of TS, VS [volatile solids] is 85% of TS. Normal values for urine are 1 to 1.6 liters volume per day, average pH 6.0, 4% to 6% TS, with a C/N of 0.8, nitrogen 15% to 18% of TS, VS is 72% of TS. (That's 1.06 to 1.69 quarts volume produced daily). Every liter of urine weighs about 1,020 grams. Every quart of urine weighs about 2.9 pounds." [The Complete Biogas Handbook, p 69.]

73-75reported variations
95-96reported variations

Using the median values for the range of urine data means 1.3 liters urine/day (very close to the 1.26 liters I used in my other calculations), with 5% of the urine as "total solids", and 16.5% of that as nitrogen. 1 liter of urine weighs 1,020 grams, so we have 1,020 * 1.26 * .05 * .165 = 10.6029 grams nitrogen/day from urine. (Lower than the 14.112 grams nitrogen/day I used in my other calculations.)

Using the median values for the range of feces data means 175 grams feces per day for adults on "ordinary diets", and 350 grams feces per day for adults on vegetarian diets, with 25.5% as "total solids", and 5% of that as nitrogen. So for "ordinary diet" we have 175 * .255 * .05 = 2.23125 grams nitrogen/day. For vegetarian diets, double the figure for 4.4625 grams nitrogen/day.

So by these numbers, adults on "ordinary diets" produce a grand total of about 12.8 grams nitrogen/day. Vegetarian adults produce a grand total of about 15 grams nitrogen/day. Using the same assumptions I used the other day, the former could provide enough nitrogen for 3730-4660 square feet of forest garden (for canopy demands of 8 grams/square meter to 10 grams/square meter). Vegetarians could provide enough nitrogen for 4370-5460 square feet.

Obviously we don't have the precision of numbers with any of this to make truly exact calculations, but having numbers from two different sources give us results in the same ballpark gives me more confidence in the general idea: one adult can provide the nitrogen for around 4000-5000 square feet of forest garden.

1 comment:

Darren (Green Change) said...

That's very interesting, and matches up with claims I've read elsewhere that there's almost enough nitrogen in one person's urine to fertilise all the plants required to provide them with food. It very neatly closes the nitrogen loop!

When you then add in additional nitrogen sources (nitrogen fixers, compost from inedible parts of the plants, poop from visiting birds, poop from your chickens and ducks, mulch breaking down, etc), nitrogen should never be a problem. There'd even be enough left over to grow a few useless ornamentals! :-)