Saturday, December 08, 2007

Further thoughts on liquid gold

A few thoughts on factors which would reduce the amount of coverage one person's urine could theoretically fertilize as number-crunched in last night's post:


  • Not peeing at home 100% of the time, pretty much unavoidable unless you live the life of a recluse
  • Composting urine in a humanure pile. As I understand it, some nutrients unavoidably leach out of compost piles. I don't know how much you lose how fast, but presumably the more urine you put into the pile, the more rain that falls on it, and the longer the pile sits before being used, the more nitrogen you lose.
  • Loss of nitrogen to the air as ammonia. We primarily pee in a bucket (though of course some goes into the humanure bucket with our poop). If I wait too long to dump the bucket outside, an ammonia smell develops indicating nitrogen loss to the air. Again, I don't know how much how fast you lose nitrogen.
  • Inability of plants to use the nitrogen supplied. I mentioned yesterday my assumption that plants can't fully use the nitrogen supplied in winter during the dormant or at least slower-growing season. I made up a number of 25% efficiency for 4 months out of the year. I assume the same situation can occur even in the active growing months, where too much nitrogen in one spot at one time will lead to leaching of excess nitrogen. Careful application of water-diluted pee over large areas should avoid or minimize this problem. Still, I wonder how much nitrogen plant roots can grab how quickly and just how much square footage you need to cover with say a gallon of urine to make sure your plants can use it all...especially in our quick-draining soil at our house, or in soils with low organic matter content to help bind nutrients. I guess with a mature forest garden the deeper tree and shrub roots and rich soil should catch almost all the nitrogen applied, but younger immature plant communities with mostly shallow roots might require more careful application.
  • Even if you carefully apply your pee to a large enough area each time you fertilize, you still have to make sure you rotate your applications evenly throughout your whole food forest. If you never apply pee to your patch of perennial greens which you harvest every day for salad and don't want to contaminate, then you can't really count that square footage as part of your coverage. Maybe you could apply extra pee to other areas nearby and have the overstory tree(s) capture the extra nitrogen and eventually return it to the perennial green patch via leaf litter. Or maybe your extra nitrogen applications elsewhere will leach away and get lost. Or maybe you apply the soil from your completed humanure compost piles in those spots.


I don't feel too worried about quantifying the loss of nitrogen efficiency from the factors above. I think that with some care to avoid obviously wasteful application of your wastes (haha), the square footage coverage shouldn't decrease by very much, so we can still use the numbers from yesterday as ballpark figures. For our purposes, I'll assume for now that the extra nitrogen from our poop via the humanure pile offsets the losses from inefficiencies in our urine application, and use the numbers from yesterday as the total recycled nitrogen.

One more complicating factor: Obviously not every pee contains exactly 5.6 grams of nitrogen, and I don't know what level of water dilution Crawford used for that figure. Piss clear as a stream? Dark yellow? Presumably somewhere in between, but where exactly? And I assume the nitrogen content varies based on how much food you're eating, since urine disposes of the excess nutrients. So if you live an active lifestyle and eat a subsistence diet, you'll have less nitrogen in your urine than if you overeat and sit around all day. (The book Dirt by David Montgomery mentioned historic Chinese farmers in a certain area who had healthy land which yielded more food than in other areas; those farmers regularly gorged themselves on the excess food to recycle the nutrients back into the land via their wastes.) But I don't know how to quantify any of that so I'll just run with Crawford's 5.6 grams figure.

1 comment:

all of us at said...

A book on this very topic:
Liquid Gold: The Lore & Logic of Using Urine to Grow Plants