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I am wondering what is the most efficient use of my garden waste. Is it:
Since I can make good use of bio-briquettes, I would really like to weigh the pro's and con's of the two options. Could you help me out?
In terms of benefit to the garden, the two biggest things you lose by burning the waste instead of composting are nitrogen and carbon (organic matter). The calcium, phosphate, and potash (among other things) will remain behind in the ash. Ash will also tend to raise the pH of the soil, whereas compost will have less effect.
In terms of sustainability and the environment, burning the waste adds particulate matter and CO2 (among other pollutants) to the air. (Composting adds CO2 to the air as well; my guess is that burning is worse.) However, if your next best alternative for generating heat is to use imported petroleum or even biomass that comes from far away, you may reduce your overall CO2 output by using your own briquettes. On a larger scale -- i.e. if this was a community decision -- if your alternative to burning garden waste is to clearcut forest for wood to heat with, then it makes sense to burn your own briquettes and limit your woodcutting to sustainable levels.
Ash is going to raise the pH of the soil. I'm not familiar with the use of biomass briquettes but I could see a negative effect of adding that ash to the soil if the quantity were significant.
Nitrogen is lost (along with other elements) as a result of the burning process, so there's a difference for sure.
A benefit of the compost is that it helps develop a much more complex soil structure, making a sandy soil less sandy and a clay soil less dense. It also provides a much more hospitable environment for microbes and worms.
Ash is going to really just provide a certain set of minerals and elements (calcium carbonate, potash, etc.) without improving the soil structure. I would say it is akin to spreading lime or bagged fertilizer on the ground rather than compost. The former will change the chemical composition but the latter will do that and develop better soil structure.
Your question "what would I lose if I used ash only" is (at least) the following: potential soil building of the compost, the nitrogen that is lost during the burning process, etc. Ash is, however, useful particularly if you are trying to raise the pH of the soil.
From a work-standpoint, well, ash weighs considerably less than compost. It is dried and has released much of its original contents as heat and gas so there's that. It requires less of your energy to spread about, that's for sure.
Me? I'd probably opt to compost it because I'm trying to improve the native clay soil I have here in Virginia. But I'm not strictly opposed to adding ash to the soil. It's a time-tested means of changing the pH and encouraging some plants to grow well.
It isn't, however, an all-or-nothing affair. You could, of course, do both.
Update: One more thing I thought about was weed seeds. If the plant waste you are talking about includes weeds, those would be better as ash unless the compost pile gets hot enough to kill them. Just a thought.
I'm another one who would naturally choose composting because it keeps more of "good stuff" - especially the organic humus, but also many of the salts (Nitrates have been mentioned).
However, if you have a problem with plant pathogens in your soil, then this strikes me as an extremely good reason to prefer briquettes and incineration. Pathogens could be things like viruses, wilts, fungi, etc.
