Solar cooking alternative to Slash and Burn horticulture.

I looked up on Wikipedia the other day and it turns out that  Slash and Burn is still used to some extent all the way from the polar regions of Finland to the tropics of India.  Even in Ireland where I grew up, they burn heather and gorse off the hills to let sheep graze on new shoots and on grass.  Millions of tonnes of peat has burned when they do this.  In agriculture, typically, they cut down trees and brush, dig up the roots and weeds and  pile it up to let it dry, and then burn it and spread it.  (or spread and burn it).  The ash and "biochar" improves the soil for a few years and the burning heats the soil and kills off significant amounts of weed seeds and pests and plant viruses in the first few inches of the soil.  So anyway, I started doing a solar cooking version of this last year.   This year, (to protect the reflective plastic tape on my solar cooker, I used less water so it didn't bubble out and spill and melt or damage the coating).   When I cooked  damp weeds and also when  I mixed damp soil and weeds, some of the weeds burned in the pot.  So, yeah, I got biochar.  BUT, the cooking was slow.  Next,  I put a small chamber in the pot with wire mesh over it so I could have water boiling in the chamber and a layer of chopped weeds over that then layers of soil and weeds till the pot was filled.  I reasoned that the steam from the boiling water would rise up through the soil, and very quickly transfer the heat and get faster cooking.  This worked! I have been using the soil weed mix for a few months now and plants do really well in it.  There are no weed seeds and that is a bonus. Saves a lot of time.  In my curved planter with  6 inches of solar cooked soil,  I was surprised because vetch seeds have grown up through the 6 inches! But it is easy to pull them because they have such long skinny stems.  The solar cooker that I used is approximately 0.9 horsepower,  and it has tracking and full sun from 11 am till 7 pm right now.  So its an 8 hour day for my little horse on sunny days, and days off when it is cloudy.  I steam 3 ten liter pots of soil per day now, and when the sun was higher and I got an extra hour or so, I was able to steam 4 pots a day.   Anyway, I made a video about the process.  I guess first crop can be stuff like lettuce, tomatoes,  onions, cabbage and later on, root crops.    

This is very interesting.

I was especially surprised when you mentioned the mushrooms growing in one batch! But we do steam mushroom media usually before adding spores or spawn.

My theory as to why it seems to work so well, despite what we know about soil microbes helping plants, is that the seed contains epiphyte and endophyte microbes that are able to colonise the soil more easily without competition. However, it might be beneficial to try adding a little soil in from the wherever the plant is growing very well unsterilized so that there are more of these beneficial microbes.

There is a similar system that used to be used in Europe. I forget what it is called but they burnt sod, mixed it with manure to compost, and then spread it on the fields. Apparently this built up an extremely rich soil that is still in existence to this day. However it was not highly sustainable because it stole the topsoil from heath-land.

One term for this form of agriculture is swidden. I learned a lot about the history and ecological relevance of this practice when I read Stephen Pyne's Fire: A Brief History. in environments where the native forests regrow quickly after disturbance, fire, cropping, and grazing can be parts of a rotational succession that builds soil fertility - or at least avoids degrading it over time. Cultures that did swiddening in prehistorical Europe were semi-nomadic and moved on periodically. When they did, their cleared fields reverted to forest, and the whole cycle would be repeated sometimes decades later.

Maieshe Ljin wrote:This is very interesting.

I was especially surprised when you mentioned the mushrooms growing in one batch! But we do steam mushroom media usually before adding spores or spawn.

My theory as to why it seems to work so well, despite what we know about soil microbes helping plants, is that the seed contains epiphyte and endophyte microbes that are able to colonise the soil more easily without competition. However, it might be beneficial to try adding a little soil in from the wherever the plant is growing very well unsterilized so that there are more of these beneficial microbes.

There is a similar system that used to be used in Europe. I forget what it is called but they burnt sod, mixed it with manure to compost, and then spread it on the fields. Apparently this built up an extremely rich soil that is still in existence to this day. However it was not highly sustainable because it stole the topsoil from heath-land.

Hi Maieshe,  ink caps came up in several places pretty quickly. I put lots of crumpled mushroom block in with steamed grass clippings into a half barrel  and so far, one ink cap (and I forgot to photograph it).  But you never know, fall is coming,  I might have a massive bloom of edible mushrooms under my tomatoes,  or maybe nothing at all.  Its the first time for me growing mushrooms. I was also helping my friend grow mushrooms and she is having good success.  BUT she is envious of my weed free planters!  And she would like a tracking solar cooker!  I can't make one for her because I don't have simple tracking yet. (If I do it long enough, someone in China will notice, and they will start making timing trackers for about 15 dollars each.  Anyway, she is not going to be around to swap out soil 2 or 3 times a day either, plus the neighbour children can get into her garden.    You can't have experimental stuff with kids around  until all the kinks are ironed out.  

I feel that the modern term for this is controlled burn or prescribed burn:

Fire creates diverse wildlife habitats by opening the forest canopy and creating opportunities for new vegetation to grow.

https://www.nationalforests.org/blog/what-is-prescribed-fire-and-why-is-it-important-for-forest-health

In another forum, on the topic of steaming soil and chopped weeds,  someone posted a link to terra preta and from that link  I found "Slash and Char"  and that was interesting, (actually a lot more like what happens when you solar cook the woody material).  It probably needs some sort of equipment to distill off the wood "steam" to collect the chemicals (methane and wood acid, etc.) in it.   Probably some are insecticidal, etc. I think that if you are using solar heat to do the job,  it can be more controlled and also more likely to get the carbon trading money that is on offer in some poverty stricken countries at the moment.   https://en.wikipedia.org/wiki/Slash-and-char  

I talked to a guy last week who has made a prototype solar concentrating pyrolysis contraption that can process wet biomass. This wouldn't be too hard to try.

I found a very interesting wikipedia entry about soil steam sterilizing.  In my case, I got 20% faster growth in a month.  Does that mean that we could use 20% less land to grow crops is we just steamed the soil?   And if we used the same equipment to do both the soil steaming AND make bioChar,  it could be a really great new thing!    Soil steaming on a big scale