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.