Friday, 29 July 2016

Steam Cleaning the Soil



I've been wanting to write about this machine for ages, but it is not easy to photograph. This photo is the best I've been able to manage. As you can see, it was taken from a moving vehicle (Célestine, in fact).

The place is the big market garden in the Cher Valley at Saint Martin le Beau. The sign on the machine says 'Delahaye [the name of the market gardeners]. In order to respect the environment this machine sanitises the soil with steam, naturally'. We often see it in action, trundling up and down the plots that will then have yet another crop of lettuce, lettuce, lettuce, Florence fennel, potatoes, celery, leeks or lettuce planted in them.

The way it works is that dry superheated steam is injected into the soil under low pressure to a specified depth, depending on the crop to be planted. The soil temperature is raised to 80-85°C and 'cleaned' of weeds, fungi and other pests whilst preserving the qualities needed to raise healthy crops. The soil has to be prepared by being ploughed to a fine tilth before the steamer goes over it, to make sure penetration is sufficient and consistent.

The 'steam clean' allows a non-selective disinfection of the soil and a rapid rotation of crops. It is versatile because there is apparently no development of resistance to the treatment. It's simple to use and doesn't involve special training to deal with pesticides. The manufacturers of the machine claim that it gives better crops both in quantity and quality and allows earlier crops due to the heat being retained in the soil. The technique does not involve any danger to the user, plants or soil. The treatment kills weeds, fungi, nematodes, soil dwelling insect larvae, bacteria and viruses.

This all sounds excellent, but nowhere can I find any research into what good bacteria and fungi the steam also kills. I also wonder if some organisms will eventually develop a tolerance to the heat. Nevertheless, it is probably on balance better than using the cocktail of pesticides that much of our fruit and vegetables are exposed to.

Further Reading: CM Regero Industries (in French).

3 comments:

  1. I've heard of putting potting soil in the oven to heat it through and then letting it cool down.before planting in it. Same concept, I think.

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  2. Replies
    1. I suspect worms aren't threatened too much. Many worms live quite deep and are quite mobile. I think that if they happen to be at the surface when the machine trundles over they just dive. I doubt more worms are harmed than with ordinary digging or ploughing. The machine heats to a depth of 25cm.

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