A prototype soil steaming machine has been developed as part of CP 6, a LINK/HDC project. Steam is an exceptionally efficient method of heat transfer, which is why it is used to sterilise surgical instruments in hospitals.
High-pressure soil steaming machines have been manufactured but they are slow, heavy and require huge amounts of energy — plus they don't disinfect to depth.
The project developed a novel method of generating low-pressure steam in which thin films of water are passed close to burning fuel. Tests determined the minimum temperatures needed to control a range of soil-borne pests, diseases and weed seeds. A threshold of 70 degsC was sufficient to control all weed seeds, fungal pathogens and potato cyst eelworm.
A prototype was successfully designed and trialled on a range of soil types. Based on a potato de-stoning machine, the soil is lifted onto a conveyor and subjected to an upward flow of steam from an efficient electric steam generator. The machine was field tested at seven sites ranging from Sussex to Inverness on a range of crops including carrot, baby leaf spinach, iceberg lettuce and onion.
The steaming machine proved capable of operating in a wide range of soil types and moisture conditions. Weed control was very good on some sites. The steam treatment also resulted in improved crop vigour. This was probably due to increased nitrogen mineralisation caused by the steam, potentially reducing the need for fertilisers.