Steam-treated soil gives healthier carrots
Scientists in Denmark are developing a less labour-intensive method to control weeds, disease and nematodes in organic vegetables.
Less hassle, expenditure and labour input to control weeds and other nuisances in organic carrots may sound like a dream for the organic farmer. But that is what scientists at the Faculty of Agricultural Sciences at Aarhus University are in the process of developing.
The method consists of heating a small strip of soil using steam. The heat kills weed seeds and shoots, nematodes and harmful microorganisms – which benefits high-value vegetable crops such as carrots, onions and leeks.
Compared with untreated soil you can reduce the weed pressure by 90 per cent if you steam the soil first, says the leader of the project, senior scientist Bo Melander from the Department of Integrated Pest Management at the Faculty of Agricultural Sciences.
The method of steaming the soil – where the soil is heated to around 80°C – is in itself nothing new. It has been used for several years, but mainly to steam large areas. This is the so-called mobile soil steaming process that sterilises the soil down to 15 cm depth. But this method is not permitted in organic farming.
- Sterile soil is not particularly ecological. The steaming certainly removes the weed seeds, but it also removes the organisms you would like to encourage, explains Bo Melander. This is why in organic vegetable growing the steam is applied in strips only to spare as many beneficial soil organisms as possible. The strip corresponds to the area where the crop plants are growing which otherwise would require substantial manual weeding
- The manual bit takes an awfully long time – for example more than 500 hours per hectare in leeks. It is costly and takes up manpower in a period that is very busy anyway, and so keeps the farm workers from doing other valuable tasks on the farm, says Bo Melander.
Steaming for organic farmers
Scientists are therefore busy developing a steaming process that can be used in organic vegetable farming. Initially, scientists have studied the effect of restricting soil steaming to the small strip where the crop is later sown. This has already led to considerable savings in energy consumption compared to sheet steaming.
When the ground is sheet steamed it is sterilised to a depth of 15 cm throughout the field. This uses a whopping 4000 l of oil per hectare. With steaming in strips the soil is sterilised to a depth of 5-8 cm and to a width of 12-13 cm. This uses only 500-600 l oil.
Machines for strip steaming have been developed and are used by a handful of Danish and Norwegian vegetable producers, but they are both expensive and slow. Scientists are therefore trying to improve the technique. They have, for example, supported the development of a machine that manages 0.2 ha in an hour and as it has an in-built GPS it needs no driver input until it has to turn. The farmer normally has to check the machine every other hour – depending on the length of the rows – in order to turn it, and to refill with water and fuel. The machine can do nine rows at a time and the GPS works to a precision of 1-2 cm.
Life returned to the soil
The improved technique will also return some life to the sterile soil – but the right kind of life. In place of the troublesome weed seeds and nematodes that are removed in the steaming process, the sterile soil is supplied with beneficial organisms subsequent to steaming and sowing. This is done to re-establish the microbiological environment and to prevent the invasion of pathogens.
The soil supplement consists of bacteria, various beneficial fungi and pea flour. Pea flour has a high content of nitrogen, which is nectar to microorganisms. The beneficial fungi have a symbiotic relationship with plant roots and improve the plant uptake of phosphorus.
This part of the practical investigation takes place at two carrot producers.
The project receives financial support by the Innovation Law under the Ministry of Food, Agriculture and Fisheries.
Date: Wednesday 13th January 2010