Steam soil disinfection in vegetable crops

Steam disinfection is a physical (thermal) control method that allows controlling most of the bioagressors present in the soil (fungi, nematodes, weed seeds…). This method consists of disinfecting the soil by injecting water vapor at 180°C to raise the soil temperature to 85-90°C. It is mainly practiced for crops in greenhouses and helps reduce the seed bank.

Equipment and implementation

Steam injection is done by different techniques. Either under plastic sheets for deep disinfection, or using metal plates called bells or boxes for surface disinfection. Traditionally, these bell or sheet systems required regular movement, but currently various self-propelled models have been developed.

Below is an example for a generator with a power of 1400 kg of steam/hour

First, the soil must be prepared before disinfection as for planting or sowing at 25-30 cm depth. It must have a loose and fine structure and should not be too wet nor contain residues of crops and roots.

Then, the steam is produced by a generator that heats water to 170-180 °C at 1 kg/cm² pressure. The generator normally runs on fuel oil/diesel but can be equipped with a gas burner. It requires electrical power.

The technique is more effective with a high water pH (> 8) to limit metal corrosion. The water should be low in mineral content to avoid scale deposits.

Steam distribution is done through insulated pipes that transport steam from the generator to the area to be disinfected. Plastic sheets are placed on the soil and held down by sand bags. Steam is introduced for several tens of minutes to several hours until the desired temperature is reached at the desired depth. For shallower depths and surfaces, bells or boxes (metal plates) are used.

The average application time varies depending on the depth to disinfect, equipment (application surface, power, pressure…): 5 to 10 minutes are needed to raise the temperature to 90 °C over about ten centimeters with bells, while to reach the same temperature at 30 cm depth over 100 m², the required time is 1 h 30. For 50 cm depth, an application time of 2 h 30 is necessary. The application time also varies depending on the targeted bioagressor: it will be shorter to combat surface fungi (Rhizoctonia solani for example) and longer for fungi living deeper (Verticillium dalhiae for example).

) and longer for fungi

Example of application:

With a machine equipped with a 300 m² sheet surface, it will take 4 h 40 min to disinfect these 300 m² at 90 °C to a depth of 30 cm, 6 hours for 40 cm, 7 h 30 min for 50 cm, and 9 hours for 60 cm. For shallower depths, bells or boxes are used. These are metal plates made of aluminum and galvanized steel, 3 to 4 m long and 1.30 to 1.4 m wide, partially inserted into the soil. Treated widths can vary from 2.50 m to 10 m for greenhouses and shelters and up to 25 m for open field. The surface of the bells depends on the generator power. A load of 50 kg of steam per hour per m² is optimal. Average application times vary depending on the disinfected depth. For surface disinfection (10 cm) with bells or boxes, it is 5 to 10 minutes with an average diesel consumption of 0.5 to 1 l/m². Deep disinfection with sheets requires an average time of 3 hours with a consumption of 2 l/m². The hourly surface disinfected with bells at 12 cm depth varies according to generator power from 15 to 350 m² per hour.

Source: Le Point Sur les méthodes alternatives « La désinfection vapeur » (See the sheet)

Technical details

Although steam disinfection can be performed year-round, the summer period is the most favorable to implement this practice because the colder and wetter the soil, the more heat is required.

Steam soil disinfection does not introduce any persistent harmful factors into the soil. The delay before replanting after steam soil disinfection is very short. However, after steam disinfection, an increase in ammoniacal nitrogen due to destruction of nitrifying bacteria or a modification of the soil pH may be observed, which can lead to phytotoxicities requiring some delay before replanting. Moreover, the temperature increase kills both harmful and beneficial soil organisms non-selectively, and following the biological void caused by disinfection, producers may face rapid recolonization by bioagressors. Steam disinfection is also very fuel-consuming and costly to implement, which is long and tedious.

Implementation period During intercrop

Spatial scale of implementation Plot

Application of the technique to...

All crops: Easily generalizable

Easily generalizable

The technique can be used both under shelter and in open field. However, caution is advised as steam disinfection can cause strong pH modification or ammoniacal nitrogen increase, which may cause growth problems or phytotoxicity for some crops (e.g. lettuce).

All soil types: Easily generalizable

Easily generalizable

All soil types can be disinfected with steam. However, in clay or silty soils, disinfection deeper than 30 cm can be difficult. Moreover, disinfection duration must be adapted to soil texture, structure, moisture, and temperature.

All climatic contexts: Easily generalizable

Effects on the sustainability of the cropping system

"Environmental" criteria

Effect on air quality: Decreasing

phytosanitary emissions: DECREASE

GHG emissions: INCREASE

Effect on water quality: Increasing

N.P.: NEUTRAL

pesticides: DECREASE

turbidity: NEUTRAL

Effect on fossil resource consumption: Increasing

fossil energy consumption: INCREASE

phosphorus consumption: NEUTRAL

Comments

Phytosanitary product emissions: Steam disinfection reduces the need for fungicides, nematicides, and herbicides.

GHG emissions: increase in atmospheric pollution due to CO2 emissions (and sulfur if fuel oil is used).

Pesticide residues in water: reduction of pollutant transfer to water due to decreased fumigants and herbicides.

Fossil energy: technique very fuel-consuming (0.5 to 2 L of fuel oil per m²).

"Agronomic" criteria

Productivity: Variable

Variable

Modification of pH and ammoniacal nitrogen level in soil can disturb plant growth through phytotoxicity phenomena.

Production quality: No effect (neutral)

no effect (neutral)

Soil fertility: Variable

Variable

Possible modification of soil physico-chemical characteristics and nitrification.

pH modification may cause phytotoxicities linked to excess manganese or copper.

Increased risk of nutrient leaching during long disinfections.

Water stress: No effect (neutral)

No effect (neutral)

Functional Biodiversity: Decreasing

Decreasing

Non-selective method, possible impact on soil biodiversity

"Economic" criteria

Operating costs: Increasing

Increasing

Very high intervention cost (labor + fuel).

Mechanization costs: Increasing

Increasing

Purchase of specific equipment

Margin: No knowledge on impact

No knowledge on impact

Other economic criteria: Increasing

High energy costs because technique is very fuel-consuming (0.5 to 2 L of fuel oil per m²)

"Social" criteria

Working time: Variable

Variable

Very variable (from 30 to 670 h/ha) depending on soil type and moisture, target, and equipment.
Reduction in the number of passes for weed control and crop protection.
Increase in overall working time.

Peak period: Variable

Variable

In principle, the delay before replanting after disinfection is very short. However, if too strong modifications of soil pH and ammoniacal nitrogen occur, a delay between steam disinfection and sowing the next crop must be respected.

Favored or disfavored organisms

Disfavored bioagressors

Organism	Impact of the technique	Type	Details
Verticillium dalhiae	MEDIUM	pathogen (bioagressor)	The technique can impact this bioagressor provided long and deep disinfection is done
weeds	HIGH	weeds	effective from 70-80 °C
botrytis cinerea	HIGH	pathogen (bioagressor)	effective from 50-60 °C
vegetable crop flies	HIGH	pest, predator or parasite	the technique may have an impact on flies species whose part of the life cycle occurs in the soil (pupal or nymph stage). For example, Delia platura (seedcorn maggot), Delia radicum (cabbage fly), Delia antiqua (onion fly), Psila rosae (carrot fly)
nematode (bioagressor)	MEDIUM	pest, predator or parasite	effective from 50-60 °C
brown rhizoctonia	HIGH	pathogen (bioagressor)	effective from 50-60 °C
sclerotinia	HIGH	pathogen (bioagressor)	effective from 60-70 °C
virus	HIGH	pathogen (bioagressor)	effective from 90 °C

Disfavored beneficials

Organism	Impact of the technique	Type	Details
Spiders	HIGH	Natural enemies of bioagressors	All beneficials that have part of their life cycle in the soil can be impacted by this technique (carabids, spiders, rove beetles…), as well as some hymenoptera pollinators living in the soil such as mason bees.
Predatory and granivorous carabids	HIGH	Natural enemies of bioagressors	All beneficials that have part of their life cycle in the soil can be impacted by this technique (carabids, spiders, rove beetles…), as well as some hymenoptera pollinators living in the soil such as mason bees.
Fungi (beneficial)	HIGH	Natural enemies of bioagressors	Antagonistic fungi naturally present in the soil are impacted by the technique (Coniothyrium sp. and trichoderma sp. for example)

For more information

Information initially from the Practical guide for designing vegetable cropping systems saving phytopharmaceutical products (2014) / Technical sheet T9.

Steam soil disinfection - Ministry of Agriculture and Food - Website, 2012 Link to website

Soil steam disinfection - Céline Gilli, Vicent Michel - Agroscope, Technical brochure, 2016 Technical sheet available

Le Point Sur les méthodes alternatives: Steam disinfection - Icard C. et al. CTIFL, Technical brochure, 2010 Link to brochure

Annexes

S'applique aux cultures suivantes

Vegetable crops

Défavorise les bioagresseurs suivants

Défavorise les auxiliaires