Soil-Steaming and Steam-Boiler Blog – for healthy soil and plants

Due to its composition and inherent organisms, soil has a fundamental impact on plant growth. For healthy development, seedlings depend on optimal growth conditions provided by soil. In particular intensely used horticultural soils have problems, since they amass many pathogenic organisms and substances.

Practical experiences of applying hot steam have shown that conservative methods of heating soil in general have a positive effect on plant growth. Hot steam has a physical, biological and chemical effect on intensely used soils and solves the majority of issues without chemicals.

Nevertheless until today not all effects of hot steam on each plant type is scientifically clarified in detail due to the complex dependencies between plant types and soil components.

In practice hot steam kills the majority of diseases and releases abundant nutrients, in particular nitrate, which is made available for plants in soluable form through condensation. After steaming plants are healthier: A culture shows more equality in growth and can be planted earlier in temperate zone, since soil is heated up and resting period is shorter in comparison to chemical usage. However there are exceptions where steamed soil can cause growth depressions e.g. with lettuce. The reason for which can not be clarified yet. It is assumed that in some soils the rate of nutrients has been negatively changed by heat. Normally soil born organisms compensate this effect. In steamed soil however it takes time until those organisms have resettled.

Hence it is recommended, in particular for sensitive cultures, to either wait two to four weeks before planting by extending the cooling period or – in order to cut down on the duration of the cooling period – to inject active beneficial micro organisms into the soil in order to facilitate and accelerate the recreation of soil equilibrium. Furthermore those beneficial organisms hinder pathogenic organisms to resettle in particular those coming from deeper soil layers which have not been steamed.

Each type of soil is a mixture of different components, in particular organic and mineral substances, which serve as biotope for many different organisms. Heat achieves a comprehensive effect: it has an impact on soil life and chemical, biological and physical processes. In the following there is a short list of the most important processes which are triggered by hot steam:

A) Biological and chemical impact:
Degeneration of organic material, in particular of structures based on proteins. Killing of organic and inorganic substances as well as solving of bound agents.

B) Physical impact:
Changing of soil structure, capillarity and the absorbency of salts and water, flushing and dissolution of chemical agents.

Research has shown that physical changes of soil are not very dependent on its specific composition. Notable are only the decrease of capillary flow conditions of water and a slight increase of siltation inclination. Both can be traced back to colloid structure changes. The acid-base metabolism of soil is not affected by heat.

The mentioned biological, chemical and physical effects have a direct impact on soil life and the growth of its plants. Diseases are fought and soil fatigue removed. After steaming many economic plants find better starting conditions and develop healthily.

In general the effectiveness of thermal soil sterilization methods such as steaming with hot water vapor, depends on the applied energy respectively temperature and duration of exposure. This means that similar results can be achieved either by applying lower energy over a longer period of time or by applying higher energy over a shorter time.
General data on the required time of exposure for certain temperatures to kill specific phytopathogens is hard to provide, in particular since in accordance to their different stages of development they show different degrees of heat resistance. When hybernating these organisms are extremely resistant. Hence an exact killing temperature for many organisms can not be identified. The following list shows approximations of killing temperature at 30 minutes of steam exposure of soil borne organisms (G.B. Bollen):

1. Up to 55°C:
   Parasitic nematodes (except Pratylenchus), saprophagous nematodes, Verticilium albo-atrum, Didymella lycopersici, Cylindrocarpon destructans, Thilaviopsis basicola, Phytiumarten, Phytophtora, Pratylenchus, Commor Ragwort, Chickweed
2. Up to 65°C:
   Fusarium oxysporum, Fusarium redolens, Verticilium dahliae, Botytis, cinerea, Phialophora cinerescens, Rhizoctonia solani, most Penicillium- and Aspergillus- species, Ascomyceten, Algaes, Insects , Worms, Snails, Centipede, Mosaic virus
3. Up to 75°C:
   plant pathogenic bacteria, Penicillium- and Aspergillus- Species, Potato X-Virus
4. Up to 90°C:
   Tomato Mosaic Virus, Cucumber Virus, five mesophilic molds
5. Over 90°C:
   Spore forming bacteria

In comparison pathogens harmful to plants are more sensitive than humus forming organisms. When using chemicals the same phenomenon occurs.  Higher organisms such as saprohagous or parasitic nematodes already die at temperatures higher than 55 degrees.

Certain types of Phytium, Rhizoctonia and Botrytis which cause molding die starting from temperatures up to 55°C after 30 minutes of exposure. It’s mentionable that fungi which act as counterparts to pathogenic types and are important to revitalize soil such as Aspergillus- and Penicillium-types are more heat resistant.

The same can be said for bacteria: Spore forming types, important to soil, are extremely resistant to high temperatures and are able to regenerate a new population even after having been exposed to more than 100°C. Amongst them in particular Bacillus subtilis is notable, which can control and even fight pathogenic fungi such as Rhizoctonia.

In contrast phytopagogenic bacteria are quite heat sensitive. It is unknown that these types survive temperatures higher than 70°C at over 30 minutes of exposure.

Research shows that the predominant majority of phytopathogenic oganisms die at temperatures up to 75°C as long as there are exposed to heat long enough. Therefore it is sufficient to heat soil to temperatures up to 98°C for sterilization in order to remove all diseases and preserve spore forming bacteria and cellulose decomposing fungi, which provide a certain natural protection against the resettling of phytopathogens.

Steaming with hot water vapor meets all general requirements. The high specific heat of water ensures high temperatures over a long period of time to achieve an effective and conservative sterilization. Other thermic methods such as the usage of hot air can only achieve similar results by applying extremely high temperatures of more than 2000°C or by longer time of exposure which always comes with the risk to dry out or even burn soil and negatively affect its fertility.

In a nutshell, steaming with hot water vapor is not sterilization in the traditional sense, since many beneficial organisms survive. A total sterilization is not desirable instead, a partial disinfection is achieved.

Illustration of energy absorption of water during evaporation

Water vapor annihilates plant pests such as weeds, fungus, bacteria and viruses merely through its physical thermal energy by degenerating cell structures.

Thereby water vapor is highly effective due to the following two reasons:
On the one hand the majority of organic pathogens are heat-sensitive and die when overheated.
On the other hand due to its high energy content but low temperature of merely 100°C water vapor is able to emit the required heat to the surroundings during condensation in order to kill substances and organisms harmful to plants without burning and deteriorating the soil.
From a biological point of view steaming with hot water vapor is nevertheless considered a partial disinfection. Important heat resistant spore forming bacteria revive soil after cooling down. Hence nutrients are unblocked which counteracts soil fatigue.
Thereby steaming leads to a better starting position and faster plant growth as well as a better resistance against diseases and pests.

Many practitioners and scientists consider the application of hot steam the best and most effective method to sanitize sick soils, out substrate and compost.