Archive for August, 2009

The danger of recontamination

Sonntag, August 23rd, 2009

A special source of danger of reinfecting soil with pathogens after steaming is deeper soil. Depending on the root depths of the planted culture, phytopathogenic organisms can reach and contaminate deeper layers of soil. It can happen that steamed higher layers of soil are reinfected by such deep lying pathogens.

Hence when soil is heavily contaminated it is recommended to take soil samples from different soil depths depending on the root depth of the planted crop and check on diseases in order to identify the necessary steaming depth.

Furthermore the injection of beneficial active micro organisms after steaming into the soil can significantly strengthen the resistance against intruding pathogens and immensely inhibit recontamination.

The danger of recontamination on the surface by carry over e.g. when using foreign substrates or planting contaminated plants can be limited through the injection of beneficial microorganisms.

The effect of heat in soil

Samstag, August 22nd, 2009

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.

Hot steam and plant growth

Samstag, August 22nd, 2009

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.

Reactivation of decontaminated soil

Sonntag, August 16th, 2009

After decontamination of soil with hot steam a quick reactivation with microorganisms takes place. At the beginning, harmful as well as beneficial organisms resettle. But beneficial bacteria and fungi find better conditions and gain a considerable head start. In general beneficial organisms will prevail. The quick revival can be traced back to many different reasons: Most important is low competitive pressure from other species as well as the availability of nutrients and other beneficial chemical substances which were dissolved by steaming.

The first wave of reactivation comes from heat resistant species e.g. spore forming bacteria. The effect of heat shock on the termination of latency is well known in particular for bacteria and fungi. Furthermore microorganisms from deeper areas which were untreated move up.

Furthermore germinable spores arrive by air, most of them come from fungi. In most of the cases a new barrier against the spread of pathogens is formed quickly and naturally.

In rare cases, pathogenic organisms may prevail after steaming due to unbeneficial circumstances, which can lead to enormous damage. In order to prevent the spread of pathogenic organisms it’s recommended to seed beneficial microorganisms into the soil right after steaming.

Water vapor for soil steam sterilization

Samstag, August 15th, 2009

In contrast to other agents (such as air) water has the ability due to its high specific heat to absorb a tremendous amount of energy at a constant temperature of 100°C when transforming from water to steam. This energy is released in the soil for disinfection.

As a result this method obtains a particularly high degree of efficiency with which all organic pathogens are killed at sufficient time of exposure.

Due to the relatively low temperatures of just up to 100°C during the condensation process, soil is prevented from damage. In contrast to the usage of dry heat (e.g. hot air) soil can not be burned and its fertility harmed.
Compared to other chemical agents, water vapor has a comprehensive sterilization effect on soil. All organic pathogens are affected by the humid heat and even killed after sufficient exposure.

Chemical agents only partially take effect, since they are focused on single pathogens only. If soil suffers from different diseases a chemical cocktail is necessary which can lead to enormous risks for the health and environment.

Soil Fatigue

Samstag, August 15th, 2009

Soil fatigue in general describes all general growth constraints of cultivated plants after repeated cultivation on the same piece of land. In particular it characterizes the phenomenon that yields decrease gradually despite fertilization and other soil preparation efforts.

In particular soil fatigue occurs after long lasting cultivation of one crop at the same location. In general soil fatigue is limited to one plant family and appears in vegetable production as well as in horticulture and fruit growing. All other plants thrive whereas the desired plant which formerly grew well on that plot, hardly develops.
The reasons are manifold and not completely understood. Different processes between plants and soil are considered:

  1. Specific deprivation of nutrients (e.g. depletion of special micronutrients)
  2. Accumulation of pests in the soil
  3. Metabolic excretions of roots, which inhibit growth or attract vermin
  4. Decline of soil living species and as a result changes of soil quality
  5. Change of pH-value in soil

In general soil fatigue can be avoided by continuous crop rotation in proper order. Furthermore the regular application of organic fertilizers can antagonize the occurrence of soil fatigue.
In conventional horticulture with intensive soil usage that makes proper continuous crop rotation impossible, the fatigued soil can be either be disposed or reactivated by hot steam.

The mode of action of steam in the soil

Samstag, August 15th, 2009
Verdeutlichung der Energieaufnahme von Wasser während der Verdampfung
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.

Killing temperatures for pathogenic organisms of plants

Samstag, August 15th, 2009

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.

Soil sterilization against soil born diseases in agri- and horticulture

Montag, August 10th, 2009

The intense cultivation of agricultural crops challenges nature, since it has a negative impact on the biological equilibrium and strongly promotes the spread and growth of organisms harmful to plants. The culture of resistant high-yielding varieties alone does not solve the issue.
Therefore plant protection and pest control is essential in order to ensure high yields and guarantee the food supply of a steadily growing world population.

Thus soil plays a central role. Under extensive use soil is disabled to control pests though its balanced biological activities. In particular in green houses, soil is exposed to many different diseases which leads to an increased occurrence of wither and root sicknesses, nematodes and weeds.
Soil sterilization is a highly effective treatment method in the area of plant protection.
Two different methods are in use at present: The chemical and the physical/thermal treatment of soil.

The chemical sterilization of soil

Montag, August 10th, 2009

Chemical compounds for soil sterilization are easy to use and need little time for application.
Modern compounds have a relatively low effect range, only one or a few specific diseases or pests are overridden. Furthermore the application of chemical agents always comes with a long period of rest which is necessary to degrade or flush them out.

In particular if chemical means for soil sterilization are not applied properly harmful residues might stay in the soil and concentrate in plants which have still unknown effects on plants and man.
Besides the risk of resistance formation can not be ruled out when applying chemical agents which can even aggravate the symptoms of a disease.

Due to the uncertainty of bad side effects chemical agents for soil sterilization should only be applied very carefully, in particular when it comes to vegetable foods.