Professor Yaakov Katan, recipient of the 2014 Israel Prize in Agricultural Research and Environmental Sciences, developed an environmentally friendly method for disinfecting soils with the help of the sun
American Supreme Court Justice Louis Brandeis wrote as early as 1913, in an article published in Harper's Weekly, that sunlight is the best disinfectant. He meant that exposure to the public by the media is the best means to fight corruption and his words have since been quoted in hundreds of court rulings. But Professor Yaakov Ketan, from the Department of Plant Diseases and Microbiology in the Faculty of Food and Environmental Agriculture of the Hebrew University, used the proverb literally and in 1976, together with his students and colleagues, developed a solar method for soil disinfection. Since then, solar disinfection and research on it has spread to more than 70 countries all over the world. In addition to Israel, this method has been well established in sunny countries, such as Jordan, Iraq, Morocco, Turkey, Italy, Greece, the United States and Mexico. This is, for the time being, the only method of disinfecting soil without chemical means that is applied on a wide scale. For his research and his contribution to agriculture around the world, Prof. Katan won the Israel Prize in the field of agricultural research and environmental science research for 2014.
Root of the problem
An agricultural field is not supposed to be a sterile place, so why should soils be disinfected anyway? The reason, Katan explains, is that there are many pathogens (pathogens) in the soil such as bacteria, viruses, fungi and even nematodes, which cause root diseases and heavy damage to agricultural crops. The soil also has weed seeds that compete with the crops. One of the best methods to fight all these pests is to disinfect the soil before sowing the crop. The most common method of disinfecting the soil, even today, is the use of chemicals, but this method causes considerable environmental pollution. For example, the main substance used for soil disinfection until 2005 was methyl bromide (CH3Br), which turned out to cause damage to the ozone layer and therefore its use was stopped completely and to this day it has no satisfactory alternatives. Another disadvantage of the chemical disinfection is that it also harms the beneficial microorganisms in the soil, which on their own exterminate a considerable part of the pathogens, and therefore can sometimes even cause the flourishing of certain pathogens.
Another way of disinfection is with the help of steam, but this is an expensive and complicated method intended mainly for small areas. A more innovative option is to develop varieties resistant to certain diseases, or to use biological control, but despite the clear advantages of genetic and biological strategies, their development is expensive and slow, and their main drawback is that they do not serve as a general solution, but treat a specific plant and a specific pathogen.
Katan and his students therefore developed a third way to disinfect the soil from pests and weed seeds, with the help of the sun. The initial idea came from Avihai Greenberger, one of Katan's students, who noticed that when the polyethylene sheets used to heat and protect the crops are accidentally left in the field for a long time, the ground heats up considerably. At first he believed that the method would not be effective, because preliminary calculations he made, which were based on the knowledge that was available at the time, suggested that the solar heating would be able to heat the soil to the required temperatures only to a depth of 10 cm, while a successful disinfection should affect the soil to a depth of 40 50 cm.
Even so, Katan conducted feasibility tests and revealed that the method does succeed in disinfecting the soil to the required depth. How does this happen? It turns out that the solar heating not only heats the ground, but also encourages biological processes and the culture of beneficial microorganisms, that is, actually combines heat control with biological control. This is possible because the heating is moderate. Solar heating involves covering moist soil with transparent polyethylene sheets for a whole month, during which the temperature of the upper layer of soil gradually increases by 5-10 degrees. As you go deeper into the ground, a temperature drop is created that encourages the development of beneficial biological processes. The slow activity of the solar heating is also one of the main disadvantages of the method - because it involves shutting down the field for a month.
New Middle East
Another characteristic of the solar disinfection is, of course, that it is mainly suitable for hot countries. It is common in all parts of the country, especially for intensive crops such as vegetables and flowers, for which the method is economical. For example, all anemone growers in Israel use solar disinfection. The method is most suitable, obviously, also for the neighboring Arab countries, and indeed one of the first countries to adopt the method and study it was Egypt. In fact, Katan, who was born in Baghdad, was the first Israeli to give, in 1981, a seminar at the leading Egyptian institute for agricultural research, the National Institute for Agricultural Research in Giza. This seminar resulted in a fruitful collaboration that lasted for 10 years. Today, the method is widespread in many Arab countries, especially in Iraq and Jordan, and international conferences on solar disinfection were held in Jordan and Syria under the auspices of the Food and Agriculture Organization of the United Nations (FAO).
Continued research on solar disinfection brought up additional positive features of the method. For example, it turned out that solar disinfection of the soil significantly increases the amount of the crop, and not only due to pest control. The accelerated growth in disinfected soils is also related to the encouragement of beneficial microorganisms, the decomposition or neutralization of toxic organic substances that accumulate in the soil and the increased release of mineral nutrients.
Disinfection can be combined with other pest control methods to increase efficiency. One possibility is to combine the solar disinfection with pesticides at a reduced dose, so that the pollution of the environment is small. Alternatively, it is possible to carry out a combined pest control with the help of solar disinfection and compost, which is used both to improve the soil and to prevent the growth of weeds. However, there are pathogens and weeds that are not killed by the solar fumigation, therefore, Katan explains, it is recommended to change the soil fumigation method from time to time, in order to avoid cultivating certain pests.
Experiment in the field: a green carrot field that underwent solar disinfection and in it a plot that did not undergo disinfection (at the front of the photo on the right) and was damaged by the parasitic weed leech.
A method to combat the method
One of the plants in Israel whose seeds are damaged by the solar fumigation is the invasive plant acacia blue. Many people are familiar with these very common roadside trees, which, despite their name (given to them due to the bluish tint of the leaves), are loaded with yellow blossoms in the spring. The plant was brought to Israel from Australia at the beginning of the 20th century for the purposes of afforestation and stopping migrating sands. Today, this is the most violent and problematic invasive plant in Israel. It cannot be destroyed by felling or burning, because not only does it regenerate easily, but its seeds are adapted to germination after fires, as was seen in the Judean Mountains after the great fire in 1995. The war with the resistant method using pesticides will be expensive and cause unreasonable environmental pollution. Fortunately, it turns out that blue wheat seeds are sensitive to solar disinfection, even though they are heat resistant. This is because the solar disinfection brings them out of dormancy, and the sprouts that grow are sensitive to being killed by heat. The combination of solar fumigation for damaging the seeds with excision and extermination of the mature plants with other methods achieves good results.
Recent studies on solar disinfection focus on several aspects. They are not only concerned with a better understanding of the mechanisms activated in the soil following the fumigation, but also in testing the effectiveness of the fumigation under different conditions and developing mathematical models that will accurately predict the warming of the soil under the different conditions. These studies, of which Ketan is a participant, lay the foundation for the development of more sophisticated plastic materials for covering the ground, on which it will be possible to register patents.
Such studies have led to the fact that in recent years the solar disinfection method is used to disinfect various substrates, and not only agricultural land. For example, Dr. Yael Hellman from the Department of Plant Diseases and Microbiology, in the Faculty of Agriculture of the Hebrew University, and Prof. Avraham Gamliel from the Volcanic Institute, are researching the use of solar disinfection for the purpose of disinfecting settled water, that is, wastewater that has been introduced into the aquifer and re-produced. Solar disinfection is also used to disinfect organic manure from E. coli bacteria, and in Morocco it is used to sanitize dahlia columns.
Solar disinfection has become an integral part of agriculture in many places around the world, it has become one of the basic methods in agriculture in general and organic agriculture in particular, and its uses grow and diversify every year. There is no doubt that in a world saturated with industrial methods that are not sustainable, the method developed by Prof. Katan and his students is essential for the future of agriculture.
The article was published with the permission of Scientific American Israel
