Soil has significant functions for securing the maintenance of life on earth. The food and water that plants need are provided via soil. A wide variety of living species which find shelter in the bosom of the soil performs vital functions for securing life in the continental ecosystem. Bacteria, mushrooms, ants, larvae, spiders, earthworms, snails and rodents are only some of this variety. These living species exist abundantly in the soil. In a soil layer of one hectare (2,47 acres or 10.000 m2) and 30 cm (approx. 1 feet) thickness, for instance, approximately 25 tones of subsoil organisms are sheltered.
The conservation of soil is also important. But as its value has not been properly appreciated until recently, soil has become seriously polluted with chemical substances which can hurt the living species in it. These poisonous chemical substances include cadmium, arsenic, chrome, and mercury at excessive levels, lead, nickel, molybdenum and fluorine at medium levels and boron, copper, manganese and zinc at low levels.
There are a number of sources of these substances which accumulate in subsoil. Tones of such substances are dispersed into the atmosphere as a result of the burning of fossil fuels, the casting of minerals and other industrial activities. Atmospheric processes cause these substances to disperse and mix with the soil first and penetrate into plants later; polluted water and soil consequently cause disastrous problems for construction, the environment, and health.
The level of borax which is available in soil, for instance, is becoming overwhelming due to the extensive use of detergents and fertilizers. Super phosphate, which is a kind of fertilizer, and limestone, which is found in soil, are generally composed of small quantities of cadmium, copper, manganese, nickel and zinc. Cadmium and chrome are utilized in producing metal coatings; cadmium is also used in battery production; and arsenic is utilized in cotton, tobacco and fruit plantations as an insect and weed killer. As products in which these substances are found have been increasingly used in recent years, these substances have been consumed by human beings in higher proportions in their daily air, water and food intakes.
Although presently a partial solution, a method of improving soil by the use of plants has recently been developed and introduced as a clean and permanent solution. Research conducted has proved that plants have important functions in cleaning chemically polluted soil, and significant scientific findings have been obtained about how plants survive in an environment replete with poisonous chemicals.
Plants: The volunteer soil cleaners
The use of plants to eliminate substances which are unfriendly to the environment or to diminish their negative effects on the environment is called “phytoremediation.” Cleaning polluted soil with techniques that necessitate engineering processes is a rather costly operation. On the other hand, certain plant species have been granted the ability to concentrate heavy metals such as zinc, cadmium and nickel, which they take up from the soil, in their stems, shoots and leaves. The parts of these plants containing the concentrations of heavy metals are collected, reduced in volume and are stored for future use.
Plants which are capable of storing metals in themselves are used for soil-cleaning purposes in the mining industry. Bio-mining is defined as obtaining minerals by way of growing plants in polluted or mineralized soils and then harvesting them as soon as they have concentrated a sufficient amount of minerals in their tissues. Plants are burned after being baled and their residual ashes are being sold as mineral ores. Zinc has been produced at a rate of 30–40 % from ashes of the plant thlaspi caerulescens which was grown in a plantation rich in zinc in Pennsylvania.
How plants are employed in soil cleaning
Lately, a number of research projects have initiated on the plant thlaspi caerulescens, which is seen as useful in terms of soil cleaning. Thlaspi is a member of the broccoli and cabbage family and it grows in soils containing high rates of zinc and cadmium. These plants develop wide root formations in soils containing heavy metals; they transfer these heavy metals via their hair-like roots first to their stems and later to other parts, and store those heavy metals in their leaves. Storers like thlaspi are a good model of the mechanism of metal-storing and even shed light onto the biological system which plays a role in this process. The biological composition of these plants has been enriched with genes so that they assume a role in increasing the solubility of heavy metals in soil, in the transfer of metals to their roots, and in producing proteins to play the intermediary in such transfers.
While a typical plant is equipped with a storage capacity of 100ppm (gr per ton) zinc and 1ppm cadmium, thlaspi is created with a storage capacity of up to 30,000 ppm of zinc and up to 1,500ppm of cadmium, without any signs of being poisoned, whereas an ordinary plant may be poisoned with a zinc level of only 1,000ppm or cadmium between 20 and 50ppm. What would it have meant for living beings which are nourished by plants, if all plants had been created with as high a storage capability for heavy metals as that of thlaspi?
It has been noted, while researching the zinc-storage mechanism of thlaspi, that certain parts of the plant have been stimulated for the purpose of transferring zinc. While in ordinary plants gene-decoding of proteins which are charged with zinc transfer is regulated according to their zinc contents, in thlaspi synthesizing of the carrier proteins continues until the zinc contents of its tissues reaches very high levels.
How radioactive cesium is cleaned
It has been found, as a result of research into soils polluted with radioactive cesium-134 and cesium-137, that the area polluted by cesium-137 is under the threat of radioactive pollution, even if the effects of pollution that it caused over the soil surface would be lessened. One of the most important reasons for this is that cesium-137 is a long-lasting radioactive isotope with a half-life of 32.2 years.
Phytoremediation is preferable to alternative cleaning methods, which cost much more due to high energy inputs. However, cesium, in the form it is found in soil, is not absorbed by most plants, and ammonium ions cause the dissolution of cesium-137 in soil.
However, amaranthus retroflexus, which is a member of the goose-foot plant family, has been found to be forty times more efficient than other plants tested in cleaning the soil of radioactive cesium. Thus, polluted lands are expected to be cleaned within fifteen years if this plant is grown and harvested two or three times annually.
A plant fed by arsenic
Arsenic is utilized in the production of agro-chemicals that are used to kill weeds and insects in the sub-soil. The fern named pteris vittata has been found to be created with the capacity to store arsenic. When this plant fern was discovered to contain two hundred times more arsenic than the surrounding soil, it was understood that it is fed by arsenic. This discovery is expected to open new horizons in cleaning the agricultural land, especially in industrial and mining areas.
How damage caused by aluminum can be decreased
Aluminum, which is one of the most abundantly available elements (among oxygen, silicon, iron, magnesium, sodium, potassium, aluminum and calcium) in the earth, is among the main components of clay in sub-soil. It does not pose a threat to plants when it has a basic or neuter pH value. However, Al+3 which is a kind of aluminum dissolvable in acidic soils, is poisonous to the extent that it threatens plant roots.
How some plant species, among which are wheat, corn and barley, can be cultivated in acidic soils in spite of high metal rates is being researched. Studies in this context are being conducted on arabidopsis thaliana (the mustard family) whose gene map is prepared and which constitutes a model. A mutated arabidopsis, for instance, has been discovered to have been equipped with the capacity to render aluminum harmless. If the genes which play their role in this process can be determined, then gene transplantation will be possible to increase the resistance of plants which are sensitive to aluminum, such as barley, and barley production will accordingly be increased.
All these facts clearly indicate that the earth is like a great and continuously working factory or a guest house continuously becoming full and empty. The pollution which is an inevitable result of the activities of living creatures is kept under control by micro organisms, plants and animals which are the mirror-bearers of Almighty God’s attribute, al-Quddus, “the All-Pure.” The relations between these living creatures and the universe have been so perfectly programmed that all of them beautifully perform their duties.
- Brady, N.C. The Nature and Properties of Soils, 1990, 10th Edition.
- Altunay, Bedirhan. “Biotechnology of the Environment,” 2006, Sizinti, No: 335, p.526–528.