Introduction:
In the last few decades, various alternate energy sources have come into the limelight, but the hyperbolic use of coal as a prime energy source cannot be counterbalanced especially country like India where plenty of coal reserves are found. This coal combustion produces lots of byproduct whose management and disposal had become an unsolved problem hence it is the need of the hour that different method must be discovered for its proper disposal or its utilization for alternate purposes. Fly ash is disposed either by wet or dry method. In wet method the fly ash is washed out with water in artificial lagoons and is called Pond Ash, while in dry method it is dumped in landfills and fly ash basins. Both of these methods leads to degradation of soil and are harmful to human health also these methods of disposal are very costly so there is an urgent need for the techniques for safe and profitable utilization of fly ash. So various beneficial utilization of fly ash in agriculture are:
FLY ASH FOR IMPROVING SOIL PROPERTIES:
Soil texture: Alteration of the soil texture is possible through the addition of appropriate quantities of fly ash. Several experiments have been performed to measure the physical properties for a variety of soils mixed with up to 50% fly ash, which revealed that soil and fly ash mixture tend to have lower bulk density, higher water-holding capacity and lower hydraulic conductivity than soil alone due to its textural manipulation through fly ash mixing. Application of high rates of fly ash can change the surface texture of soils, usually by increasing the silt content.
Bulk density: The particle size range of fly ash is similar to silt and changes the bulk density of soil. Several experiments have been performed to measure the physical properties for a variety of soils mixed with up to 50% fly-ash which reveals that soil fly ash mixture tend to have lower bulk density, higher water-holding capacity and lower hydraulic conductivity than soil alone.
Water holding capacity: Fly ash application to sandy soil could permanently alter soil texture, increase micro porosity and improve the water-holding capacity as it is mainly comprised of silt-sized particles. A gradual increase in fly ash concentration in the normal field soil (0, 10, 20 up to 100% v/v) was reported to increase the porosity, water-holding capacity, conductivity and cation exchange capacity. This improvement in water-holding capacity is beneficial for the growth of plants especially under rain fed agriculture.
Soil pH: Depending on the source, fly ash can be acidic or alkaline, which could be useful to buffer the soil pH. The hydroxide and carbonate salts give fly ash one of its principal beneficial chemical characteristics, the ability to neutralize acidity in soils. Fly ash has been shown to act as a liming material to neutralize soil acidity and provide plant-available nutrients. Most of the fly ash produced in India is alkaline in nature; hence, its application to agricultural soils could increase the soil pH and thereby neutralize acidic soils. Researches have shown that the use of fly ash as liming agent in acid soils may improve soil properties and increase crop yield.
As a source of nutrient: Fly ash contains some mineral nutrients such as K, Ca, Mg, Fe, Zn, B, Mo and S which promote plant growth and also alleviate the condition of nutrient deficient soils. Lime in fly ash readily reacts with acidic components in soil leading to release of nutrients such as S, B and Mo in the form and amount favourable to crop plants.
Use of fly ash in composting: In sewage sludge composting, lime is used to raise the pH and thereby to kill pathogens and to reduce the availability of heavy metals enriched in sludge. Since alkaline coal fly ash contain a large amount of CaO, it can serve the purpose of lime as it reduced the availability of heavy metals by physical adsorption and precipitation at high pH. Moreover, it is also cheaper than lime.
For improving crop growth and yield: Fly ash, having both the soil amending and nutrient enriching properties, is helpful in improving crop growth and yield in low fertility acid lateritic soils. Many researchers have demonstrated that fly ash increased the crop yield of wheat (Tritiucm aestivum), alfalfa (Medicago sativa), barley (Hordeum vulgare), Sabai grass (Eulaiopsis binata), mung (Vigna unguiculata) and white clover (Trifolium repens) and improved the physical and chemical characteristics of the soil.
Saving of Chemical fertilizer: Use of fly ash along with chemical fertilizers and organic materials in an integrated way can save chemical fertilizers as well as increase the fertilizer use efficiency (FUE).
Fly ash as pesticide: In a study it is found that more than 50 species of insect pests of various major crops were susceptible to fly ash treatment. Fly ash dusting at 40 kg/ha on rice could control both chewing and sucking pests such as leaf folder, yellow caterpillar, spiny beetle, ear head bug, brown bug, black bug, grasshoppers, brown plant hopper and green leafhopper. The addition of 5% fly ash to soil was also found to significantly increase the growth of tomato plants and reduce the amount of galling on the roots caused by root-knot nematode.
Apprehension that need further research
In spite of several advantages of using fly ash in agricultural field, many are afraid of its natural radioactivity and heavy metal content. Researches have proved that the effect of this radioactivity and heavy metal content is in the safe limit if fly ash is being applied in optimum quantity. However, long-term confirmatory research and demonstration are necessary as is the case for any new agricultural input materials like fertilizers, amendments or pesticides. Simultaneously, in future attention should be given on some important areas related to fly ash utilization, like proper handling of dry ash in plants as well as in fields, ash pond management (i.e., faster decantation, recycling of water, vertical expansion rather than horizontal etc.), long term studies of impact of fly ash on soil health, crop quality, and continuous monitoring on the characteristics of soil as well as fly ash.
Authors
Navya Vishweshwar Bhat1* and Pavankumar Goudar2
Department of Fruit Science and Horticulture Technology, OUAT, Bhubaneswar, India, 751003
Department of Agronomy, COA, Pasighat, CAU, Imphal, India, 791102
*Navya vishweshwar Bhat (email- navya.vbhat@gmail.com)