Coal as a Soil Amendment

I wrote this research proposal back in 1990, when I was an Associate Professor in the University of Kentucky Department of Plant Pathology. It was time to renew my Hatch Project – a research professor’s official rationale for federal funding in colleges of agriculture. For some time, I had contemplated leaving my position at UK. And since I expected that I would never have to actually undertake the proposed research, I was free to think outside the usual boxes.

I thought about how bringing diverse fields of knowledge together often produces unexpected results. Coal as a soil amendment came to mind. One of my specialties was soil-borne plant pathogens and soil microbiology, and Kentucky is a major coal-producing state. UK had recognized specialists in coal chemistry, and there was active agricultural research on reclamation of mined lands. I did some preliminary tests, got some interesting results, and went ahead with a proposal that centered on use of coal as a soil amendment. A possibly counter-intuitive combination that had some clear underlying rationales.

So I went ahead and developed a proposal. I thought that even if I left UK, the proposal might get distributed and somehow influence someone. In 1991 I left behind the concerns of an academic job. Recently I thought about the old proposal. It does not seem to be online, and there does not seem to have been a great flowering of coal-amendment research recently. So, in the public interest, back from the grave, here is the text of a draft from 1990.  – rf


Use of Coal in the Control of Soilborne Plant Diseases
A preliminary research proposal  – May 11, 1990
R.S. Ferriss, University of Kentucky Department of Plant Pathology

Coal is an abundant and relatively inexpensive resource.  There has been a limited amount of research on the use of coal (primarily lignite) as a soil amendment or potting mix component.  Results of this research have been mixed: some reports have described increased plant growth due to alteration of soil temperature or other, unknown factors; while other reports have indicated that any plant growth increases can be explained by the content of plant nutrients in coal.  In most of this work, few details have been provided about the type of coal that was used.  The only reported uses of coal that are related to plant pathogens have involved the use of coal as a carrier for biological control organisms.  Diseases caused by soilborne pathogens are limiting factors in the production of many crops.  Although control by pesticides is feasible in some production situations, their use has been limited by environmental considerations and the evolution of fungicide-tolerant pathogen biotypes.  A number of soilborne diseases can be controlled by the use of organic amendments.  In recent years, most amendment-related research has centered on disease-suppressive composts.  Suppressiveness appears to be related to anti-fungal toxins, antagonistic microorganisms in the compost, and/or the fostering of antagonistic soil microorganisms by compost amendment.  Coal is chemically similar to highly-decomposed soil organic matter (humic acids), and creosote, a coal decomposition product, has long been used as an anti-fungal treatment for wood.  Sulfur, which is present in relatively high concentrations in some coals, is among the oldest fungicides, and is frequently used agriculturally to reduce soil pH.

The proposed research would investigate the use of coal as a soil amendment for the control of plant diseases.  Since coals vary greatly in chemical composition, tests would include a number of well-characterized coals of contrasting types.  Since particle size affects surface area, and thus chemical and physical properties, a number of coal particle sizes would also be evaluated.  Initial experiments would involve the amendment of soils that are naturally-infested with common soilborne fungal pathogens, such as Theilaviopsis basicola, Phytophthora parasitica var. nicotiana, or Rhizoctonia solani.  Populations of the pathogen and other soil microorganisms would be monitored over time, and plants would be grown to evaluate coal toxicity and effects on disease.  If initial greenhouse and growth chamber experiments indicate that coal might be of use in pathogen control, further work would evaluate effects in the field, and attempt to identify coal characteristics that are associated with disease control.

Previous Research
The use of coal and coal-based products in agriculture and horticulture has received sporadic research attention in various parts of the world.  This research can be broadly classified into four areas according to the intended commercial use:  lignite as a potting-mix component, coal as a soil amendment, coal as a mulch, and coal as a carrier for microorganisms.  The following review is based on the English language literature; a good deal of additional work has been published in Polish, Hungarian and Russian.

Lignite as a Potting-Mix Component
Media used for container production of plants are usually composed of a primary, light-weight material of high water-holding and cation exchange capacities, along with other materials that improve wettability, alter bulk density, and/or add nutrients.  Sphagnum peat is the standard primary component in much of the world; however, considerations of cost and availability have led to the use of a number of other materials, including tree barks, composts, and lignite.  Beardsell and coworkers (1979, 1982) found that lignite is difficult to wet after drying (apparently due to shrinkage), but that pinebark-lignite potting mixes have good water relations if they are not allowed to dry out.  Richards and coworkers (1986) found that addition of 20 % lignite to pine bark-sand potting mixes increased the total water holding capacity, but did not increase the amount of water available to plants.  They took these results to indicate that “brown coal is a highly porous medium and that adsorbed water is held under high tension upon, and/or within the brown coal particles”.  They also found that addition of lignite increased plant growth, and speculated that this effect was probably due to its high cation exchange capacity causing greater retention of nutrients.

Coal as a Soil Amendment
Research on amendment of soil with coals has concentrated on the use of coal-derived “humic fertilizers” or soil conditioners, and the use of lignite to increase the organic matter content of soils.  In a series of experiments, Kozhekov and coworkers (1968) studied the effects on plant growth of ‘oxidized coal’, ammonified coal, and coal treated with ammonia and superphosphate (‘humophos’).  In general, they concluded that plant growth increases were similar to those produced by fertilizers containing an equivalent amount of primary plant nutrients.  However, they noted that low rates (about 500 kg/ha) of humophos sometimes increased yield more than equivalent fertilizer, and that this material might have a longer residual effectiveness than inorganic fertilizers.  They also noted that some reports of increased plant growth with humophos might be due to soil temperature increases associated with a mulching effect, and that there still were questions about reported growth increases with small amounts of humic acids.  In response to claims that coal treated with ammonia could act as a slow-release fertilizer, Berkowitz and coworkers (1970) tested the effects of different nitrogen-treated coals on plant growth and nitrogen uptake.  They found the nitrogen-treated coals to be “agrobiologically inert”.  In pot experiments with barley seedlings, Cairns and Moschopedis (1971) found that a low grade, weathered, sub-bituminous coal and a sulfomethylated derivative increased plant growth and nutrient contents similarly to an equivalent amount of ammonium nitrate.  Dzhanpeisov and coworkers (1984) reported that coal-derived soil conditioners could greatly increase the amount of water-stable aggregates in a coarse-textured, structureless soil.  El-Abedine and Hony (1982) added lignite to Egyptian soils.  Microscopic examination indicated some physical breakdown and fraying of the lignite particles with time — possibly indicating the results of microbial activity.  The lignite increased the cation exchange capacity and rate of water infiltration of the soil.  Plant yield increases appeared to be due to nutrients in the lignite.  In a cursory overview of Hungarian research, Gati (1982) reported vague beneficial effects of amendment of sandy soils with peat-lignite mixtures.  He reported the novel use of a relatively deep ‘blanket layer’ of amendment to improve interception of leachable materials.  Iswaram and coworkers (1980) placed 0.5 g of peat, charcoal or ‘coal’ in a planting hole in soil along with a pea, soybean or mung bean seed.  All of the materials increased plant growth, with the effect being greatest for coal combined with Rhizobium inoculation.  There have been a number of reports on the effects of charcoal on plant growth and nodulation by Rhizobium.  For example, Vantsis and Bond (1950) reported increases in plant growth and nodulation with charcoal added to sand, and speculated that the effect was due to adsorption of toxins.  Conversely, Devonald (1982) found no increases in growth or nodulation when charcoal was added to potting mixes, and speculated that the peat in the mixes was sufficient to adsorb any toxins that were present.  The adsorptive ability of activated charcoal has resulted in a large number of investigations of its use as a herbicide safener (Hoagland, 1989).

Coal as a Mulch
In one of the most detailed investigations of soil-coal interactions, Fairbourn (1974) reported on the effects of a surface layer of 1-3 cm-diameter pieces of ‘stoker coal’.  A 2.5 cm-deep coal layer was applied between rows, with 10 cm strips left bare for crop rows.  The coal mulch increased soil water storage, early-season soil temperatures, and growth of corn, sorghum, and soybeans.  The coal pieces rapidly weathered to smaller particles and became integrated into the soil.  In contrast to the fractured, cracked crust of the bare soil treatment, the coal treatment had a “very friable” surface structure that facilitated planting.  The coal-soil surface layer had reduced water retention, and thus an increased water infiltration rate and wetting depth.  The coal mulch increased soil temperatures at 15 cm by approximately 2 C.  Separate experiments indicated that plant growth increases were due to this increased soil temperature, rather than soil fertility or light effects.  Sharratt and Glenn (1986) applied a slurry containing coal dust to the surface of a West Virginia peach orchard at a rate of 18 Mg coal/ha.  The coal mulch increased night-time soil temperatures by approximately 2 C, and increased bud temperature 0.5 C during radiative frost conditions.

Coal as a Carrier for Microorganisms
Lignite has been widely used as a carrier for applying Rhizobium inoculum to seeds (Tilak and Subba Rao, 1978).  Jones and coworkers (1984) used stillage (a grain-alcohol byproduct) absorbed on lignite as a substrate for growth of two biological-control fungi, Gliocladium virens and Trichoderma harzianum.  Although the substrate without antagonists increased disease due to Rhizoctonia solani, substrate colonized by G. virens reduced disease.  Harman and Taylor compared lignite, a bituminous coal, and sphagnum peat as media for the solid matrix priming (pre-planting incubation at sub-germination water potentials) of seeds in combination with bacterial and fungal biocontrol agents.  The bacterial antagonist gave best control of damping-off when mixed with bituminous coal, while the fungus gave best control when mixed with lignite.  The authors ascribed the differences to a pH effect, and commented that the peat was difficult to work with in this application.

Rationale for Further Research
The previous research on uses of coals and coal-related materials in plant production indicates that some commercial applications may be feasible, and there is limited commercial use of coal as a soil amendment in the US (Anonymous, 1985).  However, many questions remain about the actual reasons why application of coals to soils might be desirable.  If their use were based only on their limited content of plant nutrients, it is unlikely that coals could be an economically viable alternative to more traditional fertilizers.  However, consideration of documented and possible theoretical interactions of coals with soils indicates that there may be a number of additional commercial applications.  Two potential uses are considered below: use of high-sulfur coals to reduce soil pH, and use of coals in the control of soilborne plant diseases.

Soil pH has a substantial effect on the availability of soil nutrients, and thus on plant growth (Rowell, 1988).  In most agricultural situations, soil pH is below the optimum for growth of most crop plants, and thus it is desirable for it to be raised by the application of lime.  However, there are a number of situations where soil pH is higher than optimum, and there is thus a need to lower it.  One example is related to the growth of trees in urban environments (Messenger, 1983; Ware, 1990).  Many commonly planted tree species are native to forest environments where soils have a relatively high content of soil organic matter, and a relatively low pH.  Although urban soils may have an initially favorable pH, runoff from concrete surfaces increases pH, and can result in deficiencies in nutrients such as iron and manganese.  Use of sulfuric acid, elemental sulfur, or sulfate salts to raise pH can present problems of plant toxicity.  Sulfuric acid is produced from high sulfur coal when it is incubated in soil (Harrison, 1978).  Mulches of high sulfur coal might provide a buffered, slow-release source of sulfuric acid for soil acidification.  Another example is calcareous soils.  The high pH of these soils necessitates foliar application of many plant nutrients, and their low exchange capacity fosters the rapid leaching of nutrients.  Amendment with high sulfur coal might provide a way to lower the soil pH and add exchange capacity.  High pH favors bacterial activity, and thus hastens the decomposition of most applied organic amendments.  However, since coal is chemically similar to highly-decomposed soil organic matter (humic acids), it might provide a relatively long-lasting effect.  Additionally, high sulfur coal is a major world resource of sulfur, and could be used to supply this nutrient to sulfur-deficient soils (Kanwar, and Mudahar, 1986).

Soilborne plant pathogens are important limiting factors in the production of many crops.  Prominent examples in Kentucky include black shank of tobacco (caused by Phytophthora parasitica var nicotiana), and black root rot of a number of crops (caused by Theilaviopsis basicola).  Additionally, many crops are affected by sub-clinical pathogens, which can reduce yield without producing recognizable above-ground symptoms (Suslow and Schroth, 1982; Lynch, 1983).  Although pesticides are used commercially to control some soilborne pathogens, their future widespread use is made questionable by their cost, environmental concerns, the evolution of resistant pathogens, and their inactivation by interactions with soil clay and organic matter.  In recent years, there has been considerable interest in the biological control of soilborne pathogens (Cook and Baker, 1983).  Control by the addition of specific antagonists and through the use of organic soil amendments have both received widespread attention.  Although the specific characteristics of most amendments which promote control are unclear, some progress has been made in work on disease-suppressive composts (Hoitink and Fahy, 1986).  This work has identified three primary factors: the presence of organic compounds toxic to pathogens, the presence of antagonistic microorganisms in the amendment, and the fostering of antagonistic microorganisms after the amendment is added to soil.  Coals have both similarities to and differences with traditional organic amendments.  On the one hand, the basic composition of coal is very similar to that of highly decomposed soil organic matter (Waksman, 1938; Tate, 1987).  On the other hand, coal contains little readily metabolizable material, and may contain additional components which are toxic to organisms (Gormley et al, 1980).  Additionally, industrial processing of coal can produce other compounds with high toxicity or carcinogenicity, including creosote, which is widely used in the preservation of wood against attack by fungi (International Agency for Research on Cancer, 1985).  A number of fungi and bacteria have been identified which can partially decompose coal and its related precursor, lignin (Kirk and Farrell, 1987; Olson, and Brinkman, 1986; Scott et al, 1986).  Furthermore, sulfur and sulfur-containing compounds have long been used in the control of insects and fungi (Ainsworth, 1981).  Since much of the sulfur in some coals is present in organic forms (Olson, and Brinkman, 1986), it is possible that sulfur-containing organic compounds toxic to fungi might be produced in the course of microbial decomposition (Wainwright, 1988).

The above considerations indicate that it is possible that amendment of soils with coal could effect some control of soilborne plant pathogens.  Possible mechanisms include the presence of toxic materials in the coal itself, the production of toxins during microbial decomposition in soil, the provision of substrates for antagonistic microorganisms, changes in soil pH, adsorption of phytotoxins, addition of plant nutrients, and the alteration of soil water relations.  There has been little evaluation of the effects of addition of coals to soils on soil microorganisms, and apparently no research on effects on plant pathogens.  It is possible that some of the reported inconsistencies in plant response to coal amendments could be due to sporadic control of pathogens.  

Plan of Investigation
In order to adequately investigate the control of soilborne plant pathogens by coal, it would be necessary to take into consideration a number of factors.  Primary among these would be the nature of the coal used.  Coals vary widely in their ultimate sources, toxicity to macrophages, and nutrient contents (Lindahl and Finkleman, 1986; Lyons and Alpern, 1989; Seemayer and Maojlovic, 1980).  It is possible that coals may also vary greatly in their effects on soilborne pathogens, so it would be important to evaluate a representative sample of well-characterized types of coal.  Another factor which would probably influence activity in soil would be the particle size of the coal.  Smaller particles have a larger surface area per unit weight, and thus would be expected to release constituents more rapidly and be more subject to microbial degradation.  However, processing and safety considerations might place a practical lower limit on particle size.  The effects to be evaluated would also be an important consideration.  Although it is unlikely that coal would have a restricted spectrum of biological activity, it would be important to evaluate effects on a taxonomically representative sample of soil microorganisms.  Additionally, it is quite possible that some coals could be toxic to plants, particularly when first added to soil.  It would thus be essential to also evaluate the phytotoxicity of amended soil.  Finally, just as coals can vary, soils vary greatly in their physical and chemical characteristics.  It would be important to evaluate effects in different soils, particularly ones that differ in pH and buffering capacity, and to monitor soil pH after amendment.

Taking these factors into consideration, the most logical research approach would be a progressive evaluation of factors, in which the design of each experiment takes into consideration the results of previous ones.  Initial experiments would concentrate on the comparison of different coals.  Amendment treatments would include well-characterized samples of the major coal ranks (including high and low sulfur coals), a commercially-available coal-based soil amendment, and a non-coal amendment marketed specifically for control of soilborne diseases (such as a chitin-based material).  A single soil containing naturally high levels of an important soilborne pathogen would be used.  It would be amended at a moderately high rate, and incubated under greenhouse conditions.  In addition to populations of the major pathogen, variables to be monitored would include: populations of total soil bacteria, fluorescent Pseudomonads, total soil fungi, Fusarium spp., Rhizoctonia spp, and Pythium spp.; effects on tomato seedling establishment and early growth; and soil pH.  Each variable would be assessed at setup, and after 2, 4, 8, 12, and 16 weeks.  Based on the results of the initial experiment, similar experiments would be performed using a number of particle sizes and rates of coals of particular interest.  Further work would examine the effects of other coals that are related to ones identified as being of interest in the initial experiments, and effects in other soils (particularly ones for which lowering pH or increasing infiltration rate would be desirable).  With the aid of cooperators with expertise in soil chemistry and physics, additional research would evaluate the effects of particular amendments on soil nutrient availability and leaching, as well as soil water relationships.  Once a few promising amendments had been identified, effects on disease, soil microorganisms and plant growth would be evaluated in field experiments.  Ultimately, work might be directed toward development of processed coal products of commercial potential.

Literature Cited
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*Berkowitz, N., Chakrabartty, S. K., Cook, F. D., and Fujikawa, J. I. 1970. On the agrobiological activity of oxidatively ammoniated coal. Soil Science 110:211-217.
*Cairns, R. R., and Moschopedis, S. E. 1971. Coal, sulfomethylated coal and sulfonated coal as fertilizers for solonetz soil. Can. J. Soil Sci. 51:59-63.
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*Devonald, V. G. 1982. The effect of wood charcoal on the growth and nodulation of garden peas in pot culture. Plant and Soil 66:125-127.
*Dzhanpeisov, R. D., Popova, N. S., Akkulova, Z. G., Kricheskiy, L. A., Sokolova, T. M., Marchenko, A. Ye., and Ramazanova, A. R. 1984. Study of new polymeric soil conditioners based on the humic acids of coal. Soviet Soil Science 16(5):83-89.
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*International Agency for Research on Cancer. 1985. Polynuclear Aromatic Compounds, Part 4, Coal-tars and Derived Products, Shale-oils and Soots. IARC Monographs on the Evaluation of the Carcinogenic Risk of Chemicals to Humans. Vol. 35.
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*Jones, R. W., Pettit, R. E., and Taber, R. A. 1984. Lignite and stillage: carrier and substrate for application of fungal biocontrol agents to soil. Phytopathology 74:1167-1170.
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*Lindahl, P. C., and Finkleman, R. B. 1986. Factors influencing major, minor, and trace element variations in U.S. coals. Pages 61-69 in Vorres, K. S., ed. Mineral Matter and Ash in Coal. Amer. Chem. Soc. Symposium Series 301.
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*Lynch, J. M. 1983. Soil Biotechnology: Microbiological Factors in Crop Productivity. Blackwell Sci. Pub., Oxford. 191 pp.
*Messenger, S. 1983. Treatment of chlorotic oaks and red maples by soil acidification. J. Arboric. 10:122-128.
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*Richards, D., Lane, M., and Beardsell, D. V. 1986. The influence of particle-size distribution in pinebark:san:brown coal potting mixes on water supply, aeration and plant growth. Scientia Horticulturae 29:1-14.
*Rowell, D. L. 1988. Soil acidity and alkalinity. Pages 899-898 in: A. Wild, ed. Russell’s Soil Conditions and Plant Growth, 11th edition. John Wiley & Sons, NY. 991 pp.
*Scott, C. D., Strandberg, G. W., and Lewis, S. N. 1986. Microbial solubilization of coal. Biotechnology Progress 2:131-139.
*Seemayer, N. H, and Maojlovic, N. 1980. Biological effects of coal mine dusts on macrophages. Pages 5-12 in: R. C. Brown, M. Chamberlain, R. Davies, and I. P. Gormley. The In Vitro Effects of Mineral Dusts. Academic Press, NY. 373 pp.
*Sharratt, B. S., and Glenn, D. M. 1986. Orchard microclimate observations in using soil-applied coal dust for frost protection. Agric. and Forest Meteorol. 38:181-192.
*Suslow, T. V., and Schroth, M. N. 1982. Role of deleterious rhizobacteria as minor pathogens in reducing crop growth. Phytopathology 72:111-115.
*Tate, R. L. 1987. Soil Organic Matter: Biological and Ecological Effects. John Wiley & Sons, New York. 291 pp.
*Tilak, K. V. B. R., and Subba Rao, N. S. 1978. Carriers for legume (Rhizobium) inoculants. Fertiliser News 23(2):25-28.
*Vantsis, J. T., and Bond, G. 1950. The effect of charcoal on the growth of leguminous plants in sand culture. Ann. Appl. Biol. 37:159-168.
*Wainwright, M. 1988. Metabolic diversity of fungi in relation to growth and mineral cycling in soil— A review. Trans. Brit. Mycol. Soc. 90:159-170.
*Waksman, S. A. 1938. Humus: Origin, Chemical Composition, and Importance in Nature. Second Edition. Williams & Wilkins Co. Baltimore. 526 pp.
*Ware, G. 1990. Constraints to tree growth imposed by urban soil alkalinity. J. Arboric. 16:35-38.

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