Reclaiming the agricultural potential of degraded soils with Class F fly ash. Abstract

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1 Reclaiming the agricultural potential of degraded soils with Class F fly ash Wayne Truter 1, Du Toit Wilken 2, Leushantha Mudaly 1 and Richard Kruger 3 1 Department of Plant Production and Soil Sciences, Faculty of Natural and Agricultural Sciences, University of Pretoria, South Africa 2 Centre of Environmental Studies, Faculty of Natural and Agricultural Sciences, University of Pretoria, South Africa 3 Richonne Consulting, Somerset West, Western Cape, South Africa Abstract Many high potential agricultural soils in South Africa are impacted by surface coal mining and poor agricultural practices. The result is a loss in agricultural yield of croplands and/or the deterioration of natural resources over a long period, during the time the land lies fallow before intensive mining commences. This situation is aggravated further by the disturbance of topsoil during the extraction of coal followed by the problems encountered during the replacement of soils. This generally results in the subsequent lower agricultural potential of the terrain which is challenging to rectify due to the severe effect of chemical and physical degradation. Contributing factors responsible for the degradation are unsustainable fertilizer practices used during re-vegetation, the management of these production systems and the additional acidity generated in the soils/substrates by pyritic material and their oxidation products. Plant roots are unable to grow properly in these substrates and vegetative yield is severely compromised due to the hostile environment created by the changed chemical and physical properties of the soil. Relative to conventional mine treatments significant increases in biomass production were evident where Class F fly ash had been used as a soil ameliorant, over a 72 month period. The ph of the soil was the most strongly affected chemical soil parameter. It was also noted that fly ash improved the physical nature of the soil changing the soils bulk density and other associated properties. This experimental work was conducted to identify amelioration strategies for the degraded soil using Class F fly ash as the soil ameliorant. The results prove that the Class F fly ash is a viable ameliorant that will facilitate effective reclamation of degraded soils and enable the reinstatement of the optimal agricultural potential of agricultural land disturbed during mining. Keywords: Reclamation, coal mine cover soils, agricultural potential, Class F fly ash Introduction In South Africa, prime agricultural land is a limited commodity and a large proportion of crops are necessarily grown on acidic and nutrient deficient soils in order to meet the demand for increased food production (Katsur and Haubold-Rosar, 1996). Amelioration of these soils is therefore crucial as a means of improving agricultural yield. The conventional practice of liming is not always the most sustainable method of counteracting the soil acidity. Research over the past 15 years at the University of Pretoria has demonstrated the feasibility of using Class F fly ash:- a coal combustion by-product (CCB) to amend acidic substrates (Norton et al., 1998; Truter, 2002; Truter, 2007, Truter, and Rethman, 2002). The work has been successful in quantifying the improvement in various soil properties such as soil ph, bulk density and indirectly, biomass production due to improved soil fertility (Rethman et al., 2000a,b; Reynolds et al., 1999: Truter, and Rethman, 2003, Truter et al., 2001). There are several causes of soil acidity; leaching of basic cations, acidity contributed by sulphate soils, acid rain, removal or harvesting of crops, indiscriminate use of fertilizers (Mbakwe, 1

2 2008) and the contribution from acidic mine spoil.soil ph is considered a master variable as it is the most important diagnostic measurement made on soil. Soil ph controls several chemical processes; ion exchange reduction, oxidation, dissolution, precipitation as well as adsorption and complexation. (Mbakwe, 2008). The main focus of soil rehabilitation is to restore the ph to within the range (Yunusa et al., 2006).This restores plant available nutrients, increases biological activity of the disturbed or degraded soil to levels where vegetative growth can be sustained (Truter and Rethman, 2002). Lime and limestone, both non-renewable natural resources are predominantly used for the rehabilitation process of counteracting soil acidity. Although liming is usually effective it is not always economic (Truter and Rethman, 2002). Some disadvantages of using lime and limestone for rehabilitation are the high treatment cost, armoring of limestone by ferric hydroxides which reduces the efficiency, short duration of the effectiveness of the treatment and, as non-renewable natural resources are being depleted at an alarming rate (Gitari et al., 2008; Bolan et al., 2003). There have been many investigations, which have studied re-vegetation and soil conditions on reclaimed land or acidic soils. The objective of this research was to determine if alternative ways of amendment would create a more sustainable agricultural production system, in which the soil chemical properties and subsequently plant productivity can be improved. As demonstrated in this research fly ash is a cost effective alternate material for amelioration. It is shown that fly ashes have several advantages, and that their application is recommended (Katsur and Haubold-Rosar, 1996, Truter, 2002; Truter, 2007). Methodology A replicated field trial in a randomized block design, with five replications of an untreated control and six soil amendments of cover soil (consisting of a mixture of A and B horizons), with an average depth of 60 cm, was conducted over a six year period in the Mpumalanga Province of South Africa. The area is situated at 1500m above sea level and receives a summer rainfall of mm and experiences dry frosty winters. The treatments consisted class F fly ash (FA), dolomitic agricultural lime (L), an untreated control (C) and a standard mine treatment (SMT). The optimum lime application rate was based on the buffering capacity of the substrate which was determined by using a Ca (OH) 2 titration solution. The test soil had a ph (H2O) of 4.3. It was calculated, from the buffer curve, that the test soil required 10 tons ha -1 of dolomitic lime [L Opt.] to raise the ph of the soil to a ph (H2O) of 6.5, ideal for plant growth. The calculated level of fly ash [FA Opt.], 50 tons ha -1, was based on Adriano et al (1980) findings that indicated that fly ash is the chemical equivalent to 20% of reagent grade CaCO3 in reducing soil ph and supplying plant Ca needs (Truter, 2002), and hence five times the amount of CaCO 3 required neutralizing acidity. The class F fly ash treatments were compared to the aforementioned standard mine treatment, control and lime treatments. All treatments were applied once off (in the establishment season), at the beginning of the trial. Studies conducted evaluated the effect of class F fly ash on soil parameters such as soil ph, soil bulk density (kg m -3) (Hillel, 1982, Tan 2005) and indirectly biomass production (kg.ha -1 ) over a 72 month period. 2

3 Results and Discussion Soil Analyses Soil ph (H20) Soil analyses were conducted after every cropping cycle and the data presented includes the influence of the different treatments on the ph (H 2 0) of the soil. The ph of the soil (Figure 3) was strongly affected by FA and lime. An improvement of up to 2 ph units was evident after 12 months (Figure 1) after treatment, and as cropping continued, and no further soil ameliorant applications were given in the 72-month period, soil ph gradually declined especially for the L treatments. FA L C SMT Poly. (FA ) Poly. (L) Soil ph(h20) Months Figure 1: The influence of treatments, relative to C and SMT treatments, on the soil ph (H 2 0) over a 72-month period Soil bulk density (kg.m -3 ) Table 1 evidently shows that soil treated with class F fly ash had a significantly lower bulk density than the untreated control or the lime treatment. These changes in bulk density had significant effects on other properties such as the hydraulic conductivity and infiltration rate of the soil. Biomass production (kg.ha -1 ) Class F fly ash treatment showed a clear response to the level of application and this was significant in certain growing seasons. This poses the question whether the optimum level of application of these ameliorants has been achieved? Considering the effect these treatments have on soil ph, a shift from an initially acidic soil condition towards a potentially saline condition is a potential concern. 3

4 DM Production (kg ha-1) WACAU-2014, Israel Table 1: The comparative influence of soil ameliorants on the bulk density of an acidic Hutton soil with an original ph ( H2O) of 4.5 Bulk Density (x 10-3 kg m -3 ) Treatments Mean SE(+/-) Control 1.53 a (0.04) Fly ash + Organic material 1.45 b (0.07) Fly ash 1.33 c (0.08) Lime 1.50 a (0.07) *abc Column means with common alphabetical subscripts do not differ significantly (P> 0.05) (Bonferroni Test) This observation, however, indicates that more frequent applications of these ameliorants at lower levels could be considered, but this aspect requires further investigation. The data presented clearly indicates that the fly ash soil ameliorants can improve the agricultural potential of reclaimed soils to the benefit of plant growth as measured in biomass production. FA C SMT Poly. (FA) y = x x - 15 R2 = Months Figure 2: Biomass production on reclaimed soils, treated with class F fly ash, relative to the C and SMT treatments over a 72-month period. Conclusion Results from this investigation indicate that alternative ameliorants such as class F fly ash can have marked beneficial effects which are still evident in the 6 th year after establishment. This would indicate that such ameliorants produce more sustainable vegetation than current practice, and due to their chemical nature and reactivity, long-term residual soil effects are evident. It can be concluded from this experimental work, that this class F fly ash definitely has a much higher CaCO 3 equivalent than the 20%, which was originally assumed and is considered to be closer to 33%. Fly ash treatments supported significantly higher biomass production during the studies. The reinstatement of agricultural potential to support crop production on reclaimed soils has been successful in many areas where rehabilitation is regarded a core business, and much attention is 4

5 given to the amelioration of important underlying soil properties responsible for sustainable plant growth. References Adriano, D.C., Page, A.L., Elseewi, A.A., Chang, A.C. and Straughan, I Utilization and disposal of fly ash and other coal residues in terrestrial ecosystems: A review. J Environ. Qual. 9: Hillel, D Introduction to Soil Physics. Academic Press, Inc. ISBN Katsur, J. and Haubold-Rosar, M Amelioration and Reforestation of Sulfurous mine soils in Latsatia (Eastern Germany). Water, Air and Soil Pollution. 91 (1-2) pp Mbakwe, I., An Evaluation of the Effectiveness of Coal Ash on an Amendment for Acid Soils. M.Sc. Thesis, University of Stellenbosch, South Africa. Norton, L.D., Altiefri, R. and Johnston, C Co-Utilization of by-products for creation of synthetic soil. S. Brown, J.S. Angle and L. Jacobs (Eds.) Beneficial Co-utilization of Agricultural, Municipal and Industrial By-products. Kluwer Academic Publishers, Netherlands Rethman, N.F.G, du Toit, E.S., Ramagadza, E.J. and Truter, W.F., 2000a. The use of fly ash and biosolids to ameliorate soils, re-vegetate disturbed areas and improve plant productivity. Proc. 25 th Conf. Canadian Land Reclamation Assoc. Edmonton, Canada. Rethman, N.F.G., du Toit, E.S., Ramagadza, E.J., Truter, W.F., Reynolds, K.A., Kruger, R.A., 2000b. Soil amelioration using waste products. Proc. Remade Lands Recl. Conf. Perth, Western Australia. Pp Reynolds, K.A., Kruger, R.A. and Rethman, N.F.G The manufacture and evaluation of an artificial soil prepared from fly ash and sewage sludge. Proc International Ash Utilization Symposium. Kentucky, U.S.A. Tan, T.K Soil Sampling, Preparation, and Analysis. 2nd Edition. CRC Press. Taylor and Francis Group. ISBN Truter, W.F., Use of waste products to enhance plant productivity on acidic and infertile substrates. MSc (Agric) Thesis, University of Pretoria, South Africa. Truter, W.F., Reclaiming degraded mine soils and substrates with domestic and industrial by-products by improving soil chemical properties and subsequently enhancing plant growth: A greenhouse study. PhD Thesis. University of Pretoria Truter, W.F. and Rethman, N.F.G Reclaiming acidic, infertile and unstable soils with urban and industrial by-products by improving chemical properties and enhancing plant growth. In. Proc. of the Canadian Land Reclamation Association. Truter, W.F. and Rethman, N.F.G., Reclaiming mine lands in grassland areas with industrial and urban by-products. Proc. of the International Rangeland Conference, Durban, South Africa. Truter, W.F., Rethman, N.F.G., Reynolds, K.A. and Kruger, R.A The use of a soil ameliorant based on fly ash and sewage sludge. In Proceedings of the 2001 International Ash Utilization Symposium, Lexington Kentucky, USA. 5