Scientific registration n : 1 Symposium n : 40 Presentation : poster Soil Acidity Amelioration by Limestone and Gypsum Applications Correction de l acidité de sol par apport de calcaire et de gypse SHAMSHUDDIN J. Department of Soil Science, Faculty of Agriculture, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia. INTRODUCTION Rubber, oil palm and cocoa are major agricultural commodities of Malaysia. These plantation crops are grown on the soils classified as Ultisols and Oxisols. The soils occupy an area approximately 72% of the country's land surface (IBSRAM, 1995). The clay fraction of the soils is dominated by kaolinite and sesquioxides (Tessens and Shamshuddin, 1983), with charges on the mineral surfaces change with changing ph and/or ionic strength (Uehara and Gillman, 1981). The rapidly increasing population in Malaysia is forcing food crop production onto such marginal and acid soils. These soils are normally used for maize production during the early phase of rubber and oil palm cultivations. But, the yield of the crop is limited by the low ph, high Al, and Ca and/or Mg deficiencies in the topsoils (Shamshuddin et al., 1991). Subsoil acidity has been a common phenomenon on highly weathered soils of the tropics as Ca from lime application usually accumulates in the topsoil (Pavan et al., 1984; Gillman et al., 1989). As such, plant roots restrict their growth to the plow layer where lime is originally incorporated. Aluminium toxicity is a major factor in acid soil infertility as it inhibits root growth, either by inhibition of cell division, cell elongation or both (Marschner, 1991). Deficiency in Ca and/or Mg in the subsoil aggravates the limitation of root growth by Al toxicity. The objective of this study was to determine the efficacy of ground magnesium limestone (GML) and gypsum as soil ameliorants. MATERIALS AND METHODS Three acid soils were used in this study. The Bungor (Ultisol) and Prang (Oxisol) soils were used for leaching experiment in PVC columns and pot study, while the Bungor and Rengam soils were used for the field trials using maize as the test crop. The treatment for the leaching experiment were 0, 0.5, 1.0, 2.0 and 8 t ha -1 of GML and gypsum in all combinations. The treated soils were watered twice weekly and leachates were collected and analyzed every 30 days. The soils in the PVC columns were sampled after 180 days. In the pot experiment, the topsoils of the Bungor and Prang series were 1
mixed with GML, gypsum and their combinations with the ameliorants application rates of 0, 0.5, 1.0, 2.0 and 4 t ha -1. Maize was grown for 40 days before harvesting. The field plots on the Rengam soil were originally treated with GML in 1986 at rate 0 (without fertilizer), 0, 0.5, 1.0, 2.0 and 8.0 t GML ha -1 incorporated into 0-30 cm depth (henceforth referred to as T 1, T 2, T 3, T 4, T 5, T 6 and T 7 ). The current experiment started in 1991 with GML annual application at the rate of 0, 0.5, 1.0 and 2 t ha -1 in plots T 1, T 2, T 3 and T 4, respectively. The T 5, T 6 and T 7 received no further treatment and, therefore, regarded as residual plots. Four crops of maize were grown on the plots. Another field trial on the Bungor soil was conducted where various rates of GML and gypsum were applied. Maize was planted and the soils in the experimental plots were sampled at the maize harvest. The fate of Ca RESULTS AND DISCUSSION The Ca movement in the PVC columns in the leaching experiment of the GML treated soils was studied. Calcium was found to remain in the zone where the lime was originally incorporated (Fig. 1). This finding is similar to that has already been reported in other studies (Pavan et al., 1994; Gillman et al., 1989). The phenomenon is explained by the following. The highly weathered soils of Malaysia are dominated by kaolinite and sesquioxides (Tessens and Shamshuddin, 1983), and the charge on the exchange complex of these minerals increases with increasing ph (Uehara and Gillman, 1981). When the GML was applied, soil ph increased which was followed by an increase in the CEC. Hence, the Ca were held by the negatively-charged surfaces. The Ca would not moved down the soil profile until the ph drop. When GML was applied together with gypsum, the Ca moved deeper into the soil profile. In this case, the amounts of Ca in the soil were higher than those that required to neutralize the CEC raised by the increase in soil ph. The excess Ca naturally moved into the subsoil. From this study, an application of 2 t GML + 1 t gypsum ha -1 is recommended. If this is done, GML detoxify Al in the topsoil, while the Ca moves downward, alleviating Ca deficiency in the subsoil Effect of SO 4 adsorption on ph and charge The SO 4 2- from the gypsum applied onto the soils was adsorbed specifically on the oxides present in the soil. This resulted in the increase of the negative charge on the oxides present in the soil. This resulted in the increase of the ph and negative charge on the oxides. However, the resultant increase in ph was only observed in the Oxisol, the Prang soil. In the Ultisol, the Bungor soil, the ph tended to decrease. When gypsum was applied, there was another opposing reaction that took place simultaneously. This other reaction was the replacement of Al on the exchange complex by Ca. As a consequence, Al went into solution and ph was lowered accordingly. Both 2
SO 4 2- adsorption and Al replacement by Ca occurred in the Oxisol and Ultisol, but the former was more dominant in the Oxisol as the soil contains higher amounts of oxides. On the contrary, exchangeable Al was high in the Ultisol and, therefore, Al replacement was dominat. The beneficial effect of gypsum application is only observed in the Oxisol. Hence, gypsum can be used to ameliorate Oxisols, but not Ultisols. As has been shown, the Ultisol became more acid than before gypsum was applied. The presence of SO 2-4 in the soil can result in other anions being removed out of the system. This can occur for NO - 3. In the Oxisol, SO 2-4 can replace NO - 3 which, in - turn, moves downwards and accumulates in the subsoil. In the subsoil, the NO 3 is attracted and absorbed onto the positive charge sites on the surfaces of the oxides. The NO - 3 is, threfore, not lost into the groundwater but retains in the soils. Removal of soil acidity In the tropics, aluminium toxicity is one of the major factors limiting crop growth. Aluminium inhibits root growth and consequently reduces crop yield. Studies in the past indicated that Al toxicity can be overcome by GML application at the rate of 2 t ha -1 or to a limited extent by gypsum application at an appropriate rate (Shamshuddin et al., 1991; Ismail et al., 1993). Thus, a good agronomic option for farmers is to apply GML together with gypsum in the topsoil. The adsorption of SO 4 2- in the Oxisol increased ph and negative charge, which consequently reduce Al, rendering the soil conditions more conducive for plant growth. Aluminium toxicity was also reduced by gypsum application via another mechanism. The SO 4 2- from gypsum formed soluble AlSO 4 + ion pairs which found their way into the groundwater after a rain event. The presence of extra Ca in the soil can detoxify Al to a certain extent (Alva et., 1986). When GML and/or gypsum are applied onto the soils, Ca are made available in large quantities, and this consequently reduce Al toxicity. As such, there exist a Ca/Al concentration ratio above which crop yield is not affected by Al toxicity anymore. Relative top maize weight % plotted against soil solution Ca/Al concentration ratio is given in Fig. 2. A 10% drop in relative top maize weight corresponds to a Ca/Al concentration ratio of 79. It shows that Ca needs to be considerably high in the soil solution of Ultisols and Oxisols in order to alleviate Al toxicity for maize production. The Ca/Al concentration ratio can, therefore, be used as an index of soil acidity. The long-term effect of GML application The changes of exchangeable Ca with time were studied. The plots T 5 - T 7 were started in 1986 and there was no further GML was applied, whereas in T 1 - T 4 GML was applied yearly beginning in 1991. In the residual treatments (T 5 - T 7 ), the exchangeable Ca in July 1991 remained reasonably high although GML was applied 5 years earlier. After 1991, the exchangeable Ca began to decrease. In the case of T 5, where 2 t GML ha -1 was applied, the exchangeable Ca was reduced to the level of the 3
untreated soil. However, in the T 6 and T 7 where GML rate was 4 and 8 t ha -1, respectively, the exchangeable Ca was considered within the range suitable for maize growth. This means that the beneficial effect of GML at the rate 4 t ha -1 or higher can last up to 8 years. The amount of exchangeable Ca in T 3 (1 t GML ha -1 applied annually) was reasonably high. The exchangeable Ca was higher in T 4, where 2 t GML ha -1 were applied annually, than that of T 3. But, it is too costly to apply GML at 2 t ha -1 annually. Therefore, it is reasonable to recommend liming at the rate of 1 t ha -1 to be applied annually provided that labour is available and cheap. The beneficial effect of liming at this rate is comparable to those of T 6 and T 7. Applying GML at the rate lower than 1 t ha -1 annually is not effective to alleviate Al toxicity in the studied soils. Data on exchangeable Al in this study show consistent results with those of the exchangeable Ca. The amount of exchangeable Al shown in T 6 and T 7 was below the toxic level for maize growth. Maize yield was not affected by Al toxicity in all in these treatments. The exchangeable Al in T 4 was low throughout the experimental period. But in the T 3, the value fluctuated depending on whether the soils were sampled before or after GML application. Soil ph followed the changing trend of the exchangeable Al. When the exchangeable Al increased, ph decreased and vice versa. In T 7, the topsoil ph (H 2 O) remained above 5.0 until 1992, after which it decreased to less than 5.0. CONCLUSION Topsoil acidity in highly weathered tropical soils can be overcome by GML application. To alleviate the subsoil acidity in the soils, GML has to be applied together with gypsum. Gypsum alone is good for amelioration of Oxisols having high oxides content. Gypsum is not suitable to ameliorate Ultisols having high exchangeable Al. Adsorption of SO 4 2- on the surfaces of oxides increases soil ph and negative charge. Liming at the rate of 4 t ha -1 or higher is effective for about 8 years, which is comparable to the annual application of 1 t GML ha -1. ACKNOWLEDGEMENTS The author would like to record his appreciation to the Universiti Putra Malaysia, the National Council for Scientific Research and Development and the Australian Centre for International Agricultural Research for financial and technical supports. REFERENCES ALVA, A.K., EDWARDS, D.G. AND ASHER, C.J. 1986. The role of calcium in alleviating aluminium toxicity. Aust. J. Soil Res. 37: 357-383. 4
ISMAIL, H., SHAMSHUDDIN, J. AND SYED OMAR, S.R.. 1993. Alleviation of soil acidity in a Malaysian Ultisol and Oxisol for corn growth. Plant and Soil. 151: 55-65. IBSRAM. 1985. Report of the inaugural workshop and proposal for implementation of the acid tropical soil management network. International Board for Soil Research and Management, Bangkok, Thailand. GILLMAN, G.P. AND SUMPTER, E.A. 1986. Surface charge characteristics and lime requirements of soils derived from basaltic, granitic and metamorphic rocks in high-rainfall tropical Queensland. Aust. J. Soil Res. 24: 173-192. MARSCHNER, H. 1991. Mechanisms of adaptation of plants to acid soils. Plant and soil 134: 1-20. Keywords: gypsum, limestone, soil acidity, liming, acid soil Mots clés : chaulage, gypse, calcaire, sol acide, acidité du sol 5
Table 1. Relevant chemical properties of the topsoil (0-15 cm depth) and subsoil (30-45 cm depth) of Bungor, Prang and Rengam soils. Cations Soil Depth PH(H2O Ca Mg K Na Al ECEC* Saturatio Fe 2 O 3 O.C. Clay ) n cm cmol kg 1 % g kg -1 Bungor 0 15 30-45 4.29 4.76 1.05 0.83 0.30 0.18 0.22 0.06 0.02 0.02 4.02 3.98 5.16 5.07 72 79 36 38 19.5 8.0 250 300 Prang 0 15 30-45 4.86 5.04 0.40 0.60 0.07 0.03 0.08 0.04 0.02 0.01 1.62 1.09 2.19 1.77 74 62 91 117 51.7 18.2 540 590 Rengam 0 15 30-45 4.83 4.43 1.05 0.72 0.17 0.14 0.08 0.05 0.02 0.01 2.68 2.83 4.00 3.74 67 76 35 29 21.3 12.1 400 450 * ECEC = effective cation-exchange capacity 6
Table 2. Effects of gypsum application on the ph of the Bungor And Prang topsoils (8). ph(h 2 O) Rate (t gypsum ha -1 ) Bungor Prang 0 4.21 4.18 2 4.16 4.48 4 4.08 4.54 LSD 0.32 0.21 7
Table 3. Changes in exchangeable Ca in Rengam soil with time and depth as affected by GML application. Dates of soil sampling Treatment Depth (cm) JULY 91 AUG 92 FEB 93 FEB 94 Exch Ca (cmol c kg -1 ) T 1 0-15 1.35 0.69 0.36 0.67 15-30 0.89 0.80 0.15 0.12 T 2 0-15 1.87 0.86 0.48 1.24 15-30 1.28 0.16 0.14 0.35 T 3 0-15 1.86 1.35 0.73 0.98 15-30 1.16 0.31 0.22 0.43 T 4 0-15 3.10 1.64 1.50 1.21 15-30 2.15 0.90 1.07 1.28 T 5 0-15 1.17 0.53 0.58 0.43 15-30 1.44 0.32 0.53 0.38 T 6 0-15 2.33 1.16 1.12 1.15 15-30 1.71 0.80 0.99 0.84 T 7 0-15 3.88 1.64 0.85 1.24 15-30 2.73 1.27 1.38 0.75 8
Table 4. Changes in exchangeable Al in Rengam soil with time and depth as affected by GML application. Dates of soil sampling Treatment Depth JULY 91 AUG 92 FEB 93 FEB 94 (cm) (Maize 1) (Maize 2) (Maize 3) (Maize 4) Exch Al (cmol c kg -1 ) T 1 0-15 2.18 0.83 1.10 2.12 15-30 2.15 1.08 1.10 2.31 T 2 0-15 1.45 1.03 1.09 1.77 15-30 1.94 1.34 1.32 1.94 T 3 0-15 1.61 0.54 1.34 1.58 15-30 2.22 1.12 1.04 1.97 T 4 0-15 0.48 0.06 0.52 0.49 15-30 1.17 0.82 0.58 1.27 T 5 0-15 1.55 1.37 1.55 2.51 15-30 1.79 1.09 1.05 2.60 T 6 0-15 1.62 1.05 1.02 1.64 15-30 1.92 1.04 0.80 2.01 T 7 0-15 0.37 0.29 0.53 0.90 15-30 0.94 0.53 0.53 1.51 9